Competitive Inhibition Calculators

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Competitive inhibition occurs when an inhibitor molecule binds reversibly to the active site of an enzyme, directly competing with the substrate for the same binding site. The inhibitor has structural similarity to the substrate and mimics substrate binding without undergoing catalysis. Competitive inhibitors increase the apparent Km (more substrate is needed to achieve half-Vmax) but do not affect Vmax (which can still be reached at sufficiently high substrate concentrations). The effect is quantified by the inhibition constant Ki, and this type of inhibition is the basis for many important drugs including statins, ACE inhibitors, and methotrexate.

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Effect on Kinetic Parameters

With a competitive inhibitor [I] present:

Apparent Km = Km × (1 + [I]/Ki)

Vmax = unchanged

α = 1 + [I]/Ki (the factor by which Km increases). At infinite [S], all enzyme molecules have substrate bound → Vmax is achievable. The inhibitor competes — excess substrate wins.

Lineweaver-Burk Pattern

On a double reciprocal (1/v vs. 1/[S]) plot: same y-intercept (1/Vmax unchanged); different slopes (Km/Vmax increases); lines with different [I] intersect on the y-axis. This graphical pattern is the diagnostic for competitive inhibition.

Mechanism

E + I ⇌ EI (dead-end complex, cannot proceed to product). E + S ⇌ ES → E + P (normal catalysis). [EI] increases → fewer free E molecules available for substrate → apparent Km rises (more substrate needed to compete inhibitor off). The inhibitor does not alter kcat — when ES forms, the reaction proceeds normally.

Pharmacological Examples

  • Statins (atorvastatin, simvastatin): competitively inhibit HMG-CoA reductase → reduce cholesterol synthesis
  • Methotrexate: competitively inhibits dihydrofolate reductase (DHFR) → blocks folate metabolism in cancer cells
  • ACE inhibitors (lisinopril): competitively inhibit angiotensin-converting enzyme → reduce blood pressure
  • Sildenafil (Viagra): competitively inhibits PDE5 → allows cGMP to accumulate → smooth muscle relaxation

Glossary

Competitive Inhibition
Enzyme inhibition where the inhibitor binds the active site in competition with substrate; increases apparent Km = Km(1+[I]/Ki); Vmax unchanged; overcome by excess substrate.
Ki (Inhibition Constant)
Dissociation constant for the enzyme-inhibitor complex: Ki = [E][I]/[EI]; lower Ki = tighter binding = more potent inhibitor; determined by kinetic analysis at multiple [S] and [I].
Apparent Km
The observed Michaelis constant in the presence of a competitive inhibitor: Km_app = Km × (1+[I]/Ki); increases with inhibitor concentration; returns to true Km at zero inhibitor.

Frequently Asked Questions

Competitive inhibition: an inhibitor binds reversibly to the enzyme active site, competing with substrate. Effects on kinetic parameters: Apparent Km increases = Km × (1 + [I]/Ki) — more substrate is needed to achieve half-Vmax because the inhibitor occupies the active site. Vmax is unchanged — at saturating [S], substrate outcompetes the inhibitor and Vmax is reached. A hallmark: competitive inhibition is overcome by adding more substrate. This distinguishes it from noncompetitive inhibition (where Vmax decreases regardless of [S]).

On a Lineweaver-Burk (1/v vs. 1/[S]) plot: Control line: y-intercept = 1/Vmax; x-intercept = −1/Km; slope = Km/Vmax. With competitive inhibitor: y-intercept is the SAME (1/Vmax unchanged); x-intercept moves closer to zero (apparent Km increases → −1/apparent Km is less negative); slope increases. All lines with different [I] concentrations intersect at the same y-intercept point. This shared y-intercept is the diagnostic pattern for competitive inhibition.

Ki is the dissociation constant for the enzyme-inhibitor complex: Ki = [E][I]/[EI]. Lower Ki = tighter inhibitor binding = more potent inhibition. Ki is determined by measuring velocity at multiple [S] values for multiple [I] concentrations, then fitting to the competitive inhibition equation: v = Vmax × [S] / (Km × (1 + [I]/Ki) + [S]). Graphically: from Lineweaver-Burk, plot slopes vs. [I]; slope = x-intercept at −Ki. Dixon plot (1/v vs. [I] at different [S] values): lines intersect at −Ki on x-axis. Modern approach: nonlinear regression directly fitting competitive inhibition model.

Yes — this is a defining characteristic of competitive inhibition. Because the inhibitor and substrate compete for the same site, increasing substrate concentration increases the probability that substrate (not inhibitor) occupies the active site. At saturating [S], essentially all enzyme molecules have substrate bound → Vmax is achievable regardless of [I]. This is why increasing substrate can 'reverse' competitive inhibition. In contrast: noncompetitive inhibition binds a site separate from the active site → cannot be overcome by adding substrate → Vmax decreases. This distinction has pharmacological significance: for competitive inhibitor drugs, high local substrate concentrations may reduce efficacy.