active site
Think of the contoured pocket inside a baseball glove — shaped so precisely that the ball nestles in snugly, held just where the hand can grip and throw it. An enzyme has a similar specially shaped pocket, and only the right molecule fits. That pocket, where the actual chemistry happens, is the active site.
More precisely, the active site is the small region of an enzyme (or other catalyst) where the reactant binds and the reaction is catalysed. It is a precisely shaped, chemically tailored crevice — formed by particular parts of the protein folded together — that holds the substrate in just the right orientation, strains its bonds, and lowers the activation energy. Its shape and the chemical groups lining it are what give an enzyme its remarkable specificity: typically only one substrate, or a close family of them, can bind and react.
Why it matters: the active site is where an enzyme's power and selectivity live, so understanding it is central to biochemistry and to designing drugs, many of which work by plugging into an active site and blocking it. The honest caveat is that the old 'rigid lock and key' image is only an approximation: real active sites often flex and mould themselves around the substrate as it arrives, a refinement called induced fit, and the same idea of a special reactive patch also applies to surfaces in solid catalysis.
Lysozyme, an enzyme in tears and saliva, has a long groove on its surface shaped to cradle a strand of the sugar that makes up bacterial cell walls. The strand slots in, the groove bends one of its bonds until it snaps, and the wall is cut — all inside that one tailored pocket.
The tailored pocket where the substrate binds and the bond is broken.
Active site (where chemistry happens) is not the same as a binding site in general; an enzyme may have separate regulatory sites elsewhere. Drugs that block the active site are called inhibitors.