active site
The active site is the small region of an enzyme where the real work happens, the spot where the substrate binds and is chemically transformed. If the enzyme is a machine, the active site is the workbench: a precisely shaped cradle that holds the substrate exactly where the reaction needs it. It is usually a pocket or cleft folded into the surface of the protein.
The active site does two related jobs. First, it recognizes and binds the correct substrate through a complementary arrangement of shape and chemical groups. Second, it carries out catalysis, using key amino acids to make and break bonds far faster than would happen in free solution. Both functions depend on the precise three-dimensional layout of the residues lining the pocket.
Because the active site is both essential and well-defined, it is a classic place to aim a drug. A molecule that occupies the active site can block the substrate, stopping the enzyme from doing its job. The caveat is that closely related enzymes can share similar active sites, so a drug aimed there must be shaped carefully to avoid hitting the wrong enzyme.
Many antiviral drugs sit in the active site of a viral enzyme, physically blocking the natural substrate so the enzyme can no longer process it.
Occupying the active site is the most direct way to stop an enzyme.
A drug that competes with the substrate for the active site is a competitive inhibitor; one that binds an allosteric site instead works by a different mechanism.