Physical Significance of Michaelis Constants

Abstract

Briggs and Haldane¹ showed that for the reaction sequence E + S ⇌ ES → E + P, the same form of initial rate equation is obtained by the steady-state assumption as by the more restrictive equilibrium assumption of Michaelis and Menten². It was evident, therefore, that the Michaelis constant, K_m, defined³ as the substrate concentration with which half the maximum rate is attained, is not necessarily equal to the substrate constant, K_s, the dissociation constant of the enzyme–substrate complex. Nevertheless, it is still sometimes assumed^4,5 that the Michaelis constant is a measure of the affinity of the substrate for the enzyme. In a recent text-book⁶, it is stated that the Michaelis constant must be either equal to or greater than the substrate constant. It seems worth while to point out that this is not so, and to indicate relationships between K_m and K_s values derived from reaction schemes for reversible reactions involving one and two substrates.