The complexity of G protein
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.
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.