The prevention of an enzymatic reaction due to interaction of an enzyme with a substance that decreases the rate of catalysis by the enzyme. The compound causing the effect is termed an inhibitor. Enzyme inhibitors are important regulators in the normal control of enzymatic reactions in living organisms, are useful in the study of enzyme mechanisms and cellular reactions, and are important chemotherapeutic agents. Inhibitors may have chemical structures that are similar to one of the substrates of the enzyme but may also be quite different.