488
UNIT THREE
and other substances. A specific example is the action of
epinephrine to raise blood sugar during periods of physical
stress. Epinephrine acts through the second messenger cAMP
to increase the activity of the enzyme that breaks down liver
G protein,
which then activates an enzyme called
adenylate
cyclase
(ah-den
ı˘-la
¯t si
kla
¯s), an integral membrane protein
with its active site facing the inside of the cell. The activated
enzyme removes two phosphates from ATP and circular-
izes it, forming
cyclic AMP
(f g. 13.6)
. Cyclic AMP, in turn,
activates another set of enzymes called
protein kinases
(ki
na¯s-ez). Protein kinases transfer phosphate groups from
ATP molecules to protein substrate molecules. This phos-
phorylation alters the shapes of the substrate molecules, in
some cases activating inactive forms.
The activated proteins then alter various cellular pro-
cesses, bringing about the effect of that particular hormone
(f
g. 13.7)
. The response of a particular cell to such a hor-
mone is determined by the type of membrane receptors pres-
ent and by the types of protein substrate molecules in the
cell.
Table 13.5
summarizes these actions.
Cellular responses to second messenger activation
include altering membrane permeabilities, activating
enzymes, promoting synthesis of certain proteins, stimulat-
ing or inhibiting specific metabolic pathways, promoting
cellular movements, and initiating secretion of hormones
Hormone molecule
Cell
membrane
Ribosome
mRNA
mRNA
Newly forming
protein molecule
Nucleus
Intracellular
receptor molecule
Hormone-receptor
complex
DNA
1
2
3
5
4
FIGURE 13.5
Steroid hormones. (
1
) A steroid hormone crosses a cell membrane and (
2
) combines with a protein receptor, usually in the nucleus.
(
3
) The hormone-receptor complex activates transcription of speciF
c messenger RNA (mRNA) molecules from DNA. (
4
) The mRNA molecules leave
the nucleus and enter the cytoplasm (
5
) where they guide synthesis of their encoded proteins. In the bloodstream, most molecules of a particular
steroid are bound to proteins. Only the few that are not bound are free to enter cells, as shown here.
TABLE
13.5
|
Sequence of Actions of
Nonsteroid Hormone Using
Cyclic AMP
1. Endocrine gland secretes nonsteroid hormone.
2. Body ±
uid carries hormone to its target cell.
3. Hormone combines with receptor site on membrane of its target cell,
activating G protein.
4. Adenylate cyclase molecules are activated in target cell’s membrane.
5. Adenylate cyclase circularizes ATP into cyclic AMP.
6. Cyclic AMP activates protein kinases.
7. Protein kinases activate protein substrates in the cell that change
metabolic processes.
8. Cellular changes produce the hormone’s e²
ects.
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