G-protein-coupled receptor
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Think of a relay race where the runner who catches the message does not run all the way to the finish himself. Instead, the moment he is tapped, he flips a baton to a teammate (a courier) who then sprints off to deliver it. A G-protein-coupled receptor works just like that handoff: when a signal arrives, the receptor does not carry the message far itself — it activates a separate mobile helper, a G protein, that detaches and runs off to switch on the cell's machinery.
A G-protein-coupled receptor, or GPCR, is a cell-surface receptor whose protein chain weaves back and forth across the membrane seven times. On the inside it sits next to a G protein, a switch that grips a small molecule called GTP when on and GDP when off. When a ligand binds the outside of the GPCR, the receptor's shape change pokes the G protein, which swaps GDP for GTP, splits apart, and goes off to activate a target — often an enzyme that makes a second messenger, or an ion channel. The G protein then slowly switches itself off by chopping its own GTP, ending the message — a built-in timer that keeps signals from running forever.
GPCRs are arguably the single most important receptor family in your body and in medicine. They let you smell and taste, see light, feel adrenaline, and respond to a vast range of hormones and neurotransmitters; humans have hundreds of different GPCRs. Around a third of all approved drugs work by targeting them, from antihistamines to many heart and psychiatric medicines. A common misunderstanding is that 'G protein' means the receptor itself; the receptor and the G protein are two separate molecules — the receptor is the antenna, the G protein is the courier it dispatches.
The 'fight or flight' rush works through GPCRs: adrenaline binds a GPCR on a heart cell, the receptor activates a G protein, the G protein switches on an enzyme that makes cyclic AMP, and the cell beats faster. The same GPCR machinery also lets your eyes detect a single photon of light.
One GPCR family underlies smell, sight, taste, and the adrenaline rush.
The receptor and the G protein are two separate molecules — 'G-protein-coupled' means the receptor hands off to a G protein, not that it is one. The G protein turns itself off by hydrolyzing its own GTP, which times out the signal.