binding site

A binding site is the exact spot on a target where a drug latches on — the keyhole within the lock. It is a pocket or groove on the surface of a receptor, enzyme or other protein, shaped and chemically lined so that a particular molecule fits snugly, held by a constellation of weak forces such as hydrogen bonds and hydrophobic contacts.

The fit between a drug and its binding site is highly specific: the better the shape and chemistry match, the tighter and more selective the binding. Many proteins have more than one binding site. The orthosteric site is where the natural ligand normally binds, while allosteric sites are separate pockets where a molecule can bind to nudge the protein's behaviour without occupying the main site.

Knowing the precise structure of a binding site lets chemists design drugs that fit it like a tailored glove, the foundation of modern structure-based drug discovery. A practical caveat is that proteins are flexible, not rigid — a binding site can change shape as a drug enters (induced fit), so the static picture from a single crystal structure can be misleading.