Noncompetitive Inhibition

Noncompetitive inhibition occurs when an inhibitor binds at a site distinct from the active site and reduces catalysis regardless of whether substrate is bound. In its classic form it lowers the maximal velocity Vmax while leaving the apparent Km unchanged, because adding more substrate cannot displace an inhibitor that does not compete for the active site.

Definition

Noncompetitive inhibition is a form of reversible inhibition in which the inhibitor binds with equal affinity to the free enzyme and the enzyme-substrate complex at a site distinct from the active site, lowering Vmax while leaving the apparent Km unchanged; mixed inhibition is the more general case in which the two affinities differ.

Scope

The entry covers the binding logic of noncompetitive and the broader mixed inhibition, their effects on Michaelis-Menten parameters, and how they are distinguished kinetically from competitive inhibition. It is a biochemical and methodological reference, not clinical guidance.

Core questions

• Does the inhibitor bind a site other than the active site?
• Is Vmax reduced while the apparent Km is unchanged (pure noncompetitive) or also altered (mixed)?
• Can excess substrate overcome the inhibition?

Key concepts

• Binding at a site distinct from the active site
• Reduced Vmax, unchanged apparent Km (pure noncompetitive)
• Mixed inhibition as the general case
• Non-surmountability by excess substrate
• Lineweaver-Burk lines intersecting on the 1/[S] axis

Key theories

Non-mutually-exclusive binding model

In pure noncompetitive inhibition the inhibitor binds free enzyme and enzyme-substrate complex with the same affinity, so substrate and inhibitor binding are independent; the steady-state treatment predicts an unchanged Km and a Vmax reduced by the factor 1/(1 + [I]/Ki). Mixed inhibition generalises this to unequal affinities.

Mechanisms

A noncompetitive inhibitor binds a site other than the active site and can bind both the free enzyme and the enzyme-substrate complex. In the pure case the two binding events are independent, so the inhibitor lowers the effective concentration of active enzyme: Vmax falls by the factor 1/(1 + [I]/Ki) while the apparent Km is unchanged, and the inhibition cannot be overcome by adding substrate (Cornish-Bowden, 1974; Cornish-Bowden, 2012). On a double-reciprocal plot the lines for different inhibitor concentrations intersect on the 1/[S] axis. Mixed inhibition is the general case in which the inhibitor binds the free enzyme and the complex with different affinities, altering both Km and Vmax. Binding at a regulatory site links noncompetitive behaviour to allosteric models of conformational change (Monod, 1965).

Clinical relevance

Inhibitors that act away from the active site can be highly selective and are not surmountable by accumulating substrate, a property relevant to how their effects are described pharmacologically (Copeland, 2013). This entry explains the mechanism for reference and education and does not provide dosing or treatment advice.

History

The kinetic separation of noncompetitive, uncompetitive and mixed inhibition, and convenient graphical methods for estimating their inhibition constants, were consolidated in the enzyme-kinetics literature, including Cornish-Bowden's 1974 replot method (Cornish-Bowden, 1974). The allosteric model of Monod, Wyman and Changeux supplied a structural rationale for inhibition at sites distinct from the active site (Monod, 1965).

Key figures

• Athel Cornish-Bowden
• Jacques Monod
• Jean-Pierre Changeux

Seminal works

• cornish-bowden-1974
• monod-1965

Frequently asked questions

How does noncompetitive inhibition differ from competitive inhibition?

A competitive inhibitor binds the active site and raises Km while leaving Vmax unchanged, and is overcome by excess substrate; a pure noncompetitive inhibitor binds elsewhere, lowers Vmax while leaving Km unchanged, and cannot be overcome by adding substrate.

What is mixed inhibition?

Mixed inhibition is the general case in which the inhibitor binds the free enzyme and the enzyme-substrate complex with different affinities, so both the apparent Km and Vmax change; pure noncompetitive inhibition is the special case where the two affinities are equal.