enzyme

An enzyme is a protein that acts as a biological machine for speeding up a chemical reaction. Left alone, many reactions in the body would take years; an enzyme can complete them in a fraction of a second by holding the reacting molecules in just the right position. Crucially, the enzyme itself is not used up, so it can do the same job over and over.

Each enzyme has a specially shaped region called the active site where its substrate binds and the chemistry happens. By lowering the energy barrier of the reaction, the enzyme converts substrate into product without permanently changing itself. The exquisite fit between active site and substrate is what gives enzymes their specificity, often acting on only one molecule or one type of bond.

Enzymes are a favorite class of drug target because blocking one can shut down a disease-driving process. A drug that fits the active site and stops the substrate from binding is an inhibitor, and much of medicinal chemistry is the art of designing tight, selective inhibitors. The honest caveat is that the human body runs thousands of enzymes, so a useful drug must hit the intended one without crippling the others.