22.5 : GPCRs Regulate Adenylyl Cylase Activity

Some GPCRs transmit signals through adenylyl cyclase (AC), a transmembrane enzyme. AC helps synthesize second messenger cyclic adenosine monophosphate (cAMP). AC catalyzes cyclization reaction and converts ATP to cAMP by releasing a pyrophosphate. The pyrophosphate is further hydrolyzed to phosphate by the enzyme pyrophosphatase, which drives cAMP synthesis to completion. However, cAMP is rapidly degraded to 5′ AMP by the enzymes phosphodiesterase (PDE), preventing overstimulation of cells.

Two types of heterotrimeric G proteins regulate AC.

1. The stimulatory G protein (G_s) binds and activates the AC, increasing cAMP levels.
2. The inhibitory G protein (G_i) binds and inactivates AC, lowering cAMP levels.

AC consists of a small N-terminal domain, two repeated transmembrane, and a cytoplasmic domain. The cytoplasmic domain forms the catalytic core and consists of the G protein binding site. The binding of G_s protein to AC help opens its catalytic core, facilitating the binding of substrate, ATP. In contrast, binding of G_i protein leads to reorientation of catalytic residues, thereby closing the catalytic core, which prevents ATP binding.

Many pathogenic bacterial toxins disrupt this regulation mechanism of adenylyl cyclases in the host cells. For example, Vibrio cholerae enter intestinal epithelial cells and release cholera toxins. The toxin induces ADP ribosylation of the G_⍺s subunit of the stimulatory G protein. The addition of ADP-ribose inhibits intrinsic GTPase activity of the G_⍺s subunit. Thus, the modified G_⍺s subunit remains constitutively active, activating adenylyl cyclase without external stimulation. Continued AC activation prolongs cAMP synthesis. The resulting increase in cAMP concentration causes water and electrolyte efflux from the intestinal cells to the lumen. This leads to severe diarrhea and excessive dehydration, characteristic of cholera.

Another pathogen infecting the airways, Bordetella pertussis, produces a toxin that catalyzes ADP ribosylation of the G_⍺i subunit of the inhibitory G protein. This modification prevents GDP from leaving the G_⍺i subunit, thereby keeping G_i protein in the inactive state. G_i protein cannot bind and inhibit adenylyl cyclase activity. Thus, cAMP released in the airway epithelial cells leads to loss of fluids and electrolytes along with enhanced mucus secretion, causing whooping cough.