the active site

An enzyme is a protein that speeds up a chemical reaction, but it does not do so all over its surface. The actual work happens in one small, special place — a pocket or cleft on the folded protein where the reaction's ingredients are held and transformed. That working pocket is the active site.

The active site is usually a cavity in the protein's surface, lined by a few specific side chains. Two things go on there. First, binding: the molecule the enzyme acts on, called the substrate, slips into the pocket and is gripped by a network of weak interactions — hydrogen bonds, charge attractions, hydrophobic contacts — that fit the substrate's shape and chemistry like a tailored glove. Second, catalysis: once the substrate is held just so, the side chains lining the pocket do chemistry on it — nudging electrons, donating or grabbing protons, straining a bond toward breaking — and turn it into product, which then leaves. The side chains that line an active site are often far apart in the sequence and only brought together by the fold, which is why the three-dimensional shape is everything.

Two features of the active site explain much of an enzyme's behavior. Its shape and chemistry give the enzyme its specificity — why a given enzyme acts on one substrate and ignores others. And its arrangement of catalytic side chains is what makes the reaction go faster, by stabilizing the awkward in-between state the reaction must pass through. Because so much rides on this little pocket, a single mutation that alters one active-site residue can cripple an enzyme entirely, even if the rest of the huge protein is untouched.