Printed in Great Britain The Pre-eminence of k,,. in the Manifestation of Optimal Enzymic Activity Delineated by using the Briggs-Haldane Two-Step Irreversible Kinetic Model

enzymes that provide maximal rates of catalysis should be characterized by values ofK., the dissociation constant of the enzyme-substrate complex, greater than 10 times the value of the ambient substrate concentration has been examined. 2. For such enzymes, K, is not relevant, and attention is best focused on the relative numerical values of kca,. (in units of s-') and the substrate molarity. It is necessary only that the former be about 10" °-1O"l times the latter to ensure that the rate of product formation be diffusion-limited and thus maximal. Simple aspects of enzyme catalysis are sometimes discussed in terms of the kinetic model given in Scheme 1 and its characterizing parameters Km and kcat., with Km = (k-1+kc,t.)/k+L if the usual Briggs & Haldane (1925) conditions apply. For this model, it is only when substrate binding is maintained at equilibrium (Michaelis & Menten, 1913), i.e. kr k-1, that Km is equalj to K., the dissociation constan