It really doesn't require a genius to work out that a G-protein coupled receptor (GPCR) is a receptor linked to a G-protein. The most important thing about GPCRs, then, is knowing what receptors and G-proteins are.
However, there are a few things about GPCRs that are often helpful to know. I mean, for a start, what are they useful for? Well, receptors are obviously important for having an effect in the cell, but there's various different ways in which this can happen. One is to produce a molecule which can shoot off into the middle of the cell and have it's effect; and it is enzymes which the body uses to make such molecules. GPCRs are involved in activating such enzymes, as shall be discussed in a moment.
A GPCR is a protein which is coiled into several alpha helices that cross the membrane seven times. Because a protein is made of amino acids joined by peptide bonds, one end will have a carboxyl group, while the other will have a amino group. The carboxyl group is found dangling inside the cell, while the amino group is outside the cell.
Between the seven alpha helices which span the membrane, there are intracellular and extracellular loops. One of these (between the 5th and 6th time that the protein spans the membrane) is where the G-protein interacts with the receptor. Between the 2nd and 4th alpha helices outside the cell is the area to which the messenger often binds.
So, a G-protein coupled receptor is simply a particular type of receptor, which works in a particular way to achieve its effect.
A G-protein is a guanine nucleotide binding protein - that is, a protein which binds nucleotides containing guanine. It is made up of three subunits - an α subunit, a β subunit and a γ subunit. It is the alpha subunit which is bound.
In its inactivated state - that is, when it's not really doing anything - it is bound to GDP (guanine diphosphate). However, when the signal arrives at the G-protein coupled receptor (GPCR), the G-protein interacts with an intracellular loop on the GPCR and swaps its GDP for a GTP (guanine triphosphate) that is hanging around inside the cell.
Now activated, the G-protein splits to allow the α-subunit to scoot off and activate the particular enzyme which this G-protein is programmed to activate. And so happily we have an activated system, whereby the enzyme produces what is known as a second messenger to have a particular intracellular effect.
The signal won't stay attached to the GPCR forever, though. Eventually it leaves, the GTP is hydrolysed back to GDP, and the system returns back to its resting, inactivated state.
G-proteins are an effective way of activating intracellular enzymes, which are involved in a whole variety of processes, including regulation of metabolism and changing the expression of genes.