There are many endogenous messengers (hormones)
with their own specific receptors. First
the hormone binds to the receptor (formation
of a hormone–receptor complex). The intracellular
transmission of signals is mainly carried
out by special guanine-nucleotide-binding
proteins, or G proteins. By binding to guanosine
triphosphate (GTP, a nucleotide composed of
guanine, a sugar, and three phosphate groups),
the G protein becomes activated and initiates
further reactions. G proteins consist of three
subunits: !, ", and #. The ! subunit (stimulatory
G protein, Gs) binds to the effector protein. Immediately
thereafter, G! is inactivated (GTPase)
by hydrolysis of GTP to GDP (guanosine diphosphate).
This transforms the G protein back
into an inactive form (Gi).
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