490
UNIT THREE
continuing signal from hormone molecules binding receptors
in the target cell membrane.
Hormones whose actions require cyclic AMP include
releasing hormones from the hypothalamus; thyroid-stimu-
lating hormone (TSH), adrenocorticotropic hormone (ACTH),
glycogen, increasing the number of glucose molecules that
can diffuse out of liver cells and enter the bloodstream.
Another enzyme, phosphodiesterase, quickly and con-
tinuously inactivates cAMP, so its action is short-lived. For
this reason, a continuing response in a target cell requires a
FIGURE 13.7
Nonsteroid hormone action. (
1
) Body f
uids carry nonsteroid hormone molecules to the target cell, where (
2
) they bind receptor
molecules on the cell membrane. (
3
) This activates molecules oF adenylate cyclase, which (
4
) catalyze conversion oF ATP into cyclic adenosine
monophosphate (cAMP). (
5
) The cAMP promotes a series oF reactions leading to the cellular changes associated with the hormone’s action.
C
C
C
C
C
O
O
OP
P
P
HO
O
H
H
O
HH
OH
(a)
ATP
OH
H
H
C
C
C
C
C
O
O
O
P
O
HH
(b)
Cyclic AMP
OH
H
H
H
H
O
O
O
O
O
O
C
NH
2
NH
2
N
N
N
N
HC
CH
C
C
C
N
N
N
HC
CH
C
C
Adenylate cyclase
N
Membrane-bound
receptor molecule
Cell membrane
Nonsteroid
hormone
molecule
G protein
Adenylate
cyclase
Protein
kinases
(inactive)
Protein
kinases
(active)
Substrate
(inactive)
Substrate
(active)
Cellular
changes
Cytoplasm
Nucleus
Hormone-
receptor
complex
ATP
cAMP
1
2
3
5
4
FIGURE 13.6
Adenylate cyclase catalyzes conversion oF (
a
) ATP molecules into cyclic AMP (
b
). The atoms Forming the new bond are shown in red.
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