The complexity of G protein signalling
Although it is usually pointless to point out the complexity of biological systems, here it may be appropriate.
G protein coupled receptors, besides their roles in respond-ing to intrinsic signals (hormones and transmitters), are also responsible for our ability to smell and taste, which means that G protein-coupled receptors respond to extrinsic rather than intrinsic signals. Beyond the large number of recep-tors with known roles, there is an even larger number of so-called `orphan' receptors, the ligands and functional roles of which have not yet been determined. The overall number of GPCR genes in the human genome is at least 300-400 – which corresponds to about 1% of all genes. As pointed out above, the significance of G protein-coupled receptors in pharmacology is already great today, and it is likely to increase as more information on the ligands and functional roles of individual receptors will become avail-able, and the interaction of drugs and receptors will be elu-cidated in structural detail.
G protein-coupled receptors function as adapters between the virtually boundless multitude and variety of extracel-lular hormone and neurotransmitter signals and the lower (yet still considerable) number of intracellular G-proteins. Therefore, it is very common to have receptors for multiple transmitters or hormones converge onto the same type of G protein and thus trigger the same response. E.g., glucagon and epinephrine both activate adenylate cyclase in the liver, through separate receptors but the very same G protein.