Lecture 3 Flashcards
What is the membrane ?
A selectively permeable barrier between the extracellular environment and the cytosol.
Why are differences in concentrations important ?
For cell function.
What are the purpose of the mechanisms in place ?
To allow the cell to maintain the concentrations inside and outside the membrane, these mechanisms depend on the transport of ions across the plasma membrane.
What is the plasma membrane made up of ?
A lipid bilayer - contains integral, extracellular and cytosolic proteins.
What are integral proteins ?
They span the width of the membrane, there are different types.
What are examples of peripheral proteins ?
Extracellular and cytosolic proteins.
Why is there a bilayer ?
Because the plasma membrane is two molecules thick and has both hydrophobic and hydrophilic characteristics of polar lipids resulting in this arrangement.
Why is the lipid bilayer important ?
To allow the plasma membrane to function because they depend on the environment (i.e. ion concentration) of the lipid bilayer for their function.
How are drugs distributed throughout the body ?
By the use of blood and bodily fluids.
How are drugs able able to act on individual cells in the body ?
Because drugs are able to bind to protein receptors.
Where are receptors found ?
On the outer membrane of the cells but also found on enzymes within the cell.
What are receptors ?
They are typically integral membrane proteins at the plasma membrane.
How can receptors be altered ?
By altering the concentration of the agonist (ligand).
Increased: up-regulation
Decreased: down-regulation
Factors that influence how drugs work
Absorption, distribution, metabolism, excretion.
Absorption
This is dependent upon a variety of factors, e.g. influenced by the solubility of the drug in the blood.
Distribution
Drugs may bind to the plasma proteins so there is a ratio of bound/free drug, only free drug molecules can bind to the receptor to exert effect.
Metabolism
It is important for dugs to be broken down (i.e. metabolised) in the body so the drug can be inactivated and excreted.
Excretion
To ensure that the drugs are excreted and leave the body, most commonly via urine.
How are biological responses exerted ?
The drugs recognise and bind to the receptors exerting a biological response.
Ligands are…
.. drugs.
What are ligands ?
They are chemicals (synthetic or natural) that can bind to receptors, they can be classified as agonists or antagonists.
How do agonists work ?
They are able to activate receptors and the cell responds. They have two important properties: affinity and efficacy.
Affinity
The strength with which a ligand binds to a receptor.
Efficacy
Hoe effective/efficient the agonist is in inducing a response (i.e. a conformational change/3D change in the receptor).
How do antagonists work ?
They will bind to the receptor (possibly very strongly) but there will be no response (or activation/conformational change), they tend to bind to the reception to prevent an agonist from binding thus blocking the response e.g. anti-histamines, beta-blockers.
What does cell signalling do ?
They encompass the signals that move between cells, i.e. extracellular cells.
Where are cells signals generated ?
Signals are generated inside the cell as a result of a chemical stimulus or drug binding to a receptor, which causes a change in the signals that are generated inside the cell - the signals inside the cell are often different.
What underlies disease processes ?
Aberrant/abnormal cell signalling.
Many drugs are targeted to…
… cellular signalling processes.
By understanding how cellular signalling and how the processes work…
…it is possible to look at potential targets either on the cell surface or inside the cell that can be used for drug design and improved therapy.
What are signal transductions ?
Signals that pass between the cell and also are signal that generated inside the cells, they enable amplification.
What indicates a series of chemical changes in the cell ?
The binding of the signal chemical to its receptor which triggers a response in the cell, these changes alter the physiology and function of the cell.
Types of signal transduction
Direct opening of ion channels, direct activation of an enzyme, indirect activation/deactivation of enzymes or indirect opening/closing of ion channels.
Direct opening of ion channels
Chemicals that bind to receptors which are ion channels, by binding to the ion channel, the drug/chemical causes an opening of the ion.
Direct activation of an enzyme
A receptor may contain enzyme activity, which remains silent until it is activated through the binding of a chemical/drug.
Indirect activation/deactivation of enzymes or indirect opening/closing of ion channels
The agonist/drug binds to the receptor, which then uses a coupling protein (G-protein).
What are G-proteins ?
They are coupling agents, they use GTP as a source of energy.
What do coupling agents do ?
They couple two enzymes of choice.
Key intracellular second messengers involved
cAMP (cyclic nucleotides), inositol triphosphate (IP3), diacylglycerol (DAG), calcium ions.
Purpose of these key intracellular second messengers
Cause changes in the activation of intracellular membranes.
What happens when drugs target these processes ?
The drugs induce changes in the levels or activation of these enzymes.
What happens with the drug/agonist binds to the ion channel ?
This causes a conformational change therefore allowing ions to enter the cell, down their concentration gradient.
Why do the ion channels open/close ?
Due to the presence or absence of the specific agonist.
The distribution of ion channels…
… is often specific to certain specialised cell types, especially excitable cells.
What are enzyme-linked receptors ?
They are transmembrane proteins which contain enzyme activity, they have a ligand binding domain on the outer face of the plasma membrane and a catalytic/enzymatic domain on the inner face of the plasma membrane.
What happens when there is a stimulation of this type of receptor by an agonist ?
There would be an increase in the receptor’s catalytic activity.
What happens when the agonist binds to the receptor ?
This would activate the enzyme activity and would lead to the phosphorylation of that receptor, i.e. a phosphate group would be transfer onto specific amino acids in the receptor itself.
What happens as a result of the phosphorylation of a receptor ?
This would lead to the recruitment of specific cytosolic proteins which would become scaffolded to the receptor.
What is the purpose of scaffolding ?
The scaffold transmits the signalling information to the cell, this would induce a change in the cell and its function.
What are G-protein coupled receptors ?
Where receptors use a coupling protein to couple with the enzyme.
What is the process of G-protein coupling ?
The agonist/drug binds to its receptor, it triggers the activation of a G-protein, this becomes active by binding with the GTP allowing the receptor to couple with the enzyme allowing activation.
What happens as a result of this specific GTPase activity ?
The receptor can hydrolyse the GTP to GDP and deactivate itself.
Three protein components
Each receptor (GPCR) binds to its specific agonist. The G-protein acts as a molecular switch. Examples of effector enzymes.
Examples of GPCR binding to its specific agonist
Adrenaline binds to adrenoreceptors, glucagon binds to glucagon receptors.
How does the G-protein act as a molecular switch ?
The G-protein is switched on when associated with GTP and switched off when associated with GDP.
What are G-proteins ?
Enzymes that convert GTP to GDP thereby switching itself and the system off, there are several types of G-proteins that exist and only specific receptor interact with specific G-proteins.
G-protein =
Guanine nucleotide binding regulatory protein.
Examples of effector enzymes
Adenylyl cyclase, phospholipase C, cGMP phosphodiesterase.
Adenylyl cyclase
It becomes active through coupling with the G-protein which triggers conversions of ATP to cAMP.
Phospholipase C
This cuts the plasma membrane, lipid phosphatidylinostiol biphosphate into DAG and IP3.
cGMP phosphodiesterase
This breaks down cGMP to GMP.
What are second messengers ?
They are intracellular signalling molecules released by the cell in response to exposure to extracellular signalling molecules, which then go on and activate other enzymes.
Examples of second messengers
cAMP, DAG, IP3 and cGMP.
GPCR signalling - activation
The agonist activation of the receptor induces its 3D conformational change which enables the agonist to interact with a G-protein.
What is the process of GPCR signalling activation ?
The G-protein loses the GDP and gains a GTP which results in the G-protein being switched on.
The activated G-protein then interacts with the effector enzyme and increases its catalytic activity.
The effector enzyme produces second messengers.
These second messengers alter the biochemical machinery inside the cell by interacting with their specific target molecule which either phosphorylate other proteins or release intracellular stores of Ca2+.
GPCR signalling - deactivation
The agonist dissociates from the receptor.
What is the process of GPCR signalling deactivation ?
The G-proteins own GTPase converts the bound GTP to GDP and the G-protein becomes switched off.
The G-protein interaction with the effector enzyme ceases and the effector enzyme activity returns to normal.
The second messenger molecule;es are broken down and their target proteins are no longer activated.
What is the result of the agonist/drug binding to the 7 transmembrane receptor ?
This causes the coupling of the G-protein to the AC (effector enzyme).
What is the purpose of cAMP ?
cAMP causes the activation of the protein Kinase A (PKA) - cyclic AMP dependent kinase.
This results in the phosphorylation and activation of a number of different molecules and enzymes inside the cell.
This leads to a change in cellular response.
All these molecules can be subjected to changes by drugs.
What are calcium signals ?
They are very important signals that signal a number of different things in the body.
These are the first signals detected after fertilisation.
They are second messengers.
They are not produced not destroyed, they are moved between compartments.
Different process that have calcium signals
Muscle contraction, secretion, metabolism, neuronal excitability, cell proliferation.
Why is it important that the calcium ion concentration remains low ?
Because if there is an increase in calcium ion concentration, this would trigger different responses and effects in the cell.
There are different mechanisms that help maintain low levels of calcium ions.
The effect that different agonists have on different cell types are…
… concentration dependent.
How does calcium enter cells ?
Through different types of channels or transporters.
Types of channels and transporters
Calcium release activated channels (CRAC) - store operated calcium ion channels.
Calcium ion tranporters (PMCA).
Voltage operated channels (VOC).
Ligand gated channels - open in response to ligand binding.
Calcium antagonists/blockers
These drugs modulate muscle contraction, in heart conditions.
There are different types of drugs that can modify calcium entry into cells.
Good example of showing how drugs target cell signals.
Examples of calcium antagonists
Nifedipine, amlodipine, verapamil, diltiazem.
What do intracellular calcium ion release channels do ?
They release calcium to induce a reaction from the cells.
The opening of these channels trigger the release of calcium from the intracellular stores and raises the cytosolic concentration.
This is important to enable changes in the cell function that are calcium dependent, eg. contraction, proliferation.
Examples of intracellular calcium ion release channels
Inositol triphosphate receptor (IP3R) - responsive to changes in Its levels and triphosphate levels.
Ryanodine receptor - responsive to changes in intracellular calcium levels.
What are excitable cells ?
They are cells which respond to changes in voltage, e.g. muscle and nerve cells.
The process of contraction coupling in smooth muscle
This is an excitable cell which therefore responses to changes in voltage as this is a muscle cell.
The process of contraction coupling in smooth muscle - step 1
There is an excitation which causes the calcium to enter the cell.
This increase in intracellular calcium trigger the opening go the intracellular calcium release channel which releases more calcium from the intracellular store.
The process of contraction coupling in smooth muscle - step 2
The increase in calcium causes the calcium to bind with the calmodulin to form Ca-calmodulin complex which will then activate proteins/enzymes in the cell.
In this case MLCK (myosin light-chain kinase)
The process of contraction coupling in smooth muscle - step 3
The MLCK will phosphorylate the myosin light chains causing contraction of the muscle cell.
What happens when there is an increase in calcium concentration in the cell ?
This results in a change in enzyme activation which could results in a change in the cell function.
What is the majority of the muscle in the GI tract ?
Smooth muscle.
What is the function of parietal cells in the stomach ?
They are key in releasing hydrochloric acid which creates an acidic environment, which allows the enzymes in the stomach to be activated due to the acidic environment.
What are mast-like cells ?
They circulate the cell and secrete other cells that work with the parietal cell.
What happens if ACh is stimulated ?
The ACh will bind to the mAChR receptor which would result in the increase in second messengers and the activation of the proton pump.
How is the activation of gastrin receptors triggered ?
By the binding of gastrin hormones which will then activate the proton pump.
What can influence the proton pump activity ?
Drugs can be used to target each of the different processes can influence the proton pump activity which therefore influences the production of acid.
Where are the receptors activated by ACh and GR (gastrin) ?
These receptors are found on the mast-like cells.
What happens when the mast-like cells are activated ?
The cells release histamines.
What do histamines do ?
They bind to the H2R receptor in the parietal cell which will also activate the proton pump.
There are different targets which can be used by…
… drugs to interfere with the functional response.
Histamine receptors
These are targeted and result in a change in modulation of HCl secretion, e.g. ranitidine.
Why are opioid receptors targeted ?
Because they are important in effecting muscle contraction, by targeting these receptors certain types of muscle movements can be decreased, e.g. loperamide.
What is peristalsis ?
Muscle movement
