A receptor is a protein that binds to a hormone or other molecule to trigger a cellular response.A receptor is a protein, usually embedded in a cell's outer membrane, that recognises and binds a specific signalling molecule and then passes the message into the cell. The molecule it binds — a hormone, neurotransmitter, growth factor, or drug — is called a ligand. Binding is highly selective, like a lock and key, so a cell responds only to signals it is meant to hear. When the ligand docks, the receptor changes shape and sets off a chain of events inside the cell: opening an ion channel, activating an enzyme, or recruiting other proteins that relay the signal onward. Receptors are what make cells responsive to their surroundings. Without them, hormones circulating in the blood would have no effect, because a cell can only act on a signal it can detect. They also explain how the same molecule can produce different effects in different tissues: the response depends on which receptors a cell carries, not just on the signal itself.
The concept of a specific receptive substance on cells was proposed by John Newport Langley in 1905 and developed into a quantitative theory of drug action by A. J. Clark in the 1920s and 1930s. The first receptor protein sequences were determined in the 1980s.
The ligand fits into a pocket on the receptor called the binding site, held there by weak interactions such as hydrogen bonds and shape complementarity. This binding stabilises a different shape of the receptor protein. In one common class, the G-protein-coupled receptors, the shape change lets the receptor activate an associated G-protein inside the cell, which then switches on enzymes that produce small messenger molecules. In another class, the receptor itself is an enzyme, often a kinase, that adds phosphate groups to target proteins once two receptors pair up. In ion-channel receptors, the shape change simply opens a pore, letting charged particles flow and altering the cell's electrical state.
Receptors are the targets of a large fraction of prescription medicines — beta-blockers, antihistamines, and many psychiatric drugs work by blocking or mimicking the natural ligand at a receptor.
The human genome encodes roughly 800 G-protein-coupled receptors, making them the largest family of receptors and the target of about a third of all approved drugs.