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 cons