G-Protein-Coupled Receptors and Second Messengers
A bound ligand changes the receptor shape so that it activates a G protein, which switches on an enzyme that floods the cytosol with a small messenger. One bound molecule outside becomes thousands of messenger molecules inside.
What a learner can do afterwards
- Traces the path from ligand to second messenger to a changed enzyme activity.
- Explains where the amplification comes from and roughly at which steps.
- Says what turns the pathway off, and why that has to be an active step.
1 · Read
A bound ligand changes the receptor shape, and that shape lets a G protein swap GDP for GTP and split. The freed piece switches on an enzyme, and the enzyme floods the cytosol with small second messengers. Those messengers change enzyme activity deeper in the cell, as when epinephrine makes liver cells release glucose.
The flood is amplification, and it happens at every step. One receptor activates several G proteins, each switched-on enzyme makes many messenger molecules, and each messenger can act on many targets. That is how one bound molecule outside becomes thousands of changed molecules inside.
Shutoff is active, because a lingering signal would keep the cell switched on. The G protein hydrolyzes its GTP back to GDP and reassembles, while enzymes clear or degrade the second messengers. Both halves of the cleanup must work for the pathway to go quiet.
The same hormone can mean different things to different cells. Epinephrine on a liver cell releases glucose, while on a heart cell it quickens the beat. Same signal, different inner wiring, so learn the path and the wiring together.
One ligand reshapes a receptor, the relay multiplies the message, and active shutoff ends it.
2 · Watch
Take it off screen
Where it sits
This opens up
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.