G-Protein Coupled Receptors

Describe the structure of G protein-coupled receptors

As I've mentioned in an earlier post, G-protein coupled receptors have an extracellular N-terminal domain and an intracellular C-terminal domain. They have 7 transmembrane helices connecting the two, with three extracellular loops and three intracellular loops.

Describe the role of G protein-coupled receptors in physiological processes
Discuss the common themes of G protein-coupled receptor signal transduction
Describe the mechanisms by which G protein-coupled receptors cause a change in cell function

I feel like I've already pretty much done this in my previous post, but here's a refresher: when G-protein coupled receptors are bound, they remove GDP from a G-protein, allowing it to be replaced with GTP. This causes the G-protein to break into alpha and beta/gamma parts, which go on to activate other stuff.

Of importance might be the different types of G-proteins. There are three different types of alpha subunits. These are Gα_s, Gα_i and Gα_q. Gα_s work by activating adenylate cyclase, which is the enzyme that converts ATP into cyclic AMP (cAMP). This is a second messenger molecule that activates cAMP-dependent protein kinase A, which leads to effects such as bronchodilation in the airways. (β-adrenoceptors use this pathway.) Gα_i has the opposite effect: it inhibits adenylate cyclase. (α₂-adrenoreceptors use this pathway.) Gα_q is kinda special, in that it actually activates two second messengers. It cleaves phosphatidylinositol 4,5-bisphosphate to form inositol 1,4,5-trisphosphate and diacylglycerol. Inositol 1,4,5-trisphosphate goes on to release Ca²⁺ from the endoplasmic reticulum, which goes on to form a Ca²⁺-calmodulin complex, while diacylglycerol activates protein kinase C. Both of these pathways go on to produce an overall effect, such as bronchoconstriction in the airways. (α₁-adrenoreceptors and muscarinic ACh receptors use this pathway. It is the latter type that results in bronchoconstriction.)

Aaaaand believe it or not, I think I'm done on this topic! That was a short post... I guess the next two will make up for it though! D: