receptor

If a ligand is a key, a receptor is the lock — and like a real lock, it does much more than just sit there. A receptor is a protein, usually with a pocket shaped to fit one particular signaling molecule. When that molecule slots in, the receptor does not merely hold it; it physically changes shape, and that shape change is the moment the outside message becomes inside news. The lock, in other words, is wired to ring a bell deep inside the house.

A receptor is a protein that recognizes a specific ligand and, on binding it, undergoes a conformational change that initiates a response inside the cell. Receptors come in two broad families. Cell-surface receptors sit in the plasma membrane with part sticking out to catch ligands that cannot cross the membrane; binding flips a switch on the inner side without the ligand ever entering. Intracellular receptors float inside the cell and are used by ligands small or oily enough to slip through the membrane, such as steroid hormones. Either way, the defining feature is selectivity: a given receptor binds essentially one kind of ligand, which is why cells can pick one message out of the chemical noise around them.

Receptors explain why the same blood, bathing every cell, produces wildly different effects in different tissues: a cell responds only to signals for which it owns the receptor. They are also among medicine's most important targets — a huge fraction of all drugs work by binding receptors. A common misconception is that the receptor is passive, just a docking spot; in truth the receptor is the active translator that converts a binding event into a cascade of chemistry. Lose or mutate a receptor and a cell can go 'deaf' to a signal even when the signal is loud and clear.