signal Flashcards
1 Reason for needing to understand signal transduction
Cellular signalling helps maintain homeostasis (balance) and are involved in multiple system in the body (eg. hormones)
2 Reason for needing to understand signal transduction
Many medicines control cell signalling events via receptors
Origins of a signal
Signaling Cells:
Cells communicate by releasing extracellular signals (chemical messengers).
These signaling molecules are secreted by a signaling/secreting cell.
Target Cells:
The released signal then diffuses or circulates until it reaches specific target cells.
Target cells have receptors specific to the signal, which allows them to respond to it.
Once the signaling molecule binds to its receptor on the target cell, a series of events occur called signal transduction.
Signal transduction involves chemical messengers within the cell relaying the signal, eventually leading to a cellular response.
Non-Target Cells:
Non-target cells do not have receptors for the signaling molecule and thus do not respond to it.
This specificity ensures that only cells with the appropriate receptors are affected by the signal.
endocrine signaling
Hormone Secretion:
Endocrine glands release hormones (extracellular signals) into the bloodstream.
The hormones travel through the blood to reach distant target cells.
Target Cells:
These target cells have specific receptors that bind to the hormone.
Once the hormone binds to the receptor, it triggers a response in the target cell.
Characteristics of Endocrine Signaling:
It involves signaling over long distances within the body.
A hormone is a chemical signal produced by endocrine glands.
Example: Insulin is a hormone secreted by the pancreas, which then travels in the blood to regulate glucose levels in various tissues.
Paracrine signalling
Paracrine Signal Release:
In paracrine signaling, cells release signaling molecules that affect adjacent target cells. This means that the signaling molecule does not travel far in the body but acts locally.
Mechanism:
The secretory cell releases the signaling molecules, which then bind to receptors on the adjacent target cell.
This type of signaling is important for local cellular interactions and rapid, targeted responses.
Example:
A typical example is the release of acetylcholine at the neuromuscular junction, where it acts on adjacent muscle cells to induce muscle contraction.
autocrine signaling
Autocrine Signal Release:
In autocrine signaling, cells release signaling molecules that act on the same cell type that produced the signal.
This means the cell effectively “talks to itself” to regulate its own activity.
Mechanism:
The secretory cell releases a signaling molecule, which binds to receptors on the same cell or nearby cells of the same type.
This allows the cell to self-regulate, often contributing to processes like cell growth or immune responses.
Example:
Growth factors are common autocrine signals. Cells can release growth factors to promote their own survival, proliferation, or differentiation.
plasma membrane-attached proteins
Key Concepts:
Mechanism:
In this form of signaling, a signaling cell has a ligand bound to its plasma membrane.
The ligand directly interacts with receptors on the plasma membrane of an adjacent target cell.
The physical interaction between cells is essential for signal transduction.
Example:
A classic example is T-cell activation in the immune system, where T-cells interact with antigen-presenting cells (APCs).
The proteins on the surface of APCs bind to receptors on T-cells, initiating an immune response.
how hormones and other extracellular signals initiate cellular responses by interacting with receptors
Key Concepts:
Receptors:
Receptors are molecules (usually proteins) found either on the cell surface (plasma membrane) or within the cell (e.g., cytoplasm).
They bind to specific signaling molecules (e.g., hormones, neurotransmitters), initiating a series of intracellular events.
Signal Transduction:
When a signaling molecule binds to its receptor, it activates a signal transduction pathway within the cell.
This pathway leads to specific cellular responses, such as changes in gene expression, enzyme activity, or metabolic processes.
Examples:
Cell-surface receptors are used for molecules that cannot cross the plasma membrane, like antigens.
Cytoplasmic receptors interact with molecules that can diffuse through the membrane, like steroid hormones.
lock and key analogy to describe the interaction between hormones (or ligands) and their specific receptors on the cell surface
Lock and Key Analogy:
The analogy suggests that each hormone or signaling molecule (the “key”) has a specific receptor (the “lock”) that it can bind to.
The binding is specific, meaning that only the correct ligand can fit into and activate the corresponding receptor, just like a key fits into a specific lock.
Activation of Intracellular Signaling:
When the hormone or ligand binds to the correct receptor, it triggers intracellular signaling, leading to a response in the target cell.
This specificity ensures that signals are directed appropriately to the correct cells and prevents erroneous activation.
Illustration:
The image on the left shows multiple receptors in the plasma membrane, with different ligands fitting only into specific receptors.
The image on the right asks about identifying the correct ligand (A, B, or C) that fits into the receptor, emphasizing specificity in signaling.
Conformational change
Receptor Activation:
When a signaling molecule, like a neurotransmitter, attaches to its specific active site on a receptor, it triggers a conformational change in the receptor.
The receptor behaves like a lock, and the signaling molecule (the ligand) behaves like a key, fitting precisely into this active site.
Intracellular Signaling Cascade:
Once the signaling molecule binds to the receptor, it sets off an intracellular signaling cascade. This cascade involves a series of events inside the cell, ultimately leading to changes in cellular activity.
This process is critical for various cellular responses, such as gene expression, enzyme activation, or changes in ion channel activity.
Medical Implications:
The concept of conformational change is crucial in pharmacology. Many drugs act by either inhibiting or enhancing this type of signaling. For example, some drugs can mimic natural signaling molecules (agonists) or block their binding (antagonists) to regulate cellular responses.
the process of transmitting messages across the cell membrane involving a conformational change in the receptor
Inactive Receptor:
The receptor begins in an inactive state, which means it is not bound to a ligand (e.g., hormone) and has a specific shape.
Hormone Binding:
When a hormone binds to the receptor, it interacts with its specific binding site, similar to how a key fits into a lock.
Conformational Change:
Conformational change means the receptor undergoes a shape change in response to the binding of the hormone.
This shape change is essential because it activates the receptor and allows it to initiate an intracellular response without the hormone actually passing through the cell membrane.
Receptor as a Gate-Keeper:
The receptor functions as a “gate-keeper” of cellular activity, controlling when a response occurs based on external signals like hormone binding. This regulation ensures that cells respond only to appropriate signals, maintaining proper cellular function.
This mechanism is crucial for signal transduction in cells, where an external signal (e.g., hormone) is converted into a specific cellular response, which might involve processes like gene activation or ion channel opening.
how drugs exploit the lock and key mechanism of cellular receptors to create highly specific medicines with minimal side effects.
How Classic Drugs Act:
Inhibitory Drugs (Left Panel):
An inhibitory drug binds to the receptor’s active site, blocking the native signaling molecule from attaching.
This prevents the receptor from activating, resulting in no signal being transmitted into the cell.
This action is often used to suppress unwanted activities in the cell, such as overactive signaling in conditions like hypertension.
Mimics of Native Signaling Molecules (Right Panel):
A drug can also act as a mimic of the native signaling molecule, binding to the receptor just like the natural molecule would.
Once bound, this mimicking drug activates the receptor and initiates a signal inside the cell.
This action can be used to replace or enhance a natural process in the body, such as when an agonist drug mimics a hormone to compensate for a deficiency.
Summary:
Drugs take advantage of the specific lock and key mechanism of receptors to either block or mimic natural signaling.
This specificity helps ensure that the effect is targeted, which is why such drugs tend to have fewer side effects compared to non-specific treatments.
Targeting specific receptors allows pharmaceutical drugs to control cellular activities effectively, making them potent therapeutic agents.
Agonists:
Definition: Agonists are ligands that bind to a receptor and activate it, triggering a signaling pathway that results in a physiological response.
Effect: They produce the maximal response possible for a given tissue.
Partial Agonists: These are similar to full agonists but produce a submaximal response compared to the maximum response elicited by a full agonist.
2. Antagonists:
Definition: Antagonists are ligands that bind to a receptor but do not activate it. Instead, they block the receptor and prevent agonists from binding.
Effect: They produce no visible response on their own and inhibit the effects of agonists by occupying the receptor, effectively blocking the pathway.
Summary:
Agonists initiate signaling by binding to and activating receptors, while antagonists block this action by preventing agonists from attaching.
This interaction plays a significant role in drug development and therapeutic interventions, as manipulating receptor activity can regulate physiological functions or mitigate pathological conditions.
Three receptor types (classes)
- G Protein-Coupled Receptor (GPCR)
Structure: Typically contains seven transmembrane helices.
Function: GPCRs are activated by binding an external ligand, which induces a conformational change that activates associated G-proteins.
Signaling Pathway: These G-proteins then trigger the production of second messengers (e.g., cAMP), which relay the signal inside the cell. - Receptor Tyrosine Kinase (RTK)
Structure: Has a single transmembrane domain with an intracellular tyrosine kinase domain.
Function: Binding of a ligand (e.g., growth factor) induces dimerization and autophosphorylation of the intracellular tyrosine residues.
Signaling Pathway: This phosphorylation initiates a phosphorylation cascade, which ultimately results in changes in gene expression or cellular activity. - Ligand-Gated Ion Channel (LGIC)
Structure: Composed of multiple subunits that form a channel through the membrane.
Function: Binding of a ligand (e.g., a neurotransmitter) induces the opening of the channel, allowing ion flow across the cell membrane.
Signaling Pathway: This direct ion flow changes the membrane potential, leading to a rapid response, such as the initiation of an action potential in neurons.
Three receptor types (classes)
- G Protein-Coupled Receptors (GPCRs)
Description: These receptors work with the help of a G protein to relay signals inside the cell.
Mechanism: When a ligand binds to a GPCR, the receptor activates an associated G protein, which in turn activates other proteins and signaling pathways within the cell. - Receptor Tyrosine Kinases (RTKs)
Description: These receptors attach phosphate groups to tyrosines in target proteins.
Mechanism: Upon binding a ligand (e.g., growth factor), RTKs dimerize, leading to autophosphorylation of tyrosine residues on their cytoplasmic domains. This phosphorylation activates downstream signaling pathways involved in cell growth, differentiation, and metabolism. - Ligand-Gated Ion Channel Receptors
Description: These receptors act as a gate for specific ions.
Mechanism: When a ligand binds to the receptor, it causes the channel to open, allowing specific ions (e.g., Na⁺, Ca²⁺) to flow through the channel into or out of the cell, changing the cell’s membrane potential and triggering downstream responses.
Note on Intracellular Receptors
Intracellular receptors are found within the cytoplasm or nucleus of target cells and respond to small or hydrophobic chemical messengers that can readily cross the plasma membrane, such as steroid and thyroid hormones.
Overview of signal transduction
Signal Transduction Pathways:
The signal from an activated receptor is relayed through cascades of molecular interactions to target molecules inside the cell.
These pathways generally involve multiple steps, allowing for a more controlled and amplified response.
Amplification and Regulation:
Multistep pathways can greatly amplify a signal. For example, a single signaling molecule can lead to the activation of many molecules in a cell.
Multistep pathways also provide more opportunities for coordination and regulation, ensuring that the cellular response is precise and appropriate.
Mechanisms Involved:
Two common mechanisms in many signal transduction pathways are:
Second Messengers: Small molecules that spread within the cell to help relay the signal.
Phosphorylation: The addition of a phosphate group to a protein, which often activates or deactivates the protein, allowing signal propagation and regulation
Second messenges
Produced Following Receptor Activation:
Second messengers are generated after a receptor on the cell surface is activated by an external signal, like a hormone or ligand.
Chemical Signals:
Unlike primary signals (ligands) that bind to cell-surface receptors, second messengers are small, diffusible molecules within the cell. They are not embedded in the membrane, allowing them to move freely within the cell.
Concentration Changes:
The concentration of second messengers can fluctuate in response to an external signal. These changes act as information carriers, propagating the initial signal from the cell membrane to various target molecules inside the cell.
First Messenger:
The first messenger is the hormone or ligand that initially activates the receptor.
common second messengers
cAMP (cyclic adenosine monophosphate) and cGMP (cyclic guanosine monophosphate):
These molecules are derived from ATP or GTP and act as important intracellular signaling molecules. They typically activate protein kinases, leading to various cellular responses.
The structure shown includes a cyclic ring with the base marked as “A or G,” representing adenine or guanine.
Calcium ion (Ca²⁺):
Calcium acts as a versatile second messenger in many cellular processes, including muscle contraction and neurotransmitter release.
Its structure involves multiple water molecules bound around the central calcium ion, highlighting its hydrated form.
Inositol 1,4,5-triphosphate (IP₃):
IP₃ is generated from phosphatidylinositol-4,5-bisphosphate (PIP₂) and plays a key role in releasing calcium from intracellular stores.
The structure shows the inositol ring with phosphate groups attached.
Diacylglycerol (DAG):
DAG works alongside IP₃ as a second messenger, activating protein kinase C (PKC), which helps regulate various cellular functions.
The structure is composed of a glycerol backbone attached to fatty acid chains.
how common second messengers can generate different cellular responses depending on various elements of specificity in cell signaling
Response 1 (Cell A): The signaling molecule binds to its receptor, triggering a pathway that leads to a single, specific response. This type of response highlights the linear and straightforward relay of the signal through relay molecules to achieve a particular outcome.
Response 2 & 3 (Cell B): In this scenario, the same receptor activation can cause pathway branching, leading to two distinct responses. Branching enables more complex outcomes from a single stimulus, allowing a greater degree of control and versatility in cellular functions.
Response 4 (Cell C): This response involves cross-talk between different pathways. Cross-talk allows for interactions between signaling pathways, which helps integrate multiple signals and regulate cellular functions through either activation or inhibition of different pathways. This ensures that the cell can respond to a combination of stimuli in a coordinated manner.
Response 5 (Cell D): Here, the signaling molecule binds to a different type of receptor, leading to a completely different response. This highlights how the nature of the receptor determines the specific downstream effects, even if the signaling molecule is the same.
the process of phosphorylation
Phosphorylation involves the addition of a phosphate group to a protein, typically by an enzyme called protein kinase. This modification can change the protein’s shape, activity, or interaction with other molecules, effectively acting as an on/off switch or modulating activity up or down.
The diagram shows a signaling molecule binding to a receptor, which activates a relay molecule. This activated relay molecule can then activate an inactive protein kinase by transferring a phosphate group to it, converting it into an active protein kinase.
Dephosphorylation is the removal of a phosphate group by an enzyme called phosphatase, reversing the activation and allowing the protein to return to its inactive state.
Protein kinases and phosphatases are crucial players in signal transduction pathways, allowing for the control of cellular responses through a cascade of phosphorylation events.
This phosphorylation cascade amplifies the signal, providing opportunities for regulation and coordination of cellular activities. This makes it a widespread cellular mechanism for regulating protein activity, ensuring that signals can be fine-tuned in response to various conditions.
phosphorylation cascade in cell signaling-
Activation of Protein Kinase:
A signaling molecule binds to a receptor, which activates a relay molecule. This relay molecule then activates protein kinase 1 by transferring a phosphate group from ATP to the protein.
The active protein kinase 1 can then activate protein kinase 2, continuing the phosphorylation cascade.
Phosphorylation Cascade:
The process involves a series of protein kinases activating other protein kinases by phosphorylation (adding phosphate groups).
This is shown with the use of ATP, where ATP donates a phosphate to the kinase, resulting in ADP as a byproduct.
The phosphorylation cascade allows for multiple opportunities to amplify the original signal, as each kinase can activate several downstream kinases.
Specificity of Phosphorylation:
The diagram indicates that three amino acids can typically be phosphorylated, depending on the specific kinase:
Tyrosine
Serine
Threonine
These phosphorylations alter the activity or function of the protein, propagating the signal.
Activation of Target Protein:
The cascade eventually activates an inactive protein that brings about the final cellular response.
Dephosphorylation:
Protein phosphatases rapidly remove phosphate groups from proteins, effectively turning off the signal once the response is no longer needed. This ensures that cellular activity is tightly regulated.
Amplification is an important feature of signal transduction pathway
Amplification Mechanism:
When a single receptor is activated by a ligand, it generates multiple second messenger molecules (indicated by the blue circles).
These second messengers further activate multiple downstream molecules (red rectangles), amplifying the original signal.
Importance of Amplification:
At each step of the signaling cascade, the number of activated molecules is much greater than in the preceding step. This means that only a small number of initial hormone or ligand molecules need to bind to receptors to elicit a significant cellular response.
Amplification ensures that a tiny stimulus can lead to a large and efficient response, making cellular processes more sensitive and efficient.
Efficiency in Cellular Signaling:
The ability to amplify signals is critical for cells to respond to low concentrations of signaling molecules effectively.
It also allows cells to use minimal resources to produce a substantial physiological response, optimizing cellular function.
Overview of GPCR signalling
Signal: The process starts with an endocrine signal, which is epinephrine in this case. Epinephrine binds to the β-adrenergic receptor, a GPCR, embedded in the cell membrane.
Reception: The GPCR undergoes a conformational change upon binding with epinephrine, which activates the G-protein.
Transduction: The activation is transferred internally by:
G-protein Activation: The G-protein is a trimeric protein with α, β, and γ subunits. Initially, the α-subunit is bound to GDP (inactive state). Once the GPCR is activated, the G-protein exchanges GDP for GTP, activating the G-protein. This leads to the dissociation of the α-subunit from the β and γ subunits.
Primary Effector: The α-subunit activates adenylate cyclase, which is the primary effector protein in this pathway.
Second Messenger: Adenylate cyclase converts ATP to cyclic AMP (cAMP), which acts as the second messenger.
Secondary Effector Protein: cAMP activates protein kinase A (PKA), which acts as the secondary effector protein.
Phosphorylation: PKA then phosphorylates target proteins, leading to the desired cellular response.
Key Steps and Concepts:
Amplification: A single molecule of epinephrine can lead to the activation of multiple G-proteins, which in turn generate multiple cAMP molecules, amplifying the signal at each step.
Second Messenger System: cAMP is a crucial second messenger that helps relay the signal from the cell membrane to intracellular targets.
Effector proteins (in signal transduction)
Effector proteins are molecules (often enzymes) inside a cell that responds to a stimulus and can be activated and further transduce a signal.
GCPRs exist in a range of organisms and express at the cell surface to respond to diverse extracellular signals
Overview of GPCR Activation:
GPCRs are expressed at the cell surface and respond to a wide range of extracellular signals.
GPCRs are only active at the cell surface, where signals have access to them.
Types of Signals:
Small Molecules: This includes amino acids, free fatty acids, nucleotides, amines, and prostaglandins. These can bind to specific GPCRs to initiate intracellular signaling.
Ca²⁺ Ions: Calcium ions can act as signaling molecules that trigger GPCR activation.
Peptides: Small peptide molecules can serve as ligands for GPCRs.
Proteins: Larger protein ligands, such as TSH (thyroid-stimulating hormone), LH (luteinizing hormone), FSH (follicle-stimulating hormone), and chemokines, are also capable of activating GPCRs.
Light: Light can activate specific GPCRs, such as rhodopsin in photoreceptor cells, indicating their role in sensory functions like vision.
Key Points:
Diverse Ligands: GPCRs can interact with a variety of extracellular molecules, from ions and small molecules to complex proteins and environmental stimuli like light.
Versatile Function: Due to their ability to respond to numerous types of signals, GPCRs play essential roles in multiple physiological processes, including sensory perception, immune response, and hormone signaling.
structural organization of a G-protein-coupled receptor (GPCR) as it spans the cell membrane
GPCR Structure:
Transmembrane Domains:
The receptor consists of seven transmembrane helices, labeled 1 through 7.
These transmembrane segments traverse the cell membrane, with alternating loops on either side of the membrane.
Loops:
There are three intracellular loops (ICL), which are highlighted in green and labeled as ICL 1, 2, and 3. These loops are located on the cytoplasmic side of the membrane and are important for interacting with G-proteins and other intracellular signaling molecules.
There are also three extracellular loops (ECL), highlighted in red and labeled as ECL 1, 2, and 3. These loops are located on the outside of the cell and help bind ligands such as hormones or neurotransmitters.
N and C Termini:
The N-terminus (N) is located on the extracellular side of the cell membrane. This region can play a role in ligand binding or receptor regulation.
The C-terminus (C) is on the intracellular side of the membrane and is involved in interacting with intracellular signaling proteins or other components that help regulate receptor activity.
Functional Insights:
Seven-Transmembrane Structure: This helical arrangement of GPCRs is characteristic of this receptor family and is crucial for their ability to transduce extracellular signals into intracellular responses.
Intracellular Loops: The intracellular loops are important for activating G-proteins by promoting the exchange of GDP for GTP, initiating downstream signaling.
Extracellular Loops: The extracellular loops and the N-terminus help determine the specificity of the receptor for different ligands, contributing to the wide variety of signaling pathways mediated by GPCRs.
comparison of the structures of two G-protein-coupled receptors (GPCRs): rhodopsin (Panel A) and the β₂-adrenergic receptor (Panel B)
Panel A: Rhodopsin
Transmembrane Helices: The structure consists of seven transmembrane α-helices, a common feature of all GPCRs.
Ligand-Binding Site: In rhodopsin, the ligand-binding site is located within the transmembrane helices on the extracellular side. Rhodopsin is activated by light, and the ligand is retinal, which undergoes a conformational change upon photon absorption.
N and C Termini:
The N-terminus (N) is located on the extracellular side, while the C-terminus (C) is on the cytoplasmic side.
These termini play a role in ligand binding and intracellular signaling, respectively.
Panel B: β₂-Adrenergic Receptor
Transmembrane Helices: Similar to rhodopsin, the β₂-adrenergic receptor also has seven transmembrane α-helices.
Blocker-Binding Site: The blocker-binding site is highlighted here, showing that antagonists or blockers bind to a region within the transmembrane domain to inhibit the receptor’s activity. This binding prevents agonists, such as epinephrine, from activating the receptor and subsequently triggering downstream signaling pathways.
N and C Termini:
The N-terminus (N) is on the extracellular side, while the C-terminus (C) is on the cytoplasmic side.
The C-terminus interacts with intracellular proteins, including G-proteins, that mediate the cellular response.
Key Similarities and Differences:
Transmembrane Architecture: Both receptors feature seven transmembrane helices, which are crucial for signal transduction across the cell membrane.
Binding Sites:
In rhodopsin, the ligand-binding site is used for light absorption and initiating the visual signaling pathway.
In the β₂-adrenergic receptor, the blocker-binding site highlights how antagonists interact with the receptor to prevent activation by natural ligands like epinephrine.
Function:
Rhodopsin is a light-sensitive receptor involved in vision.
The β₂-adrenergic receptor is involved in the sympathetic nervous system, where it mediates responses to hormones such as epinephrine, leading to effects like increased heart rate.
How G-protein-coupled receptors (GPCRs) transmit messages across the cell membrane by undergoing a conformational change
Overview of the Process:
Receptor Activation:
The GPCR consists of seven transmembrane helices.
When a ligand (e.g., a hormone) binds to the receptor on the extracellular side, it triggers a conformational change (shape change) in the receptor structure.
Conformational Change:
The conformational change is represented as a change in the orientation of the transmembrane helices, causing internal shifts.
This change enables the receptor to interact with a G-protein located on the cytoplasmic side of the membrane.
G-Protein Activation:
The G-protein (purple) interacts with the receptor following the conformational change.
The interaction with the receptor causes the G-protein to exchange GDP for GTP, leading to its activation.
The G-protein then dissociates into its active subunits, which can activate downstream signaling pathways.
structure of a G-protein (transducer)
G-Protein Overview:
G-protein stands for guanosine-binding protein. It is called this because it binds guanine nucleotides, such as GDP (guanosine diphosphate) and GTP (guanosine triphosphate).
G-proteins are involved in transducing signals from the GPCR to downstream intracellular pathways.
Structure of G-Protein:
Trimeric Structure: G-proteins are heterotrimeric, meaning they are composed of three different subunits:
α (Alpha) Subunit:
The α subunit binds guanine nucleotides (GDP or GTP).
It is responsible for the activation and inactivation of the G-protein.
When GDP is bound, the G-protein is inactive. Upon activation by a GPCR, GDP is exchanged for GTP, which triggers activation.
β (Beta) Subunit:
The β subunit forms a stable complex with the γ subunit.
It helps to anchor the G-protein to the cell membrane and is involved in downstream signaling.
γ (Gamma) Subunit:
The γ subunit also forms a complex with the β subunit.
Together, the βγ dimer can interact with other signaling molecules and play a role in intracellular signal propagation.
Activation and Dissociation:
Activation:
The α subunit is initially bound to GDP in its inactive state.
When a GPCR is activated by a ligand, the receptor facilitates the exchange of GDP for GTP on the α subunit.
Dissociation:
Once bound to GTP, the α subunit dissociates from the βγ dimer.
Both the α subunit (now active) and the βγ dimer can independently interact with other proteins to propagate the signal within the cell.
structure of a G-protein, which functions as a transducer in G-protein-coupled receptor (GPCR) signaling
Detailed G-Protein Structure
The G-protein is composed of three subunits: α (alpha), β (beta), and γ (gamma).
α Subunit:
The GDP (guanosine diphosphate) is shown bound to the α subunit, which indicates that the G-protein is in its inactive state.
This subunit binds to guanine nucleotides (GDP or GTP) and plays a central role in G-protein activation and inactivation.
β and γ Subunits:
The β and γ subunits form a βγ dimer, which stays together during the activation and deactivation cycles of the G-protein.
The βγ dimer helps to anchor the G-protein to the plasma membrane and also participates in signaling by interacting with other intracellular targets.
Panel B: Simplified Representation
Subunits: This panel shows a more simplified cartoon representation of the G-protein, highlighting the α, β, and γ subunits.
The GDP molecule is bound to the α subunit, again indicating the inactive state of the G-protein.
The βγ dimer is represented as a single unit that interacts closely with the α subunit.
Activation Mechanism:
The G-protein is heterotrimeric, consisting of an α subunit and a βγ dimer.
In the inactive state, the α subunit is bound to GDP.
When a GPCR is activated by a ligand, the α subunit exchanges GDP for GTP, leading to the activation of the G-protein.
Once activated, the α subunit dissociates from the βγ dimer, and both parts can go on to interact with other proteins in the cell, propagating the signal.
G protein cycle
“OFF” Position:
The G protein is in its inactive form when bound to GDP (guanine diphosphate).
It exists as a trimer, consisting of the α, β, and γ subunits.
Activation:
A ligand (such as a hormone or neurotransmitter) binds to the G protein-coupled receptor (GPCR).
This causes a conformational change, leading to the release of GDP from the α subunit.
GDP is replaced by GTP (guanine triphosphate), activating the G protein.
“ON” Position:
In the active state, the Gα subunit is bound to GTP and dissociates from the βγ subunits.
The Gα subunit can now interact with downstream effectors, such as adenylate cyclase (AC), leading to the production of secondary messengers like cAMP.
Deactivation:
The Gα subunit has intrinsic GTPase activity and hydrolyzes GTP to GDP, switching itself off.
This step automatically deactivates the Gα subunit.
Reassociation:
Once GTP is hydrolyzed to GDP, the Gα subunit reassociates with the βγ subunits, returning to the inactive trimeric form.
Different families of G proteins and their respective effects in cell signaling
Gs (Stimulatory G protein):
Gs proteins activate adenylate cyclase, an enzyme that converts ATP into cyclic AMP (cAMP).
Increased cAMP levels lead to the activation of protein kinase A (PKA), which then phosphorylates various cellular targets to mediate the cell’s response.
Gi (Inhibitory G protein):
Gi proteins inhibit adenylate cyclase, leading to reduced levels of cAMP.
This prevents the activation of PKA and decreases the downstream effects associated with cAMP signaling.
Gq:
Gq proteins activate phospholipase C (PLC), an enzyme that hydrolyzes phosphatidylinositol 4,5-bisphosphate (PIP2) into two secondary messengers: inositol trisphosphate (IP3) and diacylglycerol (DAG).
IP3 increases intracellular calcium levels, while DAG activates protein kinase C (PKC), leading to further cellular responses.
1 effector of adenylate cyclase
Adenylate Cyclase Overview:
Adenylate cyclase is an enzyme located in the plasma membrane, and it plays a crucial role in converting ATP (adenosine triphosphate) into cyclic AMP (cAMP).
It is activated by Gs proteins (stimulatory G proteins) when they are bound to GTP.
Once activated, adenylate cyclase catalyzes the conversion of ATP to cAMP, which serves as a second messenger to propagate intracellular signaling.
Enzyme Activation:
The enzyme contains multiple transmembrane domains (shown as M1 and M2) that help anchor it to the plasma membrane.
It also has catalytic domains (C1 and C2), which combine to form an active site for ATP conversion.
Reaction Mechanism:
The reaction involves ATP being hydrolyzed to form cAMP and pyrophosphate (PPi).
cAMP acts as a signaling molecule that activates protein kinase A (PKA), leading to further downstream effects that modify cell behavior.
Regulation:
Adenylate cyclase is regulated by different G proteins: Gs stimulates it, while Gi inhibits its activity, balancing cellular cAMP levels.
role of cyclic adenosine monophosphate (cAMP) as a second messenger in cell signaling
cAMP as a Second Messenger:
cAMP is considered the first “second messenger,” meaning it relays signals from outside the cell to the inside, allowing for cellular responses.
It acts as a common mediator for many hormones, such as adrenaline, and has widespread functions across various cell types.
Historical Context:
The discovery of cAMP occurred in the late 1950s, which was a major advancement in understanding hormone action and signal transduction.
The Nobel Prize in Physiology or Medicine in 1971 was awarded for the discoveries related to the mechanisms of hormone action, particularly involving cAMP.
Production:
cAMP is synthesized from ATP by the enzyme adenylate cyclase, which is activated by stimulatory G proteins (Gs) upon binding of a ligand to a G protein-coupled receptor (GPCR).
Once produced, cAMP activates protein kinase A (PKA), leading to a cascade of cellular events that influence physiological functions.
Comparing the roles of Gs and Gi proteins in regulating adenylate cyclase (AC), cAMP production, and downstream signaling through protein kinase A (PKA)
Gs Protein Pathway (Stimulatory):
When a ligand binds to a stimulatory receptor, the Gs protein is activated.
Gs activates adenylate cyclase (AC), leading to increased conversion of ATP to cAMP.
Higher cAMP levels activate PKA, which then phosphorylates various cellular targets, leading to physiological effects such as increased metabolic activity or secretion.
This pathway generally promotes a cellular response through increased cAMP levels.
Gi Protein Pathway (Inhibitory):
When a ligand binds to an inhibitory receptor, the Gi protein is activated.
Gi inhibits adenylate cyclase, which reduces cAMP production.
Lower cAMP levels lead to decreased activation of PKA, reducing phosphorylation events and thus suppressing cellular responses.
This pathway essentially opposes the Gs-mediated pathway, decreasing cAMP levels and thus dampening cellular activity.
Opposing Effects:
The Gs and Gi proteins have opposing effects on adenylate cyclase activity, resulting in an increase or decrease in cAMP levels, respectively.
This dual regulation allows for precise control of cellular responses to external stimuli, ensuring that the cell can adjust its activity depending on the needs of the organism.
overview of how Gs and Gi proteins regulate adenylate cyclase (AC), which in turn affects cyclic AMP (cAMP) levels and the activation of protein kinase A (PKA)
Gs and Gi Proteins:
Gs (Stimulatory G protein): Gs proteins activate adenylate cyclase, leading to an increase in cAMP production.
Gi (Inhibitory G protein): Gi proteins inhibit adenylate cyclase, resulting in a decrease in cAMP levels.
The opposing actions of Gs and Gi provide a balanced regulation of cAMP concentrations within the cell.
Adenylate Cyclase and cAMP:
Adenylate cyclase (AC) is the enzyme that converts ATP to cyclic AMP (cAMP).
Gs activation favors a catalytically active form of AC, resulting in increased cAMP production.
Gi activation, on the other hand, favors a catalytically inactive form of AC, leading to reduced cAMP production.
Protein Kinase A (PKA):
cAMP acts as a second messenger and directly activates PKA.
PKA then phosphorylates downstream targets, leading to various cellular responses.
The level of PKA activity is directly influenced by cAMP concentration, which is controlled by Gs and Gi protein regulation.
Opposing Effects:
By regulating adenylate cyclase, Gs and Gi proteins have opposing effects on cellular signaling pathways.
Gs leads to increased cAMP and enhanced PKA activity, while Gi leads to decreased cAMP and reduced PKA activity.
This mechanism allows precise control over cellular responses, ensuring appropriate reaction to different external signals.
Gq protein signaling pathway involving phospholipase C (PLC), diacylglycerol (DAG), inositol trisphosphate (IP₃), calcium (Ca²⁺), and protein kinase C (PKC)
Activation of Gq Protein:
The process begins when a ligand binds to a G protein-coupled receptor (GPCR).
This activates the Gq protein, which dissociates and interacts with downstream targets.
Activation of Phospholipase C (PLC):
Gq activates PLC, an enzyme embedded in the plasma membrane.
PLC hydrolyzes phosphatidylinositol 4,5-bisphosphate (PIP₂), a membrane phospholipid, into two secondary messengers: DAG and IP₃.
Roles of DAG and IP₃:
DAG (Diacylglycerol): Remains within the plasma membrane and activates protein kinase C (PKC).
IP₃ (Inositol Trisphosphate): Travels to the endoplasmic reticulum (ER), where it binds to IP₃ receptors, causing the release of calcium ions (Ca²⁺) into the cytosol.
Calcium and Protein Kinase C (PKC):
The release of Ca²⁺ from the ER increases intracellular calcium levels, which, along with DAG, helps activate PKC.
PKC then phosphorylates a variety of downstream targets, leading to different cellular responses.
Key Components:
PLC (Phospholipase C): Enzyme that cleaves PIP₂.
PIP₂ (Phosphatidylinositol Bisphosphate): Substrate cleaved by PLC to produce DAG and IP₃.
PKC (Protein Kinase C): A kinase activated by DAG and Ca²⁺ that phosphorylates target proteins, contributing to a range of cellular processes.
amplification in G protein-coupled receptor (GPCR) signaling
Reception
Binding of Epinephrine: The process begins when a single molecule of epinephrine binds to a G protein-coupled receptor (GPCR) on the cell surface.
2. Transduction
Activation of G Proteins: The binding of epinephrine activates multiple G proteins. Each GPCR can activate approximately 100 G proteins, leading to a significant amplification of the signal.
Adenylate Cyclase Activation: Each active G protein then activates adenylate cyclase. Around 100 molecules of adenylate cyclase are activated, further amplifying the signal.
Production of cAMP: Adenylate cyclase catalyzes the conversion of ATP to cyclic AMP (cAMP). Each activated adenylate cyclase produces many cAMP molecules, resulting in approximately 10,000 molecules of cAMP being generated.
Activation of Protein Kinase A (PKA): The increase in cAMP levels activates PKA. About 10,000 PKA molecules are activated, which continue the signal transduction cascade.
Activation of Phosphorylase Kinase: PKA then activates phosphorylase kinase, with around 100,000 molecules becoming active.
Activation of Glycogen Phosphorylase: Phosphorylase kinase, in turn, activates glycogen phosphorylase, which is responsible for breaking down glycogen into glucose-1-phosphate. Approximately 1,000,000 glycogen phosphorylase molecules are activated.
3. Response
Glycogen Breakdown: The amplified signal results in the breakdown of glycogen, releasing around 100,000,000 molecules of glucose-1-phosphate, which can be used for energy production in the cell.
Amplification Summary
The diagram effectively demonstrates how a single initial signal (epinephrine binding) can be amplified through multiple steps in the signaling cascade, ultimately leading to a significant cellular response.
This is achieved through sequential activation of multiple signaling molecules, where each step in the cascade results in the activation of a larger number of downstream molecules.
This amplification is crucial for cells to effectively respond to low concentrations of hormones or other signaling molecules, allowing for robust physiological effects.
signaling mechanisms in different sensory perceptions
Vision:
GPCRs (G Protein-Coupled Receptors): Vision relies heavily on GPCRs, specifically rhodopsin in the retina. When light activates rhodopsin, it initiates a signal transduction pathway involving G proteins, which ultimately affects ion channels and results in visual signal perception.
2. Smell (Olfaction):
GPCRs: The sense of smell uses GPCRs called olfactory receptors, which bind to odorant molecules. This binding activates a G protein pathway, which leads to an increase in cyclic AMP (cAMP) and ultimately the opening of ion channels, triggering nerve impulses sent to the brain.
3. Taste:
GPCRs and Ion Channels: Taste involves a combination of GPCRs and ion channels. Sweet, bitter, and umami tastes are detected by GPCRs, which activate G protein pathways, while salty and sour tastes are detected directly through ion channels that respond to specific ions (e.g., Na⁺ for salty and H⁺ for sour).
4. Hearing:
Ion Channels: The perception of sound relies on mechanoreceptors and ion channels. Sound waves cause vibrations in the ear, leading to the opening of mechanosensitive ion channels in hair cells within the cochlea. This triggers action potentials sent to the brain.
5. Touch:
Ion Channels: The sense of touch relies on mechanoreceptors that activate ion channels in response to physical stimuli. These ion channels are sensitive to mechanical forces, allowing the detection of pressure, vibration, and other tactile sensations.
Resting Membrane Potential:
The neuron starts at a resting membrane potential of approximately -70 mV.
At rest, there is an uneven distribution of ions, with more sodium ions (Na⁺) outside the neuron and more potassium ions (K⁺) inside, maintained by the sodium-potassium pump.
2. Depolarization:
Influx of Na⁺: When the neuron is stimulated, voltage-gated sodium channels open, allowing Na⁺ ions to flow into the cell.
This influx of Na⁺ causes the membrane potential to become more positive, moving away from -70 mV. This is called depolarization.
The membrane potential can reach around +30 to +40 mV, at which point the sodium channels close.
3. Repolarization:
Efflux of K⁺: After reaching peak depolarization, voltage-gated potassium channels open, allowing K⁺ ions to flow out of the cell.
The efflux of K⁺ leads to a decrease in membrane potential, bringing it back towards the negative resting state. This is called repolarization.
4. Hyperpolarization:
During repolarization, the membrane potential often becomes more negative than the resting potential. This is known as hyperpolarization.
This occurs because K⁺ channels are slow to close, resulting in an overshoot where the membrane potential dips below -70 mV.
5. Return to Resting Potential:
After hyperpolarization, the neuron returns to its resting membrane potential of -70 mV, aided by the sodium-potassium pump, which restores the original ion concentrations.
Summary of Ion Movements:
Depolarization is driven by the influx of Na⁺ ions into the neuron.
Repolarization occurs due to the efflux of K⁺ ions out of the neuron.
Hyperpolarization results when K⁺ continues to leave, making the inside of the neuron more negative than its resting state.
Vision
Photoreceptors: Rods and Cones
Vision is based on the absorption of light by two types of photoreceptor cells located in the retina: rods and cones.
Rods:
Function: Rods are highly sensitive to light, making them responsible for vision in dim light conditions. They do not provide color information but are essential for contrast and detecting shades of gray.
Role in Vision:
Peripheral Vision: Rods are distributed more peripherally in the retina, making them crucial for peripheral vision.
They are highly sensitive to light, making them ideal for night vision and low-light environments.
Structure:
The outer segment contains numerous discs that have photopigments sensitive to light.
Cones:
Function: Cones are responsible for color vision and work best under bright light conditions. They provide high visual acuity.
Role in Vision:
Bright Light and Color: Cones allow us to perceive color and are responsible for sharp, detailed central vision.
Central Vision: Cones are concentrated in the central part of the retina, especially in the fovea, which provides the sharpest vision.
Structure:
The outer segment contains folds rather than discs, and each cone has photopigments sensitive to specific wavelengths corresponding to different colors (red, green, blue).
Phototransduction Process:
When light enters the eye and hits the photoreceptors, it leads to changes in the photopigments located in the outer segment, initiating a cascade of events that ultimately result in an electrical signal being sent to the brain.
Summary:
Rods: Specialized for low-light conditions, contrast, and peripheral vision.
Cones: Specialized for bright light, color vision, and central vision.
process of visible light absorption in photoreceptor cells, specifically in the rods of the eye
Rhodopsin and 11-cis-Retinal:
Rhodopsin is the photoreceptor molecule present in rod cells, and it plays a key role in the detection of light.
Rhodopsin consists of:
Opsin: A protein with seven transmembrane domains.
11-cis-Retinal: A prosthetic group (a non-protein component) covalently attached to opsin, which acts as a chromophore—a light-absorbing molecule.
Structure and Function:
Opsin:
Opsin is a 7-transmembrane protein that determines the specific wavelength of light that is absorbed by rhodopsin.
The protein’s structure plays a role in anchoring 11-cis-retinal and allowing it to change shape upon exposure to light.
11-cis-Retinal:
11-cis-Retinal is derived from vitamin A and is crucial for capturing light photons.
It is the light-absorbing group that changes configuration upon light exposure.
Phototransduction Mechanism:
Light Absorption:
When light hits rhodopsin, it causes 11-cis-retinal to undergo a change in structure. This change is called isomerization, in which 11-cis-retinal is converted to all-trans-retinal.
Isomerization:
The isomerization of retinal leads to a conformational change in the opsin protein.
This structural change activates the phototransduction cascade, ultimately resulting in an electrical signal being generated and transmitted to the brain, where it is interpreted as vision.
Link to Vitamin A:
The function of 11-cis-retinal is directly linked to vitamin A because retinal is a derivative of this vitamin.
A deficiency in vitamin A can impair the production of retinal, leading to visual issues such as night blindness, where the ability to see in low-light conditions is compromised.
Summary:
Rhodopsin, composed of the protein opsin and the chromophore 11-cis-retinal, is responsible for light absorption in rod cells.
Upon exposure to light, 11-cis-retinal isomerizes to all-trans-retinal, leading to the activation of the phototransduction cascade.
The changes in rhodopsin ultimately allow the conversion of light energy into an electrical signal, a process essential for vision.
similarity between chromophore activation in rhodopsin and ligand binding in G protein-coupled receptors (GPCRs), specifically seven-transmembrane (7TM) receptors
Chromophore and Rhodopsin Activation:
Rhodopsin Activation:
Rhodopsin, found in rod cells, is a 7-transmembrane (7TM) protein similar in structure to GPCRs.
In the left diagram, 11-cis-retinal, a light-sensitive chromophore, is bound within rhodopsin.
When light hits rhodopsin, 11-cis-retinal isomerizes to all-trans-retinal, leading to a structural change in rhodopsin.
The activated form of rhodopsin, called metarhodopsin II, undergoes a conformational change that triggers the downstream phototransduction cascade, which is critical for visual signaling.
Ligand-Bound 7TM Receptor:
Analogy to Ligand Binding:
The right diagram shows a typical GPCR (7TM receptor) in two states—inactive and ligand-bound.
A ligand (such as a hormone or neurotransmitter) binds to the receptor, inducing a conformational change in the receptor.
This change is analogous to the conformational shift in rhodopsin when light activates it.
The ligand-induced conformational change in GPCRs activates an intracellular signaling cascade, leading to cellular responses.
Similarities Between Chromophore Activation and Ligand Binding:
Both rhodopsin and GPCRs are 7-transmembrane (7TM) receptors, sharing structural similarities.
The chromophore (11-cis-retinal) in rhodopsin acts in a manner similar to a ligand binding to a GPCR. Both interactions cause structural changes in the receptor protein that are essential for activating intracellular signaling.
In rhodopsin, light acts as the activating factor, leading to isomerization of the retinal chromophore, whereas in GPCRs, the binding of an external ligand (e.g., hormone, neurotransmitter) triggers activation.
How color vision is mediated by three distinct types of cone receptors in the human eye
Cone Receptors and Color Vision:
Cone cells in the retina are responsible for detecting color. These cells contain opsins, proteins that determine the wavelength of light absorbed, enabling the perception of different colors.
These opsins are homologs of rhodopsin, the photoreceptor protein found in rod cells, but are specialized for color vision.
Three Cone Types:
In humans, there are three types of cone cells, each containing a different opsin:
Blue-Opsin:
This cone type has a peak absorption at 426 nm, corresponding to blue light.
Green-Opsin:
This cone type absorbs light with a peak at 530 nm, which corresponds to green light.
Red-Opsin:
This cone type absorbs light with a peak at 560 nm, which corresponds to red light.
Color Perception:
The three types of cones allow for trichromatic vision, meaning that different combinations of activation of these cones result in the perception of various colors.
For example, when both green and red cones are stimulated, the brain interprets this as yellow.
Absorption Spectrum:
The graph in the image shows the absorbance spectra of the three types of opsins, indicating the wavelengths of light that each cone absorbs maximally.
The blue curve peaks at 426 nm.
The green curve peaks at 530 nm.
The red curve peaks at 560 nm.
These overlapping absorption spectra allow for a wide range of color discrimination.
Summary:
Human color vision is mediated by three cone receptors, each tuned to a specific range of wavelengths corresponding to blue, green, and red light.
The combination of signals from these cones enables the perception of the full color spectrum.
The image illustrates the homology among color cone receptors, specifically focusing on their structural similarity
Color Cone Receptor Opsins:
Opsins are light-sensitive proteins found in the cone cells of the retina. They are responsible for detecting different wavelengths of light, allowing for color vision.
The diagram shows a structural representation of an opsin protein with seven transmembrane (7TM) domains, characteristic of G protein-coupled receptors (GPCRs).
Homology of Green and Red Opsins:
The green and red photoreceptor opsins are highly homologous, meaning they share a similar amino acid sequence.
Specifically, green and red opsins are 95% identical in their amino acid sequences.
This high degree of similarity explains why the absorption maxima of these opsins are relatively close (530 nm for green and 560 nm for red), allowing for overlapping but distinct sensitivity to different parts of the light spectrum.
Amino Acid Differences:
In the diagram, the transmembrane regions are depicted as cylinders, with different colored circles representing amino acid residues.
The purple and black circles highlight positions where the amino acids differ between different types of opsins.
These specific amino acid differences are crucial because they determine the wavelength of light that each opsin is most sensitive to, resulting in the ability to distinguish between different colors.
Structural Similarities:
The seven transmembrane (7TM) structure is consistent across different opsins and other GPCRs, reflecting the conserved mechanism for detecting external signals and initiating intracellular responses.
Despite their structural similarities, slight variations in the amino acid sequence enable the differentiation of color, with each type of opsin being tuned to absorb a specific range of wavelengths.
Summary:
Green and red opsins are highly similar, with 95% of their amino acid sequences being identical. This high level of homology underlies their overlapping roles in color vision.
Small differences in amino acid sequences lead to the functional distinctions that make each opsin sensitive to a specific part of the light spectrum, allowing us to perceive a wide range of colors.
The 7TM structure is a defining feature of opsins, highlighting their role as G protein-coupled receptors in the process of phototransduction.
process of signal transduction and response in photoreceptor cells
Phototransduction Process:
Light Activation of Rhodopsin:
Rhodopsin is a light-sensitive receptor protein found in rod cells. It consists of an opsin protein and 11-cis-retinal (a light-sensitive chromophore).
When light is absorbed, 11-cis-retinal isomerizes to all-trans-retinal, activating rhodopsin.
Activation of Transducin:
Transducin is a G protein that is coupled to rhodopsin.
Upon activation by rhodopsin, transducin exchanges GDP for GTP.
The activated alpha subunit of transducin, bound to GTP, dissociates from the beta and gamma subunits and proceeds to activate the next component in the signaling pathway.
Activation of Phosphodiesterase (PDE):
The activated transducin alpha subunit binds to and activates phosphodiesterase (PDE).
PDE is responsible for converting cyclic GMP (cGMP) to GMP.
This reduction in cGMP levels is crucial in controlling the activity of ion channels in the photoreceptor membrane.
Closure of cGMP-Gated Ion Channels:
In the dark, cGMP levels are high, which keeps cGMP-gated ion channels open.
These channels allow a continuous influx of Na⁺ and Ca²⁺ ions, keeping the cell in a depolarized state (around -40 mV).
When light activates the phototransduction cascade, cGMP levels decrease, causing the cGMP-gated ion channels to close.
The closure of these ion channels stops the influx of Na⁺ and Ca²⁺, leading to the hyperpolarization of the photoreceptor cell.
Hyperpolarization and Signal Transmission:
Hyperpolarization occurs as a result of the closure of cGMP-gated ion channels, and the membrane potential becomes more negative (closer to -70 mV).
This reduction in ion influx decreases the release of neurotransmitters at the synaptic terminal.
The change in neurotransmitter release is interpreted by downstream neurons as the detection of light.
Key Points:
Rhodopsin absorbs light, activating transducin, which then activates phosphodiesterase (PDE).
PDE converts cGMP to GMP, reducing cGMP levels.
Reduced cGMP leads to the closure of cGMP-gated ion channels, causing hyperpolarization of the photoreceptor cell.
Hyperpolarization results in decreased neurotransmitter release, signaling the presence of light to the brain.
Retinitis Pigmentosa (RP)
Retinitis Pigmentosa:
Retinitis Pigmentosa (RP) is a group of genetic disorders that result in the progressive degeneration of photoreceptor cells, which are essential for vision.
RP predominantly affects rod cells, which are responsible for dim light vision and peripheral vision.
Effects on Vision:
The left image shows normal vision, where both central and peripheral vision are intact.
The right image illustrates tunnel vision, which is a common symptom of RP. This occurs because rod cells are more affected in the early stages of the disease.
Rod cells are responsible for vision in low-light conditions and peripheral vision. As these cells degenerate, patients lose the ability to see in the periphery, resulting in tunnel vision.
Over time, cone cells may also be affected, leading to central vision loss and eventually complete blindness in advanced stages.
Symptoms and Progression:
Night blindness is one of the earliest symptoms of RP due to the degeneration of rod cells, which are crucial for vision in low-light conditions.
As the disease progresses, visual field is reduced, leading to the tunnel vision depicted in the image.
The progression of RP is gradual, and the rate can vary significantly among individuals.
Mechanism:
The image also includes a diagram of the signal transduction pathway that is disrupted in RP. The degeneration of photoreceptors interferes with the ability to capture light and transduce it into electrical signals that are sent to the brain, resulting in vision loss.
Summary:
Retinitis Pigmentosa is an inherited retinal disorder that affects rod cells and, subsequently, cone cells, leading to symptoms such as night blindness and tunnel vision.
Rod cell degeneration causes loss of peripheral vision, while further progression can also affect central vision.
RP is characterized by a gradual loss of vision, and its severity and progression rate vary among individuals.
Genetic basis of color blindness
Color Blindness Overview:
Color blindness is a condition where individuals have difficulty distinguishing between certain colors, usually involving the inability to properly perceive red and green colors.
The most common forms of color blindness result from defects in the opsin genes that encode the red and green photoreceptor proteins in the cone cells of the retina.
Genetic Basis:
The genes for the red (R) and green (G) photoreceptor opsins are located on the X chromosome, and their similarity makes them susceptible to recombination errors.
Recombination is a process that can occur during meiosis, where sections of homologous chromosomes exchange genetic material. Errors during recombination can lead to alterations in the opsin genes.
Types of Recombination Leading to Color Blindness:
Recombination Between Genes (A):
This type of recombination occurs between the red and green opsin genes.
In the image, the exchange between the red and green genes results in one chromosome lacking a red opsin gene, while the other chromosome ends up with multiple copies of a green gene.
This type of recombination can lead to deletions or duplications, which can disrupt normal color vision. For example, if the red opsin gene is missing, the individual will have difficulty perceiving red colors, leading to protanopia (red color blindness).
Recombination Within Genes (B):
This type of recombination occurs within the opsin genes, leading to the creation of hybrid opsins.
In the image, recombination within the red and green genes results in two hybrid genes:
A greenlike hybrid and a redlike hybrid.
These hybrid genes may produce opsins that do not function properly, leading to abnormal color perception. Such hybrid opsins might not have the correct spectral sensitivity, causing anomalous trichromacy (a less severe form of color blindness where individuals have altered but not completely missing color perception).
Effects on Color Vision:
The loss or alteration of the red or green opsin genes impairs the ability of cone cells to properly detect those colors.
Red-green color blindness is the most common form, and it occurs more frequently in males due to the X-linked nature of the opsin genes (males have only one X chromosome, so a defect in the opsin gene leads to color blindness).
process of taste signal initiation
Where is the Signal Initiated?
Taste Papillae and Taste Buds:
The first diagram shows the tongue, highlighting the different types of papillae involved in taste sensation:
Fungiform papillae: Located on the tip and sides of the tongue.
Foliate papillae: Located on the sides of the tongue.
Circumvallate papillae: Located at the back of the tongue.
These papillae contain taste buds, which are the actual sites where the signal is initiated.
Taste Bud Structure:
The second diagram zooms in on a taste bud, located within the trench of a papilla.
A taste bud is a collection of specialized taste cells (gustatory receptor cells) responsible for detecting chemical stimuli from food and drinks.
Signal Transduction in Taste Cells:
The third diagram shows a cross-section of a taste bud, detailing the interaction between taste cells and gustatory afferent neurons.
Microvilli (taste hairs) at the top of the taste cells project into the taste pore, where they come into contact with tastants (chemicals in food).
When a tastant binds to receptors on the microvilli of the taste cells, it triggers a signal transduction cascade inside the taste cells.
Synapse with Gustatory Neurons:
The taste cells synapse with gustatory afferent axons (sensory neurons).
The depolarization of the taste cells causes the release of neurotransmitters at the synapse, which then activate the gustatory afferent neurons.
These neurons transmit the signal to the brain, where it is processed, and the perception of taste is formed.
Summary:
Taste signal initiation occurs at the taste cells located within the taste buds in the papillae of the tongue.
Tastants interact with receptors on the microvilli of the taste cells, leading to a series of intracellular events that result in the activation of gustatory neurons.
The signal is then relayed to the brain, where it is interpreted as a specific taste.
five major taste sensations
GPCRs (G Protein-Coupled Receptors)
These receptors are responsible for detecting certain taste modalities:
Sweetness:
Ligands: Sugars and sweeteners.
Sweet taste is detected by binding of sugar molecules (e.g., glucose, fructose) or artificial sweeteners to specific GPCRs.
Umami:
Ligands: Amino acids (e.g., glutamate).
Umami, the “savory” taste, is associated with amino acids like glutamate, found in foods such as meat, cheese, and tomatoes.
Bitterness:
Ligands: Quinine and other bitter compounds.
Bitterness is perceived through a diverse group of compounds (e.g., alkaloids like quinine or caffeine), which are detected by various GPCRs.
2. Ion Channels
These taste modalities are sensed by ion channels rather than GPCRs:
Salty:
Ligand: Na+ (sodium ion).
The sensation of saltiness arises from the movement of sodium ions through specific channels, leading to depolarization of the taste cell.
Sour:
Ligand: H+ (protons).
Sourness is detected by the presence of hydrogen ions (protons), typically from acidic substances. These ions can pass through ion channels or modulate the function of certain ion channels, resulting in the perception of sour taste.
Breakdown of Signal Transduction Pathway
Reception:
Reception refers to the initial binding of a ligand (such as sugars for sweetness, amino acids for umami, or quinine for bitterness) to specific taste receptors.
These taste receptors are GPCRs classified into two main types, often referred to as T1 and T2 families:
T1 receptors are typically involved in detecting sweet and umami tastes.
T2 receptors are associated with the detection of bitter compounds.
Amplification:
Following ligand binding, the receptor undergoes a conformational change, leading to the activation of G proteins. This step amplifies the signal, as the activated receptor can, in turn, activate multiple G protein molecules.
Transduction:
The activated G proteins trigger downstream signaling pathways inside the cell. For taste receptors, this may involve the activation of second messengers like cAMP or IP3, which further propagate the signal.
Response(s):
Ultimately, this signaling cascade leads to changes that result in a cellular response, such as the release of neurotransmitters, which then communicate the taste sensation to the brain.
Subunits of G protein-coupled taste receptors (GPCRs) involved in taste perception, classified into T1R and T2R families
T1R Receptors:
T1R1, T1R2, T1R3:
T1R1 and T1R3 form a heterodimer that detects umami (associated with amino acids, like glutamate).
T1R2 and T1R3 form a heterodimer responsible for sensing sweetness (various sugars and artificial sweeteners).
These receptors are generally associated with pleasant (yummy) flavors, such as sweet and umami, and are involved in identifying nutrient-rich food.
T2R Receptors:
T2R1, T2R2, T2R65:
T2R receptors are primarily involved in sensing bitterness.
There are many subtypes of T2R receptors, with ~65 types recognized, allowing detection of a wide variety of bitter compounds.
Bitterness often indicates potential toxins or harmful substances (yucky), helping organisms avoid ingestion of dangerous materials.
Structural Features:
All these receptors belong to the 7-transmembrane domain family, meaning they span the cell membrane seven times.
They have an extracellular N-terminus, which plays a crucial role in ligand binding.
T1R - Sweet and Umami Receptors
Key Details:
Venus Flytrap Motif: Each subunit of these receptors has a Venus flytrap motif, which plays a role in ligand binding.
Heterodimer Formation:
The receptors are functional only as heterodimers:
T1R2 + T1R3: This combination detects sweet compounds.
T1R1 + T1R3: This combination is responsible for sensing umami.
Activation: Once activated, these receptors couple to and activate a G protein, initiating a signal transduction cascade.
T2R - Bitter Receptors
Functions as a Monomer: Unlike sweet and umami receptors (T1R), which function as heterodimers, T2R bitter receptors work as individual monomers.
Couples to G Protein: Once activated by a bitter ligand, T2R receptors couple with and activate a G protein, which then initiates downstream signaling pathways.
taste transduction pathway for sweet, bitter, or umami tastes via G protein-coupled receptors (GPCRs)
Taste GPCR Activation:
Sweet, bitter, or umami tastant binds to a GPCR on the taste cell membrane.
This activates the associated G protein, known here as gustducin.
G Protein Activation:
The activated G protein stimulates the enzyme PLCβ2 (Phospholipase C beta 2), which is the primary effector.
PLCβ2 cleaves PIP2 (phosphatidylinositol bisphosphate) to form IP3 (inositol trisphosphate) and DAG (diacylglycerol).
Intracellular Signaling:
IP3 binds to its receptor (IP3R3) on the endoplasmic reticulum, leading to the release of Ca²⁺ from intracellular stores.
Increased intracellular Ca²⁺ activates TrpM5 channels, allowing for further depolarization of the cell.
Neurotransmitter Release:
The increase in Ca²⁺ and the subsequent depolarization lead to the release of neurotransmitters (NT), which propagate the signal to sensory neurons, allowing the brain to perceive taste.
ATP Release:
Panx1 channels facilitate ATP release from the taste cell, acting as a neurotransmitter to signal to adjacent sensory nerves.
different taste disorders
Phantom Taste Sensation:
The perception of a taste without any external stimulus (i.e., taste without a signal).
Hypogeusia or Ageusia:
Hypogeusia: Reduced ability to taste (less intense taste perception).
Ageusia: Complete loss of taste sensation (no taste).
Dysgeusia:
A distortion of the sense of taste, leading to perceiving a different taste than usual or a bad/unpleasant taste.
comparison of different G-protein coupled receptors involved in sensory signaling, focusing on rhodopsin (vision), sweet receptors, and bitter receptors
Signal and Reception
Signal:
Rhodopsin is activated by light.
Sweet receptors are activated by sugars (tastants).
Bitter receptors are activated by quinine.
Target Tissue:
Rhodopsin is found in the retina photoreceptor cells.
Sweet and Bitter receptors are found in taste cells on the tongue.
Receptor Feature:
Rhodopsin: Consists of 7-transmembrane (7TM) opsin, 11-cis-retinal prosthetic group, and involves isomerization upon light activation.
Sweet Receptor: Composed of T1R2 + T1R3 subunits forming a dimer, and contains a Venus fly trap domain.
Bitter Receptor: A monomeric 7TM receptor, part of the T2R family.
Conformational Change: All these receptors undergo conformational changes upon activation, which is crucial for signal transduction.
Transduction
G-Protein:
Rhodopsin activates G-transducin.
Sweet and Bitter receptors activate G-gustducin.
Effector:
Rhodopsin activates phosphodiesterase.
Sweet and Bitter receptors activate PLC (phospholipase C).
Second Messenger:
Rhodopsin decreases cGMP levels.
Sweet and Bitter receptors involve IP3 and Ca²⁺ signaling.
Response
Rhodopsin:
Decrease in cGMP leads to closure of cGMP-gated Na⁺ channels, resulting in hyperpolarization of the photoreceptor cells, which contributes to sensing light.
Sweet and Bitter receptors:
Activation of PLC and subsequent IP3/Ca²⁺ signaling leads to depolarization of taste cells, which contributes to the perception of sweetness or bitterness.
signaling pathway involved in the regulation of blood glucose levels when glucose levels are low, focusing on glucagon and epinephrine
Signal and Reception:
Signal: When blood glucose is low, glucagon or epinephrine serves as the signaling molecule.
Reception: The signal is received by glucagon receptors (GCGR) on the target cells. These receptors are 7-transmembrane (7TM) domain receptors, commonly seen in GPCR (G protein-coupled receptor) families.
Ligands:
Glucagon: The sequence of glucagon is provided, highlighting its role in triggering the signal when blood glucose is low.
Epinephrine: This is another signaling molecule involved in raising blood glucose levels, especially during stress (the “fight or flight” response).
Signaling Pathway Overview:
Signal Reception: The glucagon or epinephrine binds to its respective GPCR on target cells such as liver cells.
Amplification and Transduction: The binding initiates amplification through a G-protein-mediated pathway, which then triggers various downstream effects leading to:
Glycogen Breakdown: Activation of enzymes that break down glycogen to release glucose into the blood.
Gluconeogenesis: Activation of pathways to produce new glucose molecules.
Diagrams:
The glucagon receptor structure shows a typical 7-transmembrane GPCR, which is characteristic of these signaling pathways.
The amino acid sequence of glucagon and the chemical structure of epinephrine are also depicted to show their molecular makeup.
transduction pathway of epinephrine and glucagon, focusing on the role of cyclic AMP (cAMP) in activating protein kinase A (PKA) and the subsequent activation of enzymes involved in glycogen metabolism
Hormone Binding: A hormone (such as epinephrine or glucagon) binds to its receptor (either adrenergic receptor for epinephrine or glucagon receptor for glucagon), which is a G-protein-coupled receptor (GPCR).
Activation of Adenylate Cyclase: The hormone binding activates adenylate cyclase via the G-protein (Gs). ATP is then converted to cyclic AMP (cAMP), which serves as a second messenger.
Activation of Protein Kinase A (PKA): cAMP binds to the regulatory subunits (R) of PKA, causing the release of the catalytic subunits (C), which become active.
Phosphorylation Events:
The active catalytic subunits of PKA phosphorylate phosphorylase b kinase, activating it.
Phosphorylase b kinase then converts phosphorylase b to the active phosphorylase a, promoting glycogen breakdown.
PKA also phosphorylates glycogen synthase, inactivating it to prevent glycogen synthesis.
Glycogen Breakdown: The active phosphorylase a catalyzes the breakdown of glycogen to produce glucose-1-phosphate, which can be used for energy.
How quickly can glucagon cause glucose increase in the liver?
Glucagon rapidly increases glucose production in the liver, with a noticeable effect beginning around 5 minutes after infusion. This reflects glucagon’s role in stimulating glycogen breakdown and gluconeogenesis in the liver, leading to increased glucose release.
how epinephrine raises blood glucose levels through its effects on glycogen metabolism and glycolysis
Chemical Structure: The structure of epinephrine is shown, indicating its identity as a hormone.
Target Organs:
Liver: Epinephrine acts on the liver to stimulate glycogen breakdown (glycogenolysis), leading to increased glucose release into the bloodstream.
Muscles: Epinephrine also acts on muscles, increasing glycolysis to provide energy for muscle activity.
Overall Effect: These combined effects increase the concentration of glucose in the blood, which is particularly useful during situations requiring immediate energy, such as the “fight or flight” response.
how glucagon and epinephrine regulate glucose metabolism in both the liver and muscle cells during fasting and exercise
Fasting: Low Glucose
Glucagon is released from the pancreas to increase blood glucose levels by targeting the liver:
Glycogen Breakdown (Glycogenolysis): Glycogen in the liver is broken down into glucose, which is then released into the blood.
Gluconeogenesis: Lactate and other precursors are used in gluconeogenesis to generate more glucose, which is then released into the bloodstream.
Exercise
Epinephrine is released from the adrenal medulla to mobilize energy stores, targeting both liver and muscle cells:
Liver:
Epinephrine stimulates glycogenolysis, releasing glucose into the blood.
Muscle Cells:
Glycogenolysis: Glycogen in muscle cells is broken down into glucose-6-phosphate for energy production.
Glycolysis: Glucose-6-phosphate is further metabolized to pyruvate.
Lactate Production: During anaerobic conditions, pyruvate is converted into lactate, which can be transported to the liver for gluconeogenesis.
Citric Acid Cycle & Oxidative Phosphorylation: Under aerobic conditions, pyruvate enters the citric acid cycle, producing carbon dioxide and water while generating ATP through oxidative phosphorylation.
Active Pathways Summary:
Glycogen Breakdown: Mobilizing glycogen stores.
Gluconeogenesis: Synthesizing glucose from non-carbohydrate precursors.
Glycolysis: Breaking down glucose for energy.
Citric Acid Cycle: Oxidizing acetyl-CoA to produce ATP.
Oxidative Phosphorylation: Generating ATP via the electron transport chain.
the regulation of glycogen breakdown and how this process can be rapidly turned off when no longer necessary
Hormone Removal: The hormones (like glucagon or epinephrine) that initially stimulated glycogen breakdown are no longer present. Without these hormones, the signaling cascade is not activated.
Inactivation of G Protein Signaling: The inherent GTPase activity of the Gα subunit inactivates the G protein by hydrolyzing GTP to GDP. This terminates the signal transmission from the receptor to adenylate cyclase, stopping the production of cyclic AMP (cAMP).
Conversion of cAMP to AMP: Phosphodiesterase (PDE) converts cAMP into AMP. Since AMP does not activate protein kinase A (PKA), the signaling cascade is halted, preventing further activation of glycogen breakdown enzymes.
Dephosphorylation of Enzymes: Protein phosphatase removes phosphate groups from key enzymes such as phosphorylase kinase and glycogen phosphorylase, thereby inactivating them and stopping glycogen breakdown. Phosphorylase b kinase and phosphorylase a are dephosphorylated, converting them back to their inactive forms.
signal and reception of the signaling pathway when blood glucose is high
Signal and Reception
Signal: High blood glucose levels trigger the release of insulin from the pancreas.
Reception: Insulin binds to its receptor on the cell membrane. The insulin receptor is a tyrosine kinase receptor consisting of α and β subunits.
The α subunits are located extracellularly and bind insulin.
The β subunits span the membrane and contain the tyrosine kinase domain.
2. Transduction and Amplification
Upon insulin binding, the tyrosine kinase domains are activated, resulting in autophosphorylation of tyrosine residues on the receptor itself.
This phosphorylation triggers a cascade of intracellular signaling pathways, amplifying the initial signal.
Key proteins such as IRS (insulin receptor substrate) are phosphorylated, leading to downstream signaling effects.
3. Response
The responses to insulin include:
Increased glucose uptake: Insulin signaling promotes the translocation of GLUT4 transporters to the cell membrane in muscle and adipose tissue, allowing glucose to enter the cell.
Glycogen Synthesis: Insulin activates enzymes like glycogen synthase, which enhances glycogen storage in liver and muscle cells.
Decreased Gluconeogenesis: Insulin inhibits gluconeogenesis in the liver, thereby reducing glucose production.
Increased Lipid and Protein Synthesis: Insulin promotes the synthesis of lipids and proteins, contributing to cellular growth and storage.
structure and function of the insulin receptor
Structure of the Insulin Receptor
The insulin receptor is a tyrosine kinase receptor that has a structure distinct from G-protein-coupled receptors (GPCRs).
It is composed of two main parts:
α Subunits: Located on the extracellular side of the cell membrane and form the insulin-binding site.
β Subunits: These span the cell membrane and contain the tyrosine kinase domain, which plays a crucial role in signal transduction.
Tyrosine Kinase Activity
Kinase as Part of the Receptor: The kinase domain is an integral part of the insulin receptor itself. When insulin binds to the α subunits, it causes a conformational change that activates the tyrosine kinase domain in the β subunits.
Upon activation, the kinase autophosphorylates tyrosine residues on the receptor, which further propagates the signal by recruiting and phosphorylating other intracellular proteins like IRS (insulin receptor substrate).
Key Points
The insulin receptor is significantly different in structure and function compared to GPCRs, as it directly has kinase activity.
Tyrosine kinase receptors, like the insulin receptor, are critical for amplifying the signal once insulin binds, leading to a cascade of cellular responses such as increased glucose uptake, glycogen synthesis, and overall regulation of glucose homeostasis.
transduction pathway of insulin
Insulin Binding
Insulin binds to the extracellular domain of the insulin receptor.
The receptor is a tyrosine kinase, and upon insulin binding, the monomers of the receptor come closer together, leading to dimerization.
2. Receptor Activation
Autophosphorylation: The kinase domains of the receptor’s β subunits phosphorylate each other on tyrosine residues, activating the receptor fully.
These phosphorylated sites allow other molecules to bind and initiate further signaling.
3. Signal Transduction and Amplification
IRS-1 (Insulin Receptor Substrate 1) is phosphorylated by the insulin receptor. This phosphorylation allows phosphoinositide 3-kinase (PI3K) to bind to IRS-1.
PI3K converts PIP₂ (phosphatidylinositol 4,5-bisphosphate) to PIP₃ (phosphatidylinositol 3,4,5-trisphosphate) in the cell membrane.
PIP₃ serves as a docking site for PDK1 (PIP₃-dependent protein kinase) and Akt (Protein Kinase B).
4. Activation of Akt
PDK1 phosphorylates and activates Akt.
Activated Akt triggers various downstream effects, leading to multiple cellular responses.
5. Cellular Responses
Insertion of GLUT4 Transporters: Akt activation leads to the translocation of GLUT4 vesicles to the cell membrane, which increases glucose uptake into cells, especially in muscle and adipose tissue.
Glycogen Synthesis: Akt also activates pathways that promote glycogen synthase activity, increasing glycogen storage.
Gene Expression Changes: Insulin signaling also results in changes in gene expression that regulate glucose metabolism and growth.
Summary of Responses
The activation of Akt is a key step that results in the increased uptake of glucose into cells, promoting glycogen synthesis and reducing blood glucose levels.
This pathway allows cells to respond effectively to high glucose levels, maintaining glucose homeostasis and ensuring efficient energy utilization and storage.
complex regulation of blood glucose by insulin and glucagon
High Blood Glucose Response
Stimulus: Blood glucose levels are too high.
Insulin Release:
The pancreas releases insulin in response to elevated blood glucose levels.
Effects of Insulin:
Glucose Uptake by Muscle Cells: Insulin promotes glucose uptake by muscle cells, which use it for energy or store it as glycogen.
Glycogen Synthesis in the Liver: Insulin stimulates the liver to use glucose to synthesize glycogen, storing excess glucose for future use.
Result: Blood glucose levels decrease, restoring normal homeostatic levels.
Low Blood Glucose Response
Stimulus: Blood glucose levels are too low.
Glucagon and Epinephrine Release:
The pancreas releases glucagon, and the adrenal glands release epinephrine.
Effects of Glucagon and Epinephrine:
Gluconeogenesis and Glycogen Breakdown in the Liver: Glucagon stimulates the liver to produce glucose through gluconeogenesis and glycogenolysis (breaking down glycogen into glucose).
Glycogen Breakdown in Muscle Cells: Epinephrine stimulates glycogen breakdown in muscles, providing glucose for muscle energy demands.
Result: Blood glucose levels increase, restoring them to normal homeostatic levels.
Homeostasis
The body’s goal is to maintain normal blood glucose levels through the balanced actions of insulin and glucagon.
Insulin and glucagon have opposite effects:
Insulin lowers blood glucose levels by increasing glucose uptake and storage.
Glucagon raises blood glucose levels by stimulating glucose production and release.
Summary
When blood glucose levels are high, insulin is released, promoting glucose uptake and glycogen synthesis.
When blood glucose levels are low, glucagon and epinephrine are released, promoting gluconeogenesis and glycogen breakdown to increase blood glucose levels.
The balance between these hormones allows the body to maintain glucose homeostasis, ensuring that energy is available when needed while preventing excess glucose accumulation.
insulin helps balance blood glucose levels and depicts insulin’s structure and its physiological effects
Insulin Structure
The left side of the image shows a molecular model of insulin, highlighting its disulfide bonds.
Insulin is composed of two peptide chains (A and B chains) that are held together by intra- and inter-chain disulfide bonds.
These disulfide bonds are crucial for insulin’s stability and functionality.
Insulin’s Role in Glucose Balance
The right side shows a representation of insulin’s effect on the body:
When blood glucose levels are high, the pancreas releases insulin.
Insulin promotes glucose uptake primarily by muscle cells and adipose tissue.
This uptake occurs via the translocation of GLUT4 glucose transporters to the cell membrane, allowing glucose to enter the cells from the bloodstream.
Result
Lower Blood Glucose: By enhancing glucose uptake and storage, insulin helps reduce blood glucose levels back to normal ranges, maintaining homeostasis.
Insulin also stimulates glycogen synthesis in the liver and muscles, storing excess glucose in the form of glycogen.
Summary
Insulin plays a vital role in balancing blood glucose levels by enabling glucose uptake into tissues.
It acts as a signal for cells to absorb glucose, leading to a decrease in blood glucose levels, which helps maintain metabolic homeostasis.
Insulin is used as a diabetes drug to mimic the natural actions of this hormone at the insulin receptor, an RTK
Insulin is administered as a treatment for diabetes, especially for type 1 diabetes and sometimes type 2 diabetes, to help regulate blood glucose levels.
Insulin mimics the natural function of the hormone by binding to the insulin receptor, which is a Receptor Tyrosine Kinase (RTK).
RTKs are a class of cell surface receptors that have intrinsic kinase activity, allowing them to initiate a signaling cascade when insulin binds.
By activating these receptors, administered insulin promotes glucose uptake, glycogen synthesis, and other metabolic processes, helping to maintain normal blood glucose levels in patients who have impaired insulin production or action.
how cholera is caused by disrupted signal transduction in intestinal cells
Cholera Mechanism
Cholera Toxin: The cholera toxin, produced by Vibrio cholerae, locks the Gαs (G-protein) into its active form, disrupting normal cellular function.
Signal Transduction Pathway Disruption
Activation of Adenylate Cyclase (AC):
The cholera toxin binds to Gαs, preventing its inherent GTPase activity from turning off. This keeps Gαs in the active GTP-bound state.
The continuously active Gαs stimulates adenylate cyclase (AC), leading to persistent production of cyclic AMP (cAMP).
cAMP and Protein Kinase A (PKA):
Increased cAMP levels lead to the constant activation of Protein Kinase A (PKA).
PKA then phosphorylates proteins involved in ion transport, including chloride channels.
Chloride Ion Secretion:
PKA activation results in the opening of chloride (Cl⁻) channels in the intestinal cells.
Chloride ions are secreted into the intestinal lumen, and to maintain electrochemical balance, sodium ions and water follow.
Water Loss and Dehydration:
This leads to excessive movement of water into the intestinal lumen, causing severe diarrhea.
The resulting water loss leads to dehydration and, if untreated, can be fatal.
Summary
Epinephrine
Secreted from: Adrenal gland
Target tissue: Muscle, liver
Receptor type and class: G-protein-coupled receptors (GPCRs), Adrenergic receptors (AR)
G-protein: Gαs (stimulates adenylate cyclase)
Effector: Primary effector is adenylate cyclase (AC), secondary effector is protein kinase A (PKA)
Second messenger(s): cAMP
Response: Increases glycogen breakdown
Glucagon
Secreted from: Pancreas (alpha cells)
Target tissue: Liver
Receptor type and class: GPCR, Glucagon receptor (GCGR)
G-protein: Gαs
Effector: Primary effector is AC, secondary effector is PKA
Second messenger(s): cAMP
Response: Increases glycogen breakdown
Insulin
Secreted from: Pancreas (beta cells)
Target tissue: Muscle
Receptor type and class: Receptor tyrosine kinase (RTK)
Effector: Kinases
Second messenger(s): Not directly listed, but typically involves phosphorylation cascades
Response: Uptake of glucose into muscle cells
Notes on Signal Dysfunctions (Right-side Annotations)
Signal example: Type 1 diabetes as a signal dysfunction.
Reception example: Conditions like color blindness, retinitis pigmentosa, and melanocortin-4 receptor (MC4R) as reception dysfunctions.
Transduction example: Cholera as an example of transduction dysfunction.
Many medicines interact with to control cell signaling events via receptors
G-protein-coupled receptors (GPCRs) (Section 14.1) – Blue: GPCRs represent a major target for medicines. These receptors play a significant role in cell signaling and are involved in numerous physiological processes, making them a common target for drug therapies.
Enzymes (Chapters 8 and others) – Green: Enzymes are another key target for drugs. Many medicines work by inhibiting or enhancing the activity of specific enzymes to regulate metabolic pathways.
Nuclear receptors (Section 32.3) – Light Blue: Nuclear receptors are intracellular receptors that, upon binding with ligands (e.g., steroid hormones), regulate gene expression. Drugs targeting these receptors can influence long-term cellular changes.
Other receptors – Blue-gray: There are additional types of receptors that drugs target to control cellular functions.
Voltage-gated ion channels (Section 13.4) – Pink: These channels are critical for controlling the electrical activity of cells, particularly in excitable tissues such as neurons and muscle cells.
Ligand-gated ion channels (Section 13.4) – Red: Ligand-gated ion channels open or close in response to binding with a specific ligand, affecting ion flow across the cell membrane and altering cell excitability.
Solute carriers (Section 13.3) – Light Pink: Solute carriers are involved in the transport of various molecules across cell membranes, and drugs may target these to modulate transport processes.
Other transporters – Magenta: Drugs can also target other transport proteins to regulate cellular influx and efflux of substances.
Other protein targets – Gray: There are a variety of other protein targets that drugs may interact with, which are not specifically listed here.
Receptor types involved
Affinity in drug design
Affinity in Drug Design
Affinity refers to how tightly a ligand (such as a drug) binds to its receptor.
It is an important parameter in drug development since higher affinity means a drug can bind effectively to its target at lower concentrations, increasing its potency.
Measuring Affinity
Affinity can be quantified by measuring the dissociation constant (denoted as K_d for binding interactions or K_i if the ligand is an inhibitor).
K_d is the ligand concentration (in nanomolar, nM) where 50% of the receptors are occupied by the ligand. This value provides an indication of the strength of the interaction.
A low K_d value indicates high affinity (strong binding), while a high K_d value indicates low affinity (weak binding).
Graph Interpretation
The graph in the image plots the fraction of receptor occupancy by the ligand (y-axis) against the ligand concentration (x-axis).
The red curve shows a ligand with a higher affinity, where receptor occupancy increases more rapidly with lower concentrations.
The green curve shows a ligand with a lower affinity, which requires a higher concentration to achieve similar receptor occupancy.
Mechanism of action for asthma medications, particularly focusing on β-agonists like salbutamol (commonly known as albuterol) and their role in opening airways
Asthma Medication Mechanism
β-Agonist Binding:
A β-agonist (e.g., salbutamol) binds to the β-adrenergic receptor on the surface of smooth muscle cells lining the airways.
This receptor is a G-protein-coupled receptor (GPCR).
GPCR Activation:
Binding of the β-agonist leads to the activation of the G-protein associated with the receptor.
The α-subunit of the G-protein exchanges GDP for GTP and dissociates from the β and γ subunits.
Adenylate Cyclase Activation:
The activated α-subunit interacts with adenylate cyclase, an enzyme in the cell membrane.
Adenylate cyclase converts ATP to cyclic AMP (cAMP), which acts as a second messenger.
Protein Kinase A (PKA) Activation:
cAMP activates protein kinase A (PKA).
PKA phosphorylates target proteins, including myosin light chain kinase (MLCK), leading to a reduction in its activity.
Muscle Relaxation:
The phosphorylation of MLCK results in the relaxation of the smooth muscle in the airways.
This causes bronchodilation, allowing the airways to open up, which improves airflow and alleviates asthma symptoms.
Diagram Key Points
The left side of the image shows the signaling pathway involving β-agonists and the downstream effects on airway smooth muscle.
The right side of the image illustrates:
Airway closed: Constricted airway due to tightened smooth muscle.
Airway open: Relaxed airway after the action of β-agonists.
Structures of adrenaline (epinephrine) and salbutamol are depicted, showing their similar structural features, which allow them to activate β-adrenergic receptors effectively.
Summary
Salbutamol is a β2-adrenergic receptor agonist used as a bronchodilator for asthma treatment.
It helps relax airway smooth muscle through a signaling cascade involving cAMP and PKA, leading to muscle relaxation and bronchodilation.
Overview of β2-adrenoceptor agonists
β2-Adrenoceptor Agonists and Asthma
β2-adrenoceptor agonists are used to reverse bronchoconstriction in asthma patients. They act on the β2-adrenergic receptors present in the smooth muscles of the bronchioles, resulting in muscle relaxation and bronchodilation, thus relieving asthma symptoms.
Examples of β2-Adrenoceptor Agonists
Salbutamol (Ventolin)
Patented in: 1966
Affinity: Low affinity for β2-adrenoceptors
Duration: Short-acting
Uses: It is commonly used for the acute relief of asthma symptoms. Due to its rapid onset, it is ideal for rescue therapy during an asthma attack.
Structure: The structure of salbutamol shows hydroxyl groups that contribute to its hydrophilicity and enable its interaction with the receptor.
Salmeterol (Serevent)
Patented in: 1983
Affinity: High affinity for β2-adrenoceptors
Duration: Long-acting
Uses: It is used for long-term control of asthma symptoms and is not intended for quick relief. Due to its long-acting nature, it helps maintain airway patency over an extended period.
Structure: Salmeterol has a more complex structure compared to salbutamol, with an extended hydrophobic side chain. This structure allows it to embed within the cell membrane, leading to prolonged interaction with the receptor, contributing to its long duration of action.
Summary
Salbutamol is a short-acting β2-agonist (SABA) used to provide immediate relief from bronchoconstriction in asthma. It has a low affinity, which is appropriate for quick onset but short duration.
Salmeterol is a long-acting β2-agonist (LABA) used for maintenance therapy. Its high affinity and long side chain allow it to provide prolonged bronchodilation.
Different strategies to manage pain, focusing on two main approaches: altering the central nervous system (CNS) modulation of pain and changing the initiator of pain at the site of injury or inflammation
Strategies to Manage Pain
Change the CNS Modulation of Pain
This approach involves altering the way pain signals are processed in the central nervous system (including the brain and spinal cord).
Pain modulation occurs in the spinal cord, particularly at the level of interneurons, which help regulate pain signals before they are sent to the brain.
Medications in this category are typically opioids, which act on the μ-opioid receptors to reduce the sensation of pain by inhibiting neurotransmitter release in the CNS.
Examples of Opioids:
Codeine
Morphine
Fentanyl
Tramadol
Oxycodone
Opioids are effective for moderate to severe pain, often prescribed for acute injuries, postoperative pain, or cancer-related pain.
Change the Initiator of Pain
This approach targets the source of pain, usually by reducing inflammation and interfering with the production of prostaglandins, which are chemical mediators that increase pain sensitivity.
By targeting the initiator of pain, these drugs help to reduce the pain signal generation at the peripheral site of injury.
Examples of Medications:
Paracetamol (Acetaminophen): Acts centrally to inhibit prostaglandin synthesis, effective for mild to moderate pain and fever.
NSAIDs (Nonsteroidal Anti-Inflammatory Drugs): Include ibuprofen and naproxen, which inhibit the enzyme cyclooxygenase (COX), reducing prostaglandin production and inflammation.
Aspirin: An NSAID that also inhibits COX enzymes, reducing pain and inflammation.
overview of the causes of pain
The image provides a simplified overview of the causes of pain and the mechanisms involved in pain perception and modulation. Here’s a breakdown of the key components illustrated:
Causes and Pathways of Pain
Nociceptors and Inflammation:
Nociceptors are specialized sensory neurons that respond to potentially harmful stimuli. They are present throughout the body in tissues such as the skin, muscles, and organs.
Inflammation can lead to the production of chemical mediators like prostaglandins. Prostaglandins are important in sensitizing nociceptors, making them more responsive to pain signals.
These nociceptors send pain signals from the periphery (e.g., an injured area) to the spinal cord.
Spinal Cord Processing:
Pain signals from nociceptors are transmitted to the dorsal horn of the spinal cord, where they interact with interneurons.
Interneurons play a crucial role in modulating pain signals before they are transmitted to the brain. This process is called pain modulation, and it involves the interplay of both excitatory and inhibitory neurotransmitters.
Central Nervous System (CNS) Modulation:
Once pain signals are processed at the level of the spinal cord, they are transmitted to the brain, where they are perceived as pain.
The brain can modulate pain perception through descending pathways that release endogenous pain-relieving chemicals, such as endorphins, which act to inhibit further pain transmission at the spinal cord level.
Key Concepts Illustrated
Prostaglandins and Inflammation: Prostaglandins are produced in response to inflammation and help sensitize nociceptors, leading to increased pain perception.
Interneuron Modulation: Pain signals are modulated at the level of the spinal cord by interneurons, which can either amplify or reduce pain signals before they reach the brain.
process of synaptic transmission of pain signals at the spinal level, comparing the normal pathway with modulation by opioids
Synaptic Transmission Without Modulation
Action Potential Arrival:
An action potential (AP) arrives at the presynaptic terminal, which causes voltage-gated calcium (Ca²⁺) channels to open.
This results in an influx of Ca²⁺ ions into the presynaptic neuron.
Release of Neurotransmitter:
The influx of calcium triggers the release of glutamate, a neurotransmitter, from the presynaptic neuron.
Glutamate binds to ligand-gated ion channels (LGIC) on the postsynaptic membrane, leading to the opening of these channels.
Postsynaptic Depolarization:
The opening of ligand-gated channels allows cation (e.g., Na⁺) influx, leading to depolarization of the postsynaptic neuron.
This initiates the transmission of the pain signal to higher centers.
Afterward, the neurons repolarize through K⁺ efflux, restoring the resting membrane potential (RMP).
Modulation by Opioids
The modulation process involves opioids which interact with specific receptors to dampen the pain signal. This is illustrated in red in the image:
Opioid Release and Binding:
Opioids (e.g., endorphins or exogenous drugs like morphine) are released and bind to G-protein-coupled receptors (GPCRs) on the presynaptic neuron.
G-protein Signaling Cascade:
The binding activates a Gi protein-mediated signaling cascade.
This leads to the inhibition of voltage-gated Ca²⁺ channels.
As a result, Ca²⁺ influx is reduced, which prevents the release of glutamate into the synaptic cleft. This means fewer excitatory signals are transmitted, effectively reducing pain perception.
Increase in Potassium Efflux:
Opioid receptor activation also leads to an increase in K⁺ efflux from the presynaptic neuron.
The increased potassium efflux hyperpolarizes the neuron, making it harder for further depolarization to occur. This further reduces the likelihood of action potentials, leading to decreased pain signal transmission.
opioid signal transduction pathway
Opioid Signal Transduction Pathway
Ligand + Receptor:
Endogenous opioids such as enkephalins and endorphins act as agonists.
These endogenous ligands bind to μ (mu), κ (kappa), and δ (delta) opioid receptors, which are G-protein-coupled receptors (GPCRs) located on the cell surface.
G-Protein Activation:
Upon ligand binding, the opioid receptors activate Gαi/o proteins.
The Gi/o subunits (inhibitory G-proteins) dissociate from the receptor and regulate downstream effectors.
Effector - Adenylate Cyclase Inhibition:
The Gαi/o subunit inhibits adenylate cyclase, an enzyme responsible for converting ATP into cyclic AMP (cAMP).
Inhibition of adenylate cyclase leads to a decrease in cAMP levels within the cell.
Second Messenger:
cAMP is a second messenger that typically plays a role in amplifying cellular responses. By decreasing cAMP, the opioid signaling pathway reduces the intracellular signaling cascade associated with pain transmission.
Cellular Response:
The reduction in cAMP causes:
Decrease in Ca²⁺ influx through voltage-gated calcium channels, leading to reduced release of excitatory neurotransmitters like glutamate.
Increase in K⁺ efflux, which causes hyperpolarization of the neuron, making it less excitable.
These combined effects lead to less depolarization of the postsynaptic neurons, effectively inhibiting the propagation of pain signals.
Summary
Endogenous opioids like enkephalins bind to opioid receptors (GPCRs).
The Gαi/o proteins inhibit adenylate cyclase, leading to a decrease in cAMP.
This results in reduced Ca²⁺ influx and increased K⁺ efflux, causing less neuronal depolarization.
Ultimately, the pathway reduces pain signal transmission.
Drawbacks
solutions to opioid overdose
Antidote: Naloxone
Naloxone is a medication used as an antidote for opioid overdose.
It is classified as a competitive antagonist at opioid receptors, such as μ (mu), κ (kappa), and δ (delta) receptors.
Key Properties of Naloxone:
Competitive Antagonist:
Naloxone competes with opioids like heroin, morphine, and fentanyl for binding to opioid receptors.
Unlike opioids, naloxone does not activate the receptor, meaning it does not produce pain relief, euphoria, tolerance, or addiction.
High Affinity:
Naloxone has a higher affinity for opioid receptors than most opioids, which allows it to effectively displace opioid molecules that are bound to these receptors.
When administered in a timely manner, naloxone can reverse the effects of an opioid overdose, such as respiratory depression, by blocking opioid receptor activity.
Effectiveness:
Naloxone’s ability to bind to receptors and block opioid effects can rapidly restore normal respiration in individuals experiencing an overdose.
This makes it a critical life-saving drug for emergency situations involving opioid toxicity.
Structural Comparison
The image includes the chemical structures of heroin and naloxone:
Both molecules have similar core structures, allowing them to bind to the same opioid receptors.
The differences in the chemical structure account for naloxone’s antagonistic properties and lack of euphoric effects.
Important Note
The image points out that while naloxone is effective in reversing opioid overdoses, it is not a solution to the opioid crisis as a whole.
Naloxone can provide temporary relief from an overdose but does not address the underlying issues of opioid addiction or the systemic factors contributing to the crisis.
Comprehensive approaches including prevention, education, access to addiction treatment, and supportive care are needed to tackle the opioid epidemic.
change in policies as a solution
Policy-Based Solutions
Use of Prescription Drug Monitoring Programs (PDMPs):
Prescription Drug Monitoring Programs are electronic systems that track the prescription and dispensing of controlled substances.
PDMPs help healthcare providers identify patients who may be at risk of misusing prescription opioids and prevent doctor shopping (when patients seek prescriptions from multiple doctors).
Enforce Rules for Drug Makers:
Enforcing regulations on pharmaceutical companies can help mitigate unethical practices, such as misleading advertising or encouraging over-prescription.
Stricter rules can ensure that opioid medications are marketed responsibly, with proper risk information provided to healthcare providers and patients.
Increase Access to Drug Abuse Treatment Services:
Expanding access to drug abuse treatment and rehabilitation services is crucial for helping individuals overcome opioid dependence.
Treatment options may include medication-assisted treatment (MAT) with drugs like methadone, buprenorphine, and naltrexone, combined with counseling and behavioral therapy.
Providing adequate support and resources for those struggling with opioid use disorder is vital for long-term recovery.
