Regulation and Integration of the Body
one of these second messengers,
cyclic AMP (cAMP)
, which is
used by neurotransmitters (Chapter 11) and olfactory receptors
The Cyclic AMP Signaling Mechanism
As you recall, this
mechanism involves the interaction of three plasma membrane
components—a hormone receptor, a G protein, and an eﬀector
enzyme (adenylate cyclase)—to determine intracellular levels of
illustrates these steps:
Hormone binds receptor.
Te hormone, acting as the
, binds to its receptor in the plasma membrane.
Receptor activates G protein.
Hormone binding causes the
receptor to change shape, allowing it to bind a nearby inac-
. Te G protein is activated as the guanosine
diphosphate (GDP) bound to it is displaced by the high-
). Te G
protein behaves like a light switch: It is “oﬀ” when GDP is
bound to it, and “on” when G±P is bound.
G protein activates adenylate cyclase.
Te activated G pro-
tein (moving along the membrane) binds to the eﬀector
. Some G proteins (G
adenylate cyclase (as shown in Figure 16.2), but others
adenylate cyclase. Eventually, the G±P bound
to the G protein is hydrolyzed to GDP and the G protein
becomes inactive once again. (Te G protein cleaves the
terminal phosphate group oﬀ G±P in much the same way
that A±Pase enzymes hydrolyze A±P.)
Adenylate cyclase converts ATP to cyclic AMP.
For as long
as activated G
is bound to it, adenylate cyclase generates
cAMP from A±P.
Cyclic AMP activates protein kinases.
cAMP, which is free
to diﬀuse throughout the cell, triggers a cascade of chemi-
cal reactions by activating protein kinases.
are enzymes that
(add a phosphate group to)
various proteins, many of which are other enzymes. Be-
cause phosphorylation activates some of these proteins and
inhibits others, it may aﬀect a variety of processes in the
same target cell at the same time.
Tis type of intracellular enzymatic cascade has a huge am-
pliﬁcation eﬀect. Each activated adenylate cyclase generates
large numbers of cAMP molecules, and a single kinase enzyme
ATP to cAMP
G protein (G
Triggers responses of
target cell (activates
opens ion channel, etc.)
Recall from Chapter 3 that
G protein signaling mechanisms
are like a molecular relay race.
Cyclic AMP second-messenger mechanism of water-soluble hormones.