G protein coupled receptors and signalling.

Ola everyone..

Today the topic that we will be discussing is kinda lengthy, so let's not waste time.

We will be learning about G protein coupled receptors today. This is considered as a monstrous topic in cell biology with most students finding it either boring or difficult and they skip it. But as this topic is gaining interest, it won't take long to appear in your vivas or your multiple choice questions. Hence let's have a simple introductory session on it so that we at least don't assume the question is from planet X when we see it.

First discussing the receptors we will talk about G proteins.

I hope this post builds your interest into G proteins. Go.

So What are G proteins?

Let's not panic, the answer is G proteins are the proteins that bind to GTP or GDP. Like any protein they too have alpha, beta and gamma subunits. They can interact with other wide varities of molecules. It's main function is to transport the signals from outside the cells to inside. See not a rocket science.

Here is the structure for G protein. Sorry guys my drawing kinda sucks!

What are G protein receptors?

G protein receptor binds to G proteins. It is a transmembrane receptor i.e located in between the cell membrane having a contact with both outer and inner part of the cell. This receptor spans the membrane 7 times and hence it is also called as 7 transmembrane segment receptors. This is because it has 7 alpha helices. It can bind to various types of ligands and thus induce a corresponding signal via G protein, most common being acetylcholine signalling.

Structure for GPCR right here.

How does the G protein signalling takes place?

This process takes place in multiple steps. Let's take a look.

First, to initiate a signal a ligand binds to the G protein receptors. This brings about some conformational changes in the receptor and now it can bind to G proteins. This protein receptor binding causes alpha subunit to let go off the GDP and bind to GTP which ultimately results in dissociation of alpha subunit. the dissociated alpha subunit now binds to the target proteins which can be an enzyme or an ion which are also known as secondary messengers. This way the G protein regulates the target and the signal either stops at the target or molecules or is carried forward if required.

When the signal reaches effectively, the cycle gets partially complete. Now GTP hydrolyses back to GDP, which causes reassociation of the alpha molecule to it's beta gamma subunits. The ligand unbinds to the receptor and thus the receptor gains it's original structure and thus the G protein moves away from the reseptor, thus completing one entire signal.

That's it. I hope I was able to simplify it.

Happy learning:D

High six from Staph <3