competitive inhibition
Summary
A competitive inhibitor resembles the substrate and usually binds the enzyme's active site reversibly, so it competes with substrate. It raises the apparent Km but leaves Vmax unchanged, because enough substrate can outcompete the inhibitor.
Detail
Meaning: A competitive inhibitor usually resembles the substrate and binds reversibly to the free enzyme, typically at the active site, forming an EI complex that makes no product. Substrate and inhibitor compete for the same free enzyme, so raising [S] shifts binding toward ES and overcomes the inhibition. Vmax can still be reached, but more substrate is needed to reach half of it, so the apparent Km rises.
Equation: v = Vmax[S] / (αKm + [S]), with α = 1 + [I]/Ki. Here v is the reaction rate and Vmax the maximum rate (mol L⁻¹ s⁻¹); [S] and [I] are substrate and inhibitor concentrations, Km is the Michaelis constant, and Ki is the inhibitor dissociation constant (all in mol/L); α is dimensionless.
On the MCAT: Passages give Michaelis–Menten curves or Lineweaver–Burk plots (1/v versus 1/[S]) with and without an inhibitor. Competitive inhibition gives the same plateau and the same y-intercept (1/Vmax), but a steeper slope and an x-intercept (−1/Km) closer to zero. Expect questions on substrate analogs and on whether adding substrate restores activity.
Common confusion: Students mix this up with noncompetitive inhibition, which binds away from the active site and, in the pure case, lowers Vmax while Km stays the same. For competitive inhibition, Km rises and Vmax is unchanged.
Sources
- Lehninger Principles of Biochemistry
- Biochemistry (Berg, Tymoczko, Stryer)
- Campbell Biology
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.