G Proteins

Question 1

The G protein coupled receptor is the largest of the receptor families.

Describe its transmembrane domains? How many are there? How are they arranged? Whats special about alpha helix 3

Answer 1

They have 7 transmembrane domains which anchor into the plasma membraneS

These transmembrane domains aren’t arranged in rows, they are folded around each other

These transmembrane domains are alpha helices

Alpha helix 3 is always orientated towards structures in the cell walls. So it points towards the middle of the cell.

Question 2

What does alpha helix 3 of g proteins do? What are typical of all of the transmembrane domains? And what Is important about the 5th and 6th transmembrane domains?

Answer 2

alpha helix 3 is responsible for transduction outside of the cell, and transferring info into the cell

These domains are made of loops that are on the intracellular side of the cell

The 5th and 6th transmembrane domains tend to be longer than the other domains

Question 3

Configurations of the transmembrane domains of G proteins? What in particular is special about the C and N terminus?

Answer 3

Their amino ‘’n’’ terminus points out of the cell

The c terminus points in

There is a difference in length of these terminals in different receptors

The n terminus contributes to ligand binding

The C terminus regulates receptor function.

Question 4

What are conserved features of all membrane receptors? Where is most diversity in different G proteins?

Answer 4

They all have signal transducing domains - these are parts of the receptors which cause changes in the cell

Most diversity of receptors is seen in the parts of the receptors which are responsible for detecting stimuli

Question 5

How are g proteins formed from there 7 transmembrane domains? And what state do these transmembrane domains oscillate between?

Answer 5

They are all folded up to create the g protein.

These transmembrane domains oscillate between a resting and active state

Question 6

What is a typical feature when G proteins isn’t active? What happens to transmembrane proteins 5 and 6 When the cell is active?

Answer 6

Transmembrane proteins 5 and 6 are close together.

The cytoplasic side of the receptor (so the side in the cell) is pinched and closed

Transmembrane 5 and 6 move slightly when active. This opens the cytoplasmic cleft.

Question 7

What do agonists do when they bind to g proteins?

Answer 7

They bind to transmembrane 3 on g proteins

This binding causes the stabile conformation of the transmembrane proteins of the g protein allowing the protein to signal.

The binding also releases energy which goes to transmembrane 5 and 6. This causes them to undergo critical movements which the agonist stabilises.

Question 8

Key note about how far GPCRS stick out of the membrane?

Answer 8

They dont stick out very much

Question 9

In what orientation must a ligand enter a cell?

Answer 9

In the correct orientation.

Question 10

In terms of agonist bind how are GPCRS different?

What is the mechanism and structure for the detection of glutamate on G proteins

Answer 10

Each agonist has different mechanisms of binding

Glutamate binds to these G proteins in a similar way to how they bind to ligand gated channels

Alike the ligand gated channels:

These receptors have very large extracellular domains made by a very long amino terminus

These also have a clam shell shape which ligands binds to.

Question 11

How is the glutamate receptor on GPCR different to normal GPCR?

Answer 11

These glutamate receptors g proteins rely on a clam shell trapping molecules

Whereas normally ligand molecules usually enter the GPCRs at the 3 transmembrane domain

Question 12

What is the biggest sub family of the GPCRS?

Answer 12

The prototypical adrenoreceptors.

These detect small molecules like dopamine and ACh

THUS most small neuropeptide transmitters often work through GPCRS

Question 13

What are odourant receptors?

Answer 13

These are GPCR receptor proteins.

There are over 700 different receptors for different odours

These odours are volatile

Different olfactory sensory neurones remember expresses different repertoires for different odours.

Question 14

What do PAR receptors detect? What are they important for?

Answer 14

They detect thrombin

They are important for blood clotting and wound healing.

Thrombin is a protease remember

Question 15

How does the thrombin protein affect PAR receptors?

Answer 15

When it comes along and encounters receptors

It cleaves amino terminus at specific of the receptor

Then the peptide leaves

The remaining part of the amino terminus folds down and goes into the ligand binding site. This activates the receptor.

Question 16

How does a blood clot work? And how do GPCRs get involved?

This is initiated when there in break in the endothelium lining of blood vessels

Answer 16

This attracts red blood cells and other factors to the area where there is a breach.

Blood escapes to try and plug the hole

GPCRs are important in the blood clotting. The thrombin receptor has to be activated (to activate the protein).

And also the ADP receptor is also needed - this is the purinergic receptor.

The GPCR recognises extracellular ATP and ADP

Question 17

GPCRs are stimulated by thrombin, how do GPCRs stop blood clots? And how can drugs which stimulate these reduce risk of thrombosis?

Answer 17

They work at the level of platelets by stopping them from sticking together. Stopping blood clots.

Drugs which interact with these GPCRS can thin blood, reducing thrombosis risk.

Question 18

Before a G protein is bound to a receptor. What state is the receptor in? What happens when the receptor becomes active? What is bound to the Alpha helix at this stage? And what is this exchanged with

Answer 18

Before a G protein complex is bound to its receptor. The receptor is in the membrane as the inactive form.

When it becomes active. This is when it is bound to an alpha and a beta gamma subunit.

At this stage the alpha helix has a GDP bound. This is exchanged for a GTP by GTPase whilst the G protein is associated with the receptor

Question 19

What is a heterotrimeric and a monomeric G protein?

Answer 19

The heterotrimeric G protein is when the alpha sub unit of the G protein is bound to the beta and gamma sub units. Think of this when the G protein is bound to the receptor

The monomeric G protein is when the alpha sub unit of a G protein is dissociated from the beta gamma sub units. Think of this as when the G protein is away from the receptor.

Question 20

What happens when the GDP is exchanged for GTP on the alpha subunit of the G protein. This is when its bound to the receptor

Answer 20

The alpha and beta gamma sub units can now dissociate from the receptor and go onto work on downstream effectors.

Question 21

Whats the similarities between the heterotrimeric G protein and the monomeric G protein

In ref to the alpha sub unit

Answer 21

The monomeric G protein (When the G protein Alpha sub unit is alone) has a GTP exchanged for a GDP by a GTPase

This is thanks to GEF - guanine exchange factors

The heterotrimeric G protein (when the G protein alpha sub unit is bound to the beta gamma sub units at the receptor) it has a GDP (note what this says) to a GTP

This is also thanks to GEFs. In this case the receptor acts as the GEF

Question 22

What controls the timing of the exchange of GTP for GDP in monoeric G protein alpha sub units?

Answer 22

This is controlled by GAPs

These are extrinsic proteins.

This likely occurs when the monomeric g proteins are bound to effectors like adenyl cyclase.

Question 23

So what does the heterotrimeric G protein have bound after GTPase has done its jobs?

And how about the monomeric G protein

Answer 23

After the GTPase done its job

The heterotrimeric G protein (when the alpha sub unit is bound to the beta gamma sub units and the receptor) the G protein has a GTP bound

The monomeric G protein has a GDP bound

(The GTP which was orignally bound is swapped for a GDP by the GTPase. This is when the monomeric G protein is bound to an effector like adenyl cyclase).

Question 24

There are also proteins which control the rate of GTPases action on the alpha sub unit of heterotrimeric G proteins (the alpha subunit bound to the receptor)

What are these proteins? And the ones that increase the rate of the GTPase’s, what do they turn off quickly? What does phospholipase C do?

Answer 24

These of RGS proteins

They either speed up or slow down the rate of GTPase action.

The ones which increase GTPase action turn off signalling quicker

Phospholipase C (an effector) can also work as a RGS to speed up GTPase action on the alpha sub unit of the heterotrimeric G protein

Question 25

Important to note about combining all the alpha beta gamma subunits of heterotrimeric G proteins?

Answer 25

There is over 1000 combinations

Different G alpha proteins differ in the downstream effectors they regulate.

Question 26

What are the odour and the visual G protein coupled receptors?

Answer 26

G alpha olf receptors are specific for odour

G alpha T is related to visual systems. This is on rhodospin channels

Question 27

What determines how long the heterotrimeric G proteins are in the GTP state

Answer 27

This is determined on the length of the associated signalling

Question 28

What can beta sub units do from G proteins?

Answer 28

These can work on regulating enzyme activity

And 2nd messengers

Question 29

What does adenyl cyclase do remember?

Answer 29

This converts ATP to cAMP