The pH dependence of the kinetic parameters of recombinant acetohydroxy acid synthase catalyzed reaction was determined in order to obtain information about the chemical mechanism, particularly acid-base chemistry. The maximum velocity and V/K for pyruvate were bell-shaped with estimated pK values of 6.5-6.7 and 8.6-8.9, respectively. The maximum velocity and V/K for 2-ketobutyrate were also bell-shaped with estimated pK values of 6.6-7.0 and 8.4-8.6. The pH dependence of 1/Ki for 3-bromopyruvate, a competitive inhibitor of pyruvate, was also bell-shaped, giving pK values almost identical with those obtained for pyruvate. Since the same pK values were observed in the $pK_{i 3-bromopyruvate}$, V/K pH profiles and $V_{max}$ profiles, both enzyme groups must be in their optimum protonation state for efficient binding of reactants. These results reflect that two enzyme groups are necessary for binding of substrate and/or catalysis.