Cell-surface receptors, as their name implies, bind hormones on the
extracellular side of the cell membrane. When the hormone binds to its
receptor on the cell surface, the hormone-bound receptor either activates or
forms a complex with a transducer protein in the membrane that will
cause the activation of some enzymatic activity in the cytoplasm of the cell.
The enzymatic activity, sometimes called the effector, catalyzes the produc-
tion of a second messenger that mediates the intracellular response.
Most transducer proteins that interact with cell-surface receptors are GTP-
binding proteins, commonly called G proteins. G proteins typically are
composed of three subunits: a, b, and g. When the receptor is in the unbound
state, all three subunits are bound together to form a heterotrimer that is in
close association with the membrane-bound receptor. In this state, GDP is
bound to the a-subunit (Figure 7-2). Binding of the hormone by the receptor
causes a conformational change such that the a-subunit is able to exchange the
bound GDP for a molecule of GTP. The GTP-bound a-subunit separates from
the Py-dimer and is able to interact with the effector enzyme in either a stim-
ulatory or inhibitory manner, depending on the nature of the a-subunit. Some
Figure 7-2. Transduction by heterotrimeric G proteins.