G protein cycle to activate adenylate cyclase

When a hormone binds to its specific receptor, a
structural change occurs (1). This activates the
! subunit of the G protein, which separates
from the " and # subunits (2). The stimulatory G
protein (Gs-!) binds to the effector protein, usually
adenylate cyclase, and activates it (3). cAMP
is then formed from ATP, while GTP is hydrolyzed
to GDP at the G-! subunit. This inactivates
the effector protein and the formation of cAMP
is terminated. Thus, the signal is of very short
duration, and the initial conditions are rapidly
restored. Several toxins exert their activity by
interrupting this cycle. For example, cholera
toxin inhibits inactivation of the Gs-! protein so
that adenylate cyclase remains activated and
large amounts of sodium and water are lost
through the intestinal mucous membranes.