Description of 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 behavior following the Michaelis-Menten equation: n = V[S]o/(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 Vmax or vmax are sometimes used for V. | Note 1. The parameters V and KM (the ‘Michaelis constant’) of the equation can be evaluated from the slope and intercept of a linear plot of 1/v vs. 1/[S] (‘Lineweaver-Burk plot’) or from slope and intercept of a linear plot of v vs. v/[S] (‘Eadie-Hofstee plot’). | Note 2. A Michaelis-Menten equation is also applicable to the condition where E is present in large excess, in which case the total concentration [E]o appears in the equation instead of [S]o. | Note 3. 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, the dissociation constant of the complex. In this case KM is a true dissociation constant (Michaelis-Menten conditions). See also rate-controlling step [3]