= Enzyme inhibitor
{wiki}
An <enzyme inhibitor> lowers an <enzyme>'s <reaction rate>. In <reversible enzyme inhibition>, reducing free inhibitor <concentration> can restore activity; slow release can make this difficult to distinguish experimentally from irreversible inactivation. <Competitive inhibition> and <mixed inhibition> describe different changes in <enzyme kinetics>; <pure noncompetitive inhibition> is a special case of <mixed inhibition>, not a name for every inhibitor that binds away from an <active site>.<Symbolism and Terminology in Enzyme Kinetics (Recommendation 1981) by Nomenclature Committee of the International Union of Biochemistry>[Symbolism and Terminology in Enzyme Kinetics, recommendation 1981, sections 6.1 and 6.4: reversibility and the ambiguity of non-competitive]{ref}
The useful question is how adding <substrate> changes the inhibition. The model below connects binding to a measurable <reaction rate>, predicts when more <substrate> can overcome inhibition, and supplies examples for comparison with an <enzyme inhibition initial-rate assay>. Its <inhibition constants> are explicitly defined; neither a binding cartoon nor one reduced <reaction rate> by itself determines a mechanism.
\Image[/enzyme-inhibition-binding-scheme.png]
{title=<Reversible enzyme inhibition>: which bound states can form?}
{description=Original branching scheme for a single <substrate>. $E$ is free <enzyme>, $S$ free <substrate>, $I$ free inhibitor, $ES$ the productive <enzyme-substrate complex>, and $EI$ and $ESI$ inactive complexes. Reversible vertical arrows show inhibitor binding and release. In <competitive inhibition> the right-hand inhibitor-binding branch is absent. <Pure noncompetitive inhibition> retains both branches with equal <inhibition constants>; general <mixed inhibition> permits unequal constants. The drawing specifies populations and allowed steps, not an atomic binding-site geometry.}
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