Most chemical reactions in cells require enzymes—biological catalysts that speed up the reaction without being consumed or permanently changed. They reduce the activation energy needed to convert the reactants into products. Enzymes are proteins, that usually work by binding to a substrate—a reactant molecule that they act upon.

Enzymes exhibit substrate specificity, meaning that they can only bind to certain substrates. This is mainly determined by the shape and chemical characteristics of their active site—the region of the enzyme that binds to the substrate.

According to the induced-fit model of enzyme activity, this binding changes the conformation—or shape—of the enzyme. This brings the substrate closer to the higher energy transition state needed for the reaction to occur, for instance, by weakening its bonds so that it can more readily react. Enzymes may also speed up a reaction by creating conditions within the active site that are more conducive for the reaction to occur than the surrounding cellular environment.

Once the products of the reaction are formed, they are released from the active site of the enzyme, and the enzyme can catalyze reactions once again.