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A competitive inhibitor is an enzyme inhibitor that produces competitive inhibition relative to a specified substrate. In the usual single-substrate kinetic description, it reduces the low-substrate response while leaving the limiting rate unchanged. A common molecular model has the inhibitor bind free enzyme and prevent formation of the enzyme-substrate complex; the kinetic classification alone does not prove a particular binding location.^[ref]

For the simple reversible model at fixed free inhibitor concentration $[I]$,Here $K_i$ is the inhibition constant for this model and $K_M$ the uninhibited Michaelis constant. Take illustrative values $V_{\max }=12\mu \mathrm{M}\mathrm{mi}{\mathrm{n}}^{-1}$, $K_M=2\mu \mathrm{M}$, $K_i=5\mu \mathrm{M}$ and $[I]=10\mu \mathrm{M}$. Then $\alpha =3$ and the apparent Michaelis constant is6micromolar. At $[S]=2\mu \mathrm{M}$, the inhibited rate is3micromolar per minute, compared with6without inhibitor.

As substrate concentration grows, both rates approach the same limiting value in this model. That is a limit, not an assertion that a particular finite substrate addition removes all inhibition. Characterizing a candidate therefore requires rates over suitable substrate and inhibitor ranges with controlled enzyme amount; one reduced rate does not establish that the inhibitor is competitive.^[ref]

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.^[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.

An enzyme-substrate complex is the bound association of an enzyme with a substrate before or during catalysis, commonly written $ES$. In the scheme $E+SightleftharpoonsESoE+P$, binding, unbinding and product formation are distinct steps. The amount of $ES$ need not increase indefinitely with substrate concentration because the supply of enzyme is finite. A bound complex also need not be the transition state or proceed to product on every encounter. ^[ref]

An inhibition constant sets an inhibitor concentration scale in an enzyme kinetics model. In the rapid-binding construction under reversible enzyme inhibition, $K_i=[E]I/[EI]$ and $K_i^{\prime }=[ES]I/[ESI]$ are dissociation constants. Smaller values favour the corresponding bound population at fixed free inhibitor concentration. More generally, fitted inhibition constants need not be true binding dissociation constants; a mechanism must justify that interpretation.^[ref]

For example, choosing $I=K_i$ makes $[EI]=[E]$ in this model, but does not imply that half of all enzyme is $EI$: $ES$ and $ESI$ also contribute to the total population.

Institutional nomenclature committee credited with Symbolism and Terminology in Enzyme Kinetics (Recommendation 1981) by Nomenclature Committee of the International Union of Biochemistry. This entry identifies an institutional body.