GPCR

A GPCR is a receptor that threads back and forth through the cell membrane seven times, like a ribbon stitched through fabric. Its outer face catches a signal from outside the cell, and its inner face passes that signal to a partner protein waiting inside. This shared seven-pass architecture defines the largest and most heavily drugged receptor family in the human body.

When a messenger binds the outer pocket of a GPCR, the protein shifts shape just enough for its intracellular loops to grab and activate a G protein. The activated G protein then splits and triggers internal cascades that change levels of small messengers, ion flow, or enzyme activity. GPCRs detect an enormous range of inputs, including light, odors, tastes, hormones, and neurotransmitters.

Roughly a third of all approved small-molecule drugs act on GPCRs, which is why understanding them is central to medicinal chemistry. Modern work goes beyond simple on or off switches: some drugs bind sites away from the natural pocket, and some preferentially activate one downstream pathway over another, an idea called biased signaling that may separate benefit from side effect.