The body uses 7 membrane-spanning serpentine receptors for an astounding variety of biological signalling functions. Receptors on the cells lining our tongue convey taste. Hundreds of distinct receptor species in the cells of our olfactory bulbs in our nose convey information about the presence of odors (odorant ligands). A carotenoid molecule related to vitamin A is bound in the ligand position of rhodopsin in the rods and cones of our eyes where it serves to pick up photons, alter its conformation, and cause the receptor to which it is bound to release signals into the rod/cone cytoplasm that result in our perception of light. These serpentine receptors are of very ancient lineage. Baker's yeast cells communicate their sexual identity to each other by release of polypeptide mating factors. The cell surface receptors that recognize these mating factors are once again 7 membrane-spanning serpentine receptors! We will explore another well-studied example of a ligand/serpentine receptor pair.
The G protein's two states (ON or OFF) are determined by the guanine nucleotide that it binds (whence the term G protein). When it is inactive it binds GDP; when active, it binds GTP. Accordingly, the resting, OFF form of the G protein sits around with its bound GDP. When a ligand- activated receptor pokes it, the G protein releases its bound GDP and allows a GTP molecule to jump aboard. This GTP-bound form of the G protein represents the active ON configuration of the G protein. While in the ON state, it releases downstream signals. After a short period of time (seconds or less), the G protein will then hydrolyze its own GTP down to GDP, thereby shutting itself off. This hydrolysis represents a negative feedback mechanism which ensures that the G protein is only in the active, signal- emitting ON mode for a short period of time.
Once this happens, the GTP-bound a subunit also loses affinity for the receptor, dissociates from it, and moves over and pokes yet another nearby protein, the enzyme adenylate cyclase, which until this time has been inactive. Once it is poked by the active, GTP-binding oc subunit of the G protein, the adenylate cyclase enzyme gets activated and does its job: it cyclizes ATP into 3'5' cyclic AMP. This reaction involves the release of the beta and gamma phosphates from the ATP and the linking of the surviving a phosphate (still attached to the 5' hydroxyl of ribose) to the 3' hydroxyl as well, forming a circular or cyclic structure, whence the term ''cyclic adenosine monophosphate'' or simply cAMP.
After a several second encounter with the adenyl cyclase enzyme, the alpha subunit of the G protein will hydrolyze its bound GTP and release the adenyl cyclase, thereby reverting to an inactive OFF signalling state. It will then rejoin the beta and gamma subunits that it deserted earlier in the game. The adenyl cyclase, no longer being poked by the activated a subunit of the G protein, will shut down and stop making cAMP from ATP. The whole cycle has resulted in only a brief pulse of signalling, in this case the production of several hundred cAMP molecules made by the adenylate cyclase during its brief period of activity.
Once made, the cAMP molecules act as intracellular glycogen, the high cAMP concentrations enable A kinase to
There is enormous signal amplification in this cascade. A single epinephrine molecule (present at 1O-10M) may cause the activation of dozens of alpha subunits of proteins. Each of these in turn will activate the synthesis of a single adenylate cyclase, and each of these in turn will synthesize hundreds of cAMP molecules. Each of these in turn can activate a cAMP-dependent kinase that will on its own right modify hundreds of target molecules in the cell.