= Competitive inhibitor
A \b[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.<Symbolism and Terminology in Enzyme Kinetics (Recommendation 1981) by Nomenclature Committee of the International Union of Biochemistry>[§§6.1–6.4: reversible inhibition, competitive classification and mechanistic interpretation]{ref}
For the simple reversible model at fixed free inhibitor concentration $[I]$,
$$
v=\frac{V_{\max}[S]}{\alpha K_M+[S]},\qquad \alpha=1+\frac{[I]}{K_i}.
$$
Here $K_i$ is the <inhibition constant> for this model and $K_M$ the uninhibited <Michaelis constant>. Take illustrative values $V_{\max}=12\,\mathrm{\mu M\,min^{-1}}$, $K_M=2\,\mathrm{\mu M}$, $K_i=5\,\mathrm{\mu M}$ and $[I]=10\,\mathrm{\mu M}$. Then $\alpha=3$ and the apparent Michaelis constant is6micromolar. At $[S]=2\,\mathrm{\mu M}$, 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.<Symbolism and Terminology in Enzyme Kinetics (Recommendation 1981) by Nomenclature Committee of the International Union of Biochemistry>[§4.2, Michaelis-Menten parameters; §6.4, operational inhibition classes]{ref}
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