The dependence of an initial rate of reaction upon the concentration of a substrate S that is present in large excess over the concentration of an enzyme or other catalyst (or reagent) E with the appearance of saturation behaviour following the Michaelis-Menten equation,
v = V[S]/(Km + [S]),
where v is the observed initial rate, V is its limiting value at substrate saturation (i.e., [S] >> Km), and Km the substrate concentration when v = V/2. The definition is experimental, i.e., it applies to any reaction that follows an equation of this general form. The symbols Vma or vma are sometimes used for V.
The parameters V and Km (the "Michaelis constant") of the equation can be evaluated from the slope and intercept of a linear plot of v-1 against [S]-1 (a "Lineweaver-Burk plot") or from slope and intercept of a linear plot of v against h/[S] ("Eadie-Hofstee plot").
A Michaelis-Menten equation is also applicable to the condition where E is present in large excess, in which case the concentration [E] appears in the equation instead of [S].
The term has sometimes been used to describe reactions that proceed according to the scheme
in which case Km = (k-1 + kcat)/k1 (Briggs-Haldane conditions). It has more usually been applied only to the special case in which k-1 >> kcat and Km = k-1/k1 = Ks; in this case Km is a true dissociation constant (Michaelis-Menten conditions). See also rate-determining step.
