receptor

A receptor is a protein that acts like a lock waiting for a specific key. When the right signaling molecule, such as a hormone or neurotransmitter, slots into it, the receptor changes shape and passes a message into or across the cell. The cell does not respond to the molecule floating around outside; it responds because the receptor was triggered.

Receptors recognize their natural messengers with great precision through a binding pocket whose shape and chemistry match the messenger. Activation typically alters the receptor's three-dimensional conformation, which then turns on a downstream chain of events: opening an ion channel, switching on an internal protein, or changing which genes a cell reads. Because so many physiological signals run through receptors, they are among the most important targets in drug discovery.

Drugs that bind receptors fall into broad behavioral classes. An agonist mimics the natural messenger and switches the receptor on, while an antagonist blocks the pocket and prevents activation without triggering it itself. It is worth remembering that a single protein labeled a receptor may have several subtypes spread across different tissues, so a drug's effect depends on which subtype it engages and where.