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Medical Term:

Michaelis constant

Pronunciation: mi-kā′lis


  1. the true dissociation constant for the enzyme-substrate binary complex in a single-substrate rapid equilibrium enzyme-catalyzed reaction (usually symbolized by Ks);
  2. the concentration of the substrate at which half the true maximum velocity of an enzyme-catalyzed reaction is achieved (when velocities are measured under initial rate and steady-state conditions); the ratio of rate constants (k2 + k3)/k1 in the single-substrate enzyme-catalyzed reaction: E + S ⇄ ES ⇄ E + products, where E represents the free enzyme, S is the substrate, and ES is the central binary complex. The expression for the Michaelis constant will be more complex for multisubstrate reactions. An apparent Michaelis constant is a constant determined either under conditions that are not strictly steady-state and initial rate or under a condition that varies with the concentration of one or more cosubstrates.

See: Michaelis-Menten equation

Synonym(s): Michaelis-Menten constant

[Leonor Michaelis]