L2 - GPCR 1 Flashcards
What are G-protein coupled receptors (GPCRs)?
GPCRs are receptors that, once activated by an agonist, couple with G-proteins and downstream effectors to produce second messengers, which stimulate further biochemical processes.
What is the historical definition of metabotropic receptors?
Metabotropic receptors are those that, once activated by an agonist, initiate metabolic changes that influence the activity of the cell indirectly.
Why are GPCRs also called 7-transmembrane receptors (7TM)?
GPCRs are called 7TM receptors because they have a characteristic structure with seven transmembrane domains.
What is the current research regarding GPCRs’ mechanisms?
Current research suggests that GPCRs may cause cellular changes through pathways that do not involve G-proteins, leading some to suggest using the term “7TM receptors” instead of GPCRs.
How are GPCRs typically classified?
GPCRs are classified based on the main neurotransmitter or agonist that activates them, such as adrenoceptors, cholinoceptors, GABAergic receptors, opioid receptors, dopamine receptors, purinoceptors, 5HT receptors, and glutamate receptors.
What are adrenoceptors?
Adrenoceptors are a subtype of GPCRs activated by adrenaline and noradrenaline, with α- and β- subtypes.
What are cholinoceptors?
Cholinoreceptors are GPCRs activated by acetylcholine, with muscarinic subtypes (M1–M4).
What are GABAergic receptors?
GABAergic receptors are GPCRs activated by GABA, with the GABAB subtypes.
What are opioid receptors?
Opioid receptors are GPCRs that include μ, κ, and δ subtypes, activated by endogenous opioids and synthetic drugs.
What are dopamine receptors?
Dopamine receptors are GPCRs, with D1 and D2 subtypes, activated by dopamine.
What are purinoceptors?
Purinoceptors are GPCRs activated by purines, including P1 and P2Y subtypes.
What are 5HT receptors?
5HT receptors are serotonin receptors, including subtypes 5HT1a-c and 5HT2.
What are glutamate receptors?
Glutamate receptors are GPCRs activated by glutamate, including mGlu1–mGlu8 subtypes.
What is the largest family of receptors in the human body?
G-protein coupled receptors (GPCRs) are the largest family of receptors in the human body, with around 800 types.
What functions are GPCRs involved in?
GPCRs are involved in metabolism, nerve activity, secretion, cardiac contraction, and pharmacology.
How many non-antibiotic prescription drugs act on GPCRs?
Roughly half of all non-antibiotic prescription drugs act at GPCRs.
What is the basic structure of all GPCRs?
All GPCRs have a basic structure consisting of seven transmembrane (7-TM) domains, with an extracellular N-terminus and an intracellular C-terminus.
What are the three main components of the cell signaling pathway in GPCRs?
The three main components are the 7-TM receptor, the G-protein, and effector molecules.
What is the role of the 7-TM receptor in GPCR signaling?
The 7-TM receptor initiates the cell signaling pathway by binding an agonist, leading to receptor activation.
How does the G-protein contribute to GPCR signaling?
Upon activation by the receptor, the G-protein moves along the membrane and activates or inhibits effector molecules.
What is the role of effector molecules in GPCR signaling?
Effector molecules, such as ion channels or enzymes, alter their basal activity to trigger a cellular response.
What is the basic structural organization of GPCRs?
GPCRs consist of an extracellular N-terminus, intracellular C-terminus, seven transmembrane (7-TM) domains, and loops exposed on both extracellular and intracellular sides.
Where is the N-terminus of GPCRs located?
The N-terminus is located on the extracellular side of the membrane.
Where is the C-terminus of GPCRs located?
The C-terminus projects into the cytosol on the intracellular side.
How many times does the GPCR polypeptide chain traverse the membrane?
The polypeptide traverses the membrane seven times, forming seven transmembrane domains.
What links the membrane-spanning segments in GPCRs?
Membrane-spanning segments are linked by three exposed loops on both the extracellular and intracellular sides of the membrane.
Where do G-proteins bind to GPCRs?
G-proteins bind to GPCRs via the intracellular loops and the C-terminal tail.
Where is the agonist binding site on GPCRs?
The agonist binding site location varies depending on the specific receptor type.
What is the function of the extracellular loops of GPCRs?
The extracellular loops often participate in ligand (agonist or antagonist) recognition and binding.
What is the role of the intracellular loops in GPCRs?
The intracellular loops are involved in G-protein binding and activation.
What does hydrophilic mean in the context of amino acids?
Hydrophilic amino acids are water-loving and are attracted to water-based environments.
What does hydrophobic mean in the context of amino acids?
Hydrophobic amino acids are water-hating and are drawn to lipid-based environments like the cell membrane.
What are the two main environments in a cell?
The cell has a water-based environment (intracellular and extracellular) and a lipid-based environment (cell membrane).
Where are hydrophobic protein segments typically located in GPCRs?
Hydrophobic segments are drawn to the lipid-based cell membrane and form the transmembrane domains (1-7).
Where are hydrophilic protein segments typically located in GPCRs?
Hydrophilic segments are usually on the protein’s exterior or interior surfaces, forming the N- and C-terminals and the intra/extracellular loops.
What gives GPCRs their characteristic 7-Transmembrane domain shape?
The twisting and turning of the receptor’s polypeptide chain, influenced by the hydrophilic and hydrophobic properties of amino acids, forms the 7-Transmembrane domain structure.
What is the common structural feature shared by all GPCRs?
All GPCRs share the standard 7-Transmembrane (7-TM) domain structure.
Name four types of agonists that can activate GPCRs.
Subatomic particles (e.g., photons)
Ions (e.g., Ca²⁺)
Small organic molecules (e.g., noradrenaline, acetylcholine)
Peptides and proteins (e.g., glutamate)
What happens to GPCRs when a ligand binds?
Ligand binding causes a conformational change in the receptor structure, opening a cavity on the intracellular side for G-protein binding.
What does the conformational change in GPCRs result in?
It allows a high-affinity interaction between the G-protein and the receptor.
Where can agonists bind on GPCRs?
Within the transmembrane domains in the membrane
To the extracellular loops joining the transmembrane domains
To the N-terminus
Why is the binding site location important in GPCR function?
The location of the binding site determines the specific interaction and activation mechanism of the receptor by different ligands.
What is a tethered ligand in the context of GPCRs?
A tethered ligand is a sequence of amino acids in the N-terminus that is part of the receptor it activates.
How is a tethered ligand activated?
Enzymes, such as proteases, cleave the N-terminus of the receptor, releasing the tethered ligand.
What happens after the tethered ligand is cleaved?
The cleaved amino acid sequence binds to the receptor and acts as an agonist.
What is the physiological effect of a tethered ligand binding to its receptor?
It activates the receptor, causing a physiological response.
Why is a G-protein needed in GPCR signaling?
The GPCR and effector molecules are often located in different parts of the cell membrane, so the G-protein shuttles messages from the receptor to the effector.
What triggers the interaction between a GPCR and a G-protein?
: Binding of an external signaling molecule to the GPCR causes a conformational change, revealing the G-protein binding site.
What are the components of a G-protein trimer?
A G-protein trimer consists of an alpha subunit, a beta subunit, and a gamma subunit.
What role does the alpha subunit of the G-protein play?
The alpha subunit contains the nucleotide binding site and binds guanosine nucleotides (GDP or GTP).
What happens to the alpha subunit during G-protein activation?
GDP bound to the alpha subunit is replaced by GTP, activating the G-protein.
What is the state of the alpha subunit in an inactive G-protein?
Inactive alpha subunit is bound to guanosine diphosphate (GDP).
How does the conformational change in a GPCR contribute to G-protein activation?
It reveals the G-protein binding site, enabling the receptor to activate the G-protein.
What happens to the G-protein after it is activated?
: The activated G-protein can move to different cellular locations to communicate the signal to effector molecules.
What is the first step in the G-protein cycle?
An agonist binds to the GPCR, activating the receptor and causing a conformational change in its structure.
What happens after the receptor undergoes a conformational change?
The conformational change allows the G-protein to bind to the receptor.
What occurs when the G-protein binds to the activated receptor?
GDP dissociates from the alpha subunit of the G-protein and is replaced by GTP, activating the G-protein.
What happens to the G-protein after it is activated?
The G-protein dissociates into two parts: an alpha subunit (with GTP) and a beta/gamma subunit.
What is the role of the G-protein alpha subunit after dissociation?
The alpha subunit, with GTP attached, moves along the membrane and interacts with an effector molecule
What happens when the G-protein alpha subunit binds to its effector molecule?
The binding changes the activation state of the effector molecule, either activating or inhibiting it
Can the beta/gamma subunit affect the effector molecule?
Yes, the beta/gamma subunit can also modulate effector molecule activity.
What is the resting state of the G-protein?
In the resting state, the alpha subunit is bound to GDP, and the G-protein is inactive.
How does the G-protein alpha subunit return to its inactive state?
GTP on the alpha subunit is hydrolyzed to GDP, causing the alpha subunit to reassociate with the beta/gamma subunit and return to the inactive trimeric form.
What happens to the GTP bound to the alpha subunit over time?
The intrinsic GTPase activity of the alpha subunit hydrolyzes GTP to GDP and a phosphate molecule, inactivating the G-protein.
What occurs after the GTP is hydrolyzed?
The alpha subunit reassociates with the beta/gamma subunit, returning the G-protein to its inactive trimeric state.
Can the G-protein cycle restart while the agonist remains bound to the receptor?
Yes, if the agonist is still bound, the G-protein can rebind to the receptor and become reactivated, initiating the cycle again.
: What happens to the G-protein if there is no agonist bound to the receptor?
The G-protein remains inactive, ready to be activated when the receptor binds another agonist.
Are effector molecules always inactive in a resting cell?
No, some effector molecules are basally active, and in such cases, the G-protein may inhibit their activity rather than activate them.
What is the role of intrinsic GTPase activity in the G-protein cycle?
t ensures that the G-protein is inactivated after a certain period, allowing for tight regulation of cellular signaling.
Why is the G-protein cycle considered dynamic?
Because it can continuously activate and inactivate based on the presence of an agonist bound to the receptor.
What denotes the subtype of a G-protein?
The alpha subunit determines the G-protein subtype.
What is the role of beta-gamma subunits in G-protein signaling?
The beta-gamma subunits are active and regulate cell signaling linked to ion channels, nuclear signaling, and cell adhesion.
Which G-protein subtype activates adenylate cyclase and calcium channels?
G-stimulatory (Gs).
Which G-protein subtype inhibits adenylate cyclase and calcium channels but activates potassium channels?
G-inhibitory (Gi).
What is the primary action of the Go subtype?
It inhibits calcium channels.
What is the primary effector molecule activated by the Gq subtype?
Phospholipase C.
What are the second messengers associated with G-protein signaling pathways?
cAMP (associated with adenylate cyclase) and IP3/DAG (associated with phospholipase C).
How do different G-protein subtypes regulate diverse signaling pathways?
By interacting with specific effector molecules and second messengers that initiate distinct cellular responses.
What are examples of signaling pathways regulated by G-proteins?
Ion channel regulation, nuclear signaling, cell adhesion, and second messenger production.
How does the alpha subunit influence cellular responses?
It interacts with specific effector molecules, determining the signaling cascade initiated by the G-protein.
What activates adenylate cyclase?
The G⍺s subunit of a G-protein activates adenylate cyclase, a membrane-bound enzyme.
What is the primary reaction catalyzed by adenylate cyclase?
Adenylate cyclase converts ATP into cyclic AMP (cAMP).
Which G-protein subtype inhibits adenylate cyclase?
G⍺i inhibits adenylate cyclase.
What is the role of cAMP in cellular signaling?
cAMP activates Protein Kinase A (PKA), which phosphorylates various intracellular proteins.
What is the structural composition of inactive PKA?
: Inactive PKA is a tetramer consisting of two regulatory (R) subunits and two catalytic (C) subunits.
How does cAMP activate PKA?
Two molecules of cAMP bind to each regulatory subunit, causing the release of the catalytic subunits, which become active.
What types of residues do active PKA catalytic subunits phosphorylate?
They phosphorylate serine and threonine residues on target proteins.
What is the overall effect of PKA activation in the cell?
PKA activation leads to the phosphorylation of target proteins, modulating their activity and triggering various cellular responses.
Which G-protein subunit activates phospholipase C (PLC)?
G⍺q activates phospholipase C (PLC).
What is the function of phospholipase C?
PLC cleaves the membrane phospholipid phosphatidylinositol-4,5-bisphosphate (PIP2) into diacylglycerol (DAG) and inositol-1,4,5-trisphosphate (IP3).
What are the two products of PIP2 cleavage, and what are their roles?
DAG: Activates protein kinase C (PKC), which phosphorylates serine/threonine residues on target proteins.
IP3: Binds to IP3 receptors (IP3R) on the endoplasmic reticulum (ER), causing the release of calcium from intracellular stores.
What is the role of DAG in cell signaling?
DAG activates protein kinase C (PKC), which regulates various cellular processes by phosphorylating target proteins.
What happens when IP3 binds to its receptor on the ER?
IP3 binding triggers the release of calcium ions (Ca²⁺) from the ER into the cytoplasm.
How does the release of calcium ions from the ER influence cellular activity?
The increased intracellular calcium levels activate downstream signaling events, such as enzyme activation and muscle contraction.
What are the two major intracellular events resulting from PLC activation?
Activation of PKC via DAG.
Release of calcium from the ER via IP3 binding to IP3R
What are the primary functions of calcium in cellular signaling?
Carrier of electrical charge (Ca²⁺ ion)
Modulation of enzyme activity
Modulation of ion channel activity
How does calcium contribute to cellular functions?
Calcium plays a role in:
Muscle contraction
Heart rhythm
Mitochondrial function
What protein does calcium bind to, and what is the significance of this binding?
Calcium binds to calmodulin (CaM), which amplifies the small signal of calcium to the scale of proteins.
How does calcium-calmodulin affect proteins?
Calcium-calmodulin binds to proteins, changing their activation state, thus regulating various cellular processes.
Provide an example of calcium-calmodulin’s role in cellular function.
Calcium-calmodulin binds to myosin light chain kinase (MLC-K), activating it, which leads to smooth muscle contraction.
What is the role of protein kinases in intracellular signaling?
Protein kinases regulate intracellular second messenger pathways by phosphorylating specific amino acids (e.g., serine and threonine) within proteins, such as receptors, ion channels, and enzymes.
What happens during protein phosphorylation?
: Protein phosphorylation involves the addition of a phosphate group (PO₄²⁻) to specific amino acids in a protein, altering the protein’s structure and function.
What are the potential effects of protein phosphorylation?
Changes protein structure, potentially activating or deactivating active sites or ligand-binding domains.
Alters ligand binding, influencing drug efficacy.
Modifies enzyme catalytic activity, either enhancing or inhibiting it.
How is the effect of protein phosphorylation ‘switched off’?
The effect of phosphorylation is reversed by phosphatases, which remove the phosphate group from the substrate protein.
What are ion channels?
Ion channels are pore-forming proteins that allow ions to cross a membrane based on electrochemical gradients between the inside and outside of the cell or organelle.
How do GPCRs modulate ion channels?
GPCRs activate signaling pathways that modulate the activity of various ion channels. This response is relatively slow but long-lasting, lasting from minutes to hours.
What types of ion channels exist?
There are many types of ion channels, each with a unique mechanism of gating, allowing them to be activated in different ways.
Do GPCRs function as ion channels themselves?
No, GPCRs are not ion channels, but they influence ion channel activity through downstream signaling pathways.
What are the two main ways GPCRs modulate ion channel activity?
Mechanism 1: Via the G-protein.
Mechanism 2: Through GPCR-activated signaling molecules (covered in the next slide).
How does Mechanism 1 modulate ion channels?
The G-protein directly binds to the ion channel and modulates its activity. Both the alpha and beta/gamma subunits of the G-protein can affect ion channel function.
How do opioid receptor agonists, like morphine, affect ion channels?
Opioids such as morphine act on mu-opioid receptors, which inhibit neuronal transmission by modulating ion channels through G-protein subunits. This contributes to their analgesic (pain-relieving) effects.
How can GPCRs modulate ion channel activity through signaling molecules?
GPCRs can modulate ion channels through molecules like PKA, cAMP, Ca²⁺, or IP3 that are part of the GPCR signaling pathway. These molecules bind to ion channels and alter their activity.
What is one example of a molecule that can modulate ion channels in GPCR signaling?
Protein kinases can phosphorylate ion channels, which modulates their function.
How does IP3 modulate ion channels?
IP3 binds to the IP3 receptor, which is an intracellular ligand-gated ion channel, triggering ion channel activity.
