Competitive inhibitor

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]