The oxygenase reaction of acetolactate synthase

In addition to the physiological reactions catalyzed by acetolactate synthase, it supports an oxygen-consuming side reaction. Although the synthase and oxygenase activities are activated to somewhat different extents by various metals (Mn2+, Mg2+, Ca2+, Co2+, Zn2+, Ni2+, Cd2+, Cu2+, Ba2+, Al3+), the modest degree of these differences (at most 6-fold) and the high degree of promiscuity of the enzyme with respect to its metal requirement suggest that the metal is not intimately involved in the chemistry of either reaction. Saturation of the oxygenase reaction occurs at pyruvate concentrations below the limit of sensitivity for the oxygen electrode (< 10 microM), at higher concentrations pyruvate inhibits the rate of oxygen consumption. At a noninhibitory concentration of pyruvate (1 mM), inhibition of the reaction is also observed with alpha-ketobutyrate. Inhibition of the oxygenase reaction by high concentrations of pyruvate or alpha-ketobutyrate is presumably due to competition between these substrates and molecular oxygen for a common carbanionic reaction intermediate, the conjugate base of (hydroxyethyl)thiamin pyrophosphate. Inhibition of the reaction indicates that the lactylthiamin pyrophosphate intermediate can decarboxylate prior to binding of the second pyruvate or alpha-ketobutyrate. At high concentrations of pyruvate or alpha-ketobutyrate, only incomplete inhibition of the oxygenase reaction is achieved (65-89% or 89-93% maximal inhibition, respectively). This incomplete inhibition of the oxygenase reaction by alpha-keto acids indicates that the reaction is not Theorell-Chance with respect to addition of the second alpha-keto acid and that oxygen has more than one route of access to the carbanionic reaction intermediate.