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Enzyme regulation refers to the processes by which cells control the activity and concentration of enzymes to match metabolic demands. Regulation occurs at two levels: activity regulation, which adjusts an enzyme's catalytic performance in real time, and abundance regulation, which changes the amount of enzyme present through synthesis and degradation.

Regulatory mechanisms include allosteric regulation, covalent modification, proteolytic activation, feedback inhibition, and control by localization or substrate availability. Allosteric regulation involves effectors binding to sites aside from the active site to up- or down-regulate activity; examples include ATP and fructose-2,6-bisphosphate modulating phosphofructokinase-1. Covalent modification, especially reversible phosphorylation, rapidly shifts activity in response to signals; glycogen phosphorylase is a classic case. Proteolytic activation converts inactive precursors into active enzymes, a common feature of digestive enzymes and coagulation factors. Feedback inhibition uses downstream products to slow earlier steps and prevent accumulation. Localization and compartmentalization place enzymes in environments with appropriate substrates or signals, while substrate availability links activity to metabolic flux. Regulation of enzyme abundance through gene expression and protein turnover enables longer-term adaptation, with hormonal signals often mediating these changes.

Examples of regulation occur in glycolysis, where enzyme activity is tuned to cellular energy status, and in the urea cycle, where CPS I is activated by N-acetylglutamate. Understanding enzyme regulation is fundamental to physiology and medicine.