fundamental molecular receptor pharmacology one

Question 1

how do cells communicate with each other in the body ?

Answer 1

Cells in the body communicate with each other and this is through chemical signalling and the use of receptors

Question 2

how are these signals commonly released ?

Answer 2

These signals are commonly released into an aqueous environment and so they need to be polar. However, as the signal reaches a cell, the cell contains a lipid bilayer membrane, and the polar molecules are insoluble. This means that the signal cannot enter the cell to produce a response, so instead there are protein receptors embedded in the plasma membrane to which the signal can bind to. Once the signal binds to the receptor this activates the receptor, and it can convey a signal to the cytosol of the cell and a response occurs.  

Question 3

exception to this ?

Answer 3

An exception to this are the steroids which can pass the lipid bilayer into a cell, their problem occurs when needing to be transported between cells as they are in insoluble in the aqueous environment. They need to be bound to protein transporters. Once the steroid has reached the target cell it can dissociate from the transporter and then cross the bilayer and influence a cell to produce a response.

Question 4

what is the inter cellular communication dependent on ?

Answer 4

The inter cellular communication used is dependent on the distance the signal has to travel. If the signal is very close then, direct contact signalling will occur, if there is distance between the cells but still close then paracrine signalling occurs. Endocrine signalling has the furthest to travel.

Question 5

direct contact ?

Answer 5

Direct contact occurs only if cells are close in contact and contain small pores composed of protein such as connexin, these are called gap junctions, through which molecules can be transferred from one cell to another. An example is cardiac muscle cells which transfers electronic charge that enable spread of depolarisation that results in a collective group of cardiomyocytes contracting. 

Question 6

paracrine ?

Answer 6

Paracrine signalling is a common communication between secretory cells which release chemical mediators acting on adjacent effector cells through binding to the cell surface receptors on the effector response, inducing a biological response in that cell

Question 7

autocrine ?

Answer 7

Autocrine signalling is similar except the chemical mediator acts back on the cell that produced it and released it. A good example of this is when chemicals are released that bind to the blood vessel cells receptor and allows the vessel to dilate or constrict.  

Question 8

endocrine ?

Answer 8

Endocrine signalling is common for cells of a gland for example beta cells/insulin release, they release a hormone that acts on a cell some distance away by travelling in the blood to promote a cellular response. 

Question 9

synaptic ?

Answer 9

Synaptic signalling is similar to paracrine signalling but involves specialised neurones.  Here, neurotransmitters are released from the neurone synaptic terminal and act on the target cells to propagate of nerve impulse or to contract skeletal muscle

Question 10

intracellular signalling ?

Answer 10

There are 4 main types of intracellular signalling these are ligand gated ion channels, G protein coupled receptors, kinase linked receptors and nuclear receptor.

Question 11

ligand gated ion channels ?

Answer 11

For ligand gated ion channels, the receptor is an ion channel, and this could be for sodium ions or chloride ions. The ion channel will normally open in response to a ligand binding to the receptor as the majority are closed and open in response to being activated. This can cause depolarisation (Na+, Nicotinic acetylcholine receptor, Cl-, GABA receptor) to cause excitation or inhibition in milliseconds. GABA receptors binds Gaba ions which allow chloride ions to enter which hyperpolarises the cell and inhibits the excitation of a particular neuron.

Question 12

GPCR simple explanation ?

Answer 12

For G-protein coupled receptors the binding of a ligand results in the formation of second messengers such as cyclic AMP which promotes the activation of protein kinase such as PKA, that phosphorylate proteins on Serine or threonine and either activates them or inactivates them. The receptors are coupled with a G protein and when a ligand binds this interacts with the effectors to produce these secondary messengers

Question 13

protein kinase ?

Answer 13

Protein kinase splits ATP and transfers the terminal phosphate to covalently modify proteins , activating or inhibiting them, this is a molecular switch to turn on or off cell response.

Question 14

IP3 secondary messenger?

Answer 14

Ins(1,4,5)P3 binds to an intracellular receptor to enable calcium to flow into the cytoplasm from the endoplasmic reticulum to activate calcium-dependent cellular processes

Question 15

activity of GPCR depedent on ?

Answer 15

The activity of the G-protein is dependent on the exchange of GDP for GTP in the G-protein itself and this is activated when the ligand binds to the receptor.

Question 16

time of GPCR response ?

Answer 16

Time taken to produce a response is between seconds and minutes.

Question 17

kinase linked receptors ?

Answer 17

Kinase-linked receptors, when a ligand binds to these receptors it activates a kinase that is part of the receptor , which is intrinsic or recruits an external kinase. These kinases phosphorylate proteins on tyrosine residues to activate them. This signalling cascade results in the activation of transcription factors that turn on genes, the protein products of which regulate processes such as mitosis and cell growth.  Responses typically take hours for a response. 

Question 18

nucealr receptors ?

Answer 18

Nuclear receptors bind steroids such as oestrogen as they can cross the bilayer membrane. Binding of ligand to these receptors promotes formation of a complex that moves to the nucleus and functions as a transcription factor to turn on or off genes, the protein products of which regulate processes such as cell growth and differentiation, responses typically take hours. 

Question 19

structure of GPCR ?

Answer 19

GPCR contain 7 transmembrane domains that span the bilayer, with an extracellular N terminus and an Intracellular C terminus. 

Question 20

3 classes of GPCR >

Answer 20

rhodopsin
secretin
meabotrophic

Question 21

rhodopsin ?

Answer 21

Rhodopsin family are the largest group and are related to the first GPCR discovered which was called rhodopsin. This receptor is activated by light not a ligand in the retina-and is responsible for visual perception. It contains a short extracellular N terminal tail. 

Question 22

secretin ?

Answer 22

Secretin family for example glucagon have an intermediate longer N terminal tail and agonist is generally a peptides which bind to N terminal tails.  

Question 23

metabotrophic ?

Answer 23

Metabotropic family such as glutamate and very long N terminal tail. 

Question 24

G protein composed of ?

Answer 24

The G-protein is composed of an alpha, beta and gamma sub-unit complex and has GDP bound to the alpha sub-unit. It is inactive in this state

Question 25

explain dissociation and agonist ?

Answer 25

At millimolar concentrations of magnesium which exist in the cell, GDP slowly dissociates from the G-protein. The agonist-receptor complex increases the rate of release of GDP.  Since GDP release is usually slow, it is this reaction that is the rate-limiting step in the activation of the effector e.g. adenylyl cyclase.  The increase in the rate of release of GDP is achieved by the agonist-receptor complex lowering the concentration requirement for the magnesium-dependent GDP release.  When the G-protein is free of GDP, GTP will bind and this reaction occurs faster than GDP re-binding. This is an exchange reaction that occurs and as a result the agonist receptor complexes can be referred to as guanine nucleotide exchange factors. It is not a phosphorylation of GDP to GTP.  Thus, the rate of GTP binding is dictated to by the rate of GDP release.  When GTP has bound to the alpha sub-unit this causes dissociation of the alpha-beta-gamma G-protein and both the complexes are free to move in the plasma membrane. The activated GTP bound alpha sub-unit then interacts with an effector enzyme such as adenylyl cyclase which stimulates cyclic AMP synthesis.  Or by interacting with the phospholipase C effector enzyme which increases protein Kinase C. The process is terminated by an intrinsic GTPase activity in the alpha sub-unit. The GTPase will cleave the terminal phosphate of GTP to GDP. The GDP reassociates with the beta gamma subunits. The rate of GTPase reaction is determined also by the rate of GDP release; in other words, it only occurs when GTP is bound to the protein.  It serves as a time-delay switch.  Thus, the degree of activation of adenylyl cyclase is governed by the rate of GTPase activity. 

 

Question 26

what is the rate of cycling directly linked to ?

Answer 26

The rate of cycling is directly linked to the efficacy of the ligand (agonist) e.g. more cycling = increased efficacy.  A full agonist will activate this cycle hundreds of times, a partial agonist (weaker) may only activate 10 times. So, the amount of effector is determined by the rate of GTP activation being cycled.  This relates to the conformation of the agonist when it binds to the receptor, you could have a conformation that’s weakly bound to the G protein or a conformation that is tightly bound to the G protein. Therefore, determining how much effector is activated and second messengers are produced which determine the biological response.

 

Question 27

conformations of GPCR ?

Answer 27

The GPCR exists in an inactive and active conformation that are in equilibrium.  The active conformation is coupled to the G-protein. Within this active state there can be conformations that are tightly coupled to the G protein or weakly coupled. The inactive conformation is not coupled to the G-protein. 

Question 28

agonist to conformation ?

Answer 28

Binding of an agonist is exclusively to the active conformation and this creates a new equilibrium of agonist bound to the active conformation. This stabilizes the active conformation so that it exists for longer. The longer the active conformation exists the more chance it has of causing GDP-GTP exchange in the G-protein and therefore productive interaction with the effector (e.g adenylyl cyclase) to produce second messenger and therefore biological response.

Question 29

equilibrium shift of adding agonist ?

Answer 29

If agonist binds to the active conformation, it will reduce the concentration of the active conformation with no ligand bound. Therefore, the rate of the conversion of active conformation back into inactive conformation reduces and the overall equilibrium shifts to the right e.g. formation of the active confirmation (because the backward rate is reduced, while the forward reaction rate remains unchanged).

Question 30

competitive antagonism ?

Answer 30

Competitive antagonism binds to both the active and inactive conformation with equal affinity. Therefore, there is no equilibrium shift towards the active conformation, and this causes no efficacy. By the antagonist binding to the active conformation, it overlaps the binding of the agonist as the binding site is blocked. If you increase the conformation of the agonist this increases the chance of the agonist binding over the antagonist.  

Question 31

non competitive antagonist ?

Answer 31

The non-competitive antagonist depresses the maximum response. Even If you increase the concentration of the agonist it cannot fix this.  

Question 32

inverse agonist ?

Answer 32

An inverse agonist binds exclusively to the inactive conformation. In doing so it creates a new equilibrium for binding ligand and reduces the concentration of free inactive conformation. Therefore, the rate of conversion to the active conformation is reduced and the equilibrium is shifted to the left (e.g. from active conformation to inactive conformation) . The active conformation will stimulate G-protein very weakly (because it is not stabilized with agonist).  There is some activity, and therefore the inverse agonist will be reduced G-protein activity (reduced GTP binding in the graph). Therefore, inverse agonists exhibit negative efficacy. 

Question 33

how is effector enzyme adenyl cyclase modulated >

Answer 33

by g protein Gs and Gi

Question 34

Gi mechanism ?

Answer 34

The beta gamma sub-units are produced in high concentrations from Gi as there is usually a ten-fold excess of Gi over Gs in the plasma membrane.  Thus, they drive the equilibrium of association of Gs alpha with beta gamma to the formation of the alpha-beta-gamma complex, thereby inhibiting the action of GTP-Gs alpha upon adenylyl cyclase. 

Question 35

stimulatory beta 2 adrenergic receptor coupled to >

Answer 35

A stimulatory receptor such as beta 2 adrenergic receptors is coupled with Gs which is composed of 3 subunits , alpha beta and gamma. The alpha s subunit is unique to the Gs protein. The dissociation of the alpha subunit  with bound GTP then collides with the effector enzyme adenyl cyclase and causes an increase in the production of cyclic AMP which is released from the membrane into the cytosol of a cell and targets protein Kinase A.  

Question 36

inhibitory receptor alpha 2 adrenergic receptor coupled to ?

Answer 36

An inhibitory receptor such as the alpha 2 adrenergic receptors is coupled with Gi , the unique alpha i subunit dissociates with the bound GTP from the beta gamma subunits and collides with the effector enzyme adenyl cyclase and inhibits it which reduces the levels of cyclic AMP and Protein Kinase A is not activated

Question 37

liver PKA ?

Answer 37

In the liver, PKA catalyses the phosphorylation of glycogen synthase converting it from an active form to an inactive form.  This switches off glycogen synthesis.   

Question 38

glucagon affect adenyl cyclase?

Answer 38

glucagon can activate adenylyl cyclase via a Gs coupled receptor to turn off anabolic processes like glycogen synthesis and turn on catabolic processes like glycogen breakdown

Question 39

adrenaline binds to beta 3 ?

Answer 39

Adrenaline binds to the beta3 adrenoceptor in adipocytes to cause activation of adenylyl cyclase/cyclic AMP/PKA which activates triglyceride lipase to promote the breakdown of triglycerides to produce fatty acids that undergo beta oxidation to produce energy (ATP)-another catabolic process 

Question 40

how is IP3 removed ?

Answer 40

Ins(1,4,5)P3 is progressively dephosphorylated and removed by phosphatases to produce Inositol which is recycled to PIP2.