The effect of pH on fumarase activity in acetate buffer.

vary in a simple way with pH. This variation in maximal initial velocity was interpreted in terms of the ionization constants of two groups in the fumarase-fumarate and fumarase-n-malate complexes. In this article, data are presented for the pH variation of the maximal initial velocities and Michaelis constants of both substrates in 10 mM acetate buffer at 25”. These data may be interpreted in terms of the ionization of two groups in the enzymatic site which have different ionization constants in the free enzyme and in the complexes with fumarate and L-malate. This interpretation has its roots in the suggestion by Michaelis et al. (3) that the bell-shaped activity curves so often found for enzymes result from ionization of groups in the enzyme. These ideas are expressed explicitly by the accompanying scheme (mechanism (2)) which we believe to be the simplest that can represent all the facts at constant buffer concentration. En+’ E”+ 1F E”+‘M -&Cl (2)