Catalysts are substances that speed chemical reactions without being used up in those reactions. In cells, catalysts are usually large globular proteins known as enzymes. There is a portion of an enzyme, its active site, that binds only to a particular molecule, its substrate. The substrate is a participant in the reaction catalyzed by the enzyme. The binding of substrate to enzyme causes a change in the shape of the enzyme which in turn facilitates the forming or breaking of bonds by the substrate. The product of the reaction is released and the enzyme is available once again. Because of the fit between substrate and active site, each enzyme is specific for a particular reaction.