Ligand* (1st messenger)
binds to the receptor.
The
receptor changes shape and
activates.
The activated receptor binds to a G protein
and activates it.
The G protein changes shape
(turns “on”), causing it to release GDP and
bind GTP (an energy source).
Activated G protein
activates (or inactivates) an
effector protein by causing
its shape to change.
The sequence described here is like a molecular
relay race. Instead of a baton passed from runner
to runner, the message (a shape change) is
passed from molecule to molecule as it makes its
way across the cell membrane from outside to
inside the cell.
Extracellular fluid
Intracellular fluid
GTP
GTP
GDP
Inactive 2nd
messenger
Active 2nd
messenger
Activated
kinase
enzymes
Ligand (1st
messenger)
Receptor
Effector protein
(e.g., an enzyme)
Receptor
G protein
Ligand
G protein
Enzyme
2nd
messenger
GTP
Cascade of cellular responses
(The amplification effect is
tremendous. Each enzyme
catalyzes hundreds of reactions.)
Activated effector enzymes
catalyze reactions that produce
2nd messengers in the cell.
(Common 2nd messengers include
cyclic AMP and Ca
2+
.)
Second messengers activate
other enzymes or ion channels.
Cyclic AMP typically activates
protein kinase enzymes.
Kinase enzymes activate other
enzymes.
Kinase enzymes transfer
phosphate groups from ATP to
specific proteins and activate a
series of other enzymes that
trigger various metabolic and
structural changes in the cell.
6
5
4
3
2
1
Ligands include
hormones and
neurotransmitters.
*
Figure 3.16
G proteins act as middlemen or relays between
extracellular first messengers and intracellular second messengers
that cause responses within the cell.
FOCUS
G Proteins
82
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