Ligand* (1st messenger)
binds to the receptor.
receptor changes shape and
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.
(e.g., an enzyme)
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
Second messengers activate
other enzymes or ion channels.
Cyclic AMP typically activates
protein kinase enzymes.
Kinase enzymes activate other
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.
G proteins act as middlemen or relays between
extracellular ﬁrst messengers and intracellular second messengers
that cause responses within the cell.