Enzymes catalyse the reactions of their substrates by initial formation of a complex (ES) between the enzyme and substrate(S) at the active site of the enzyme. This complex then breaks down, either directly of through intermediary stages, to give the products (P) of the reaction with regeneration of the enzyme:



kcat is the overall rate constant for decomposition of ES into products, k2 and k3 are the respective rate constants for formation and breakdown of the intermediate E' (i.e. kcat= k2k3/(k2+k3)). Chemical agents, known as inhibitors, modify the ability of an enzyme to catalyse the reaction of its substrates. The term inhibitor is usually restricted to chemical agents, other modifiers of enzyme activity such as pH, ultra-violet light and heat being known as denaturising agents.