Nichtkompetitive

Nichtkompetitive (noncompetitive) inhibition is a form of enzyme inhibition in which an inhibitor binds to an enzyme at a site distinct from the active site, or to the enzyme–substrate complex, reducing catalytic activity without preventing substrate binding. As a result, the maximum rate of the reaction (Vmax) is lowered, while the affinity of the enzyme for its substrate (Km) may be unchanged or only modestly affected depending on the type of noncompetitive interaction.

There are two principal forms. Pure or true noncompetitive inhibition occurs when the inhibitor binds to the enzyme and to the enzyme–substrate complex with equal affinity, so Km remains unchanged but Vmax decreases. Mixed noncompetitive inhibition happens when the inhibitor binds with different affinities to E and to ES, causing changes in Km in addition to a decreased Vmax; depending on affinities, Km can increase or decrease.

In kinetic terms, noncompetitive inhibition changes the apparent Vmax without necessarily altering Km in the pure form, and can produce characteristic patterns on Lineweaver–Burk plots such as lines intersecting on the x-axis for pure noncompetitive inhibition. This contrasts with competitive inhibition, which increases Km while leaving Vmax unchanged, and uncompetitive inhibition, which lowers both Km and Vmax.

Nichtkompetitive inhibition is relevant in biochemistry and pharmacology, where many allosteric inhibitors reduce enzyme activity without blocking substrate binding. The concept highlights how regulation of enzyme activity can occur through sites separate from the active site, influencing catalytic efficiency rather than substrate affinity.