The kinetic mechanism of beef kidney D-aspartate oxidase

The mechanism of action of the flavoprotein D-aspartate oxidase (EC 1.4.3.1) has been investigated by steady-state and stopped flow kinetic studies using D-aspartate and O₂ as substrates in 50 mM KP(i), 0.3 mM EDTA, pH 7.4, 4°C. Steady-state results indicate that a ternary complex containing enzyme, O₂, and substrate (or product) is an obligatory intermediate in catalysis. The kinetic parameters are turnover number = 11.1 s^-1, K(m(D-Asp)) = 2.2 x 10^-3 M, K(m(O₂)) = 1.7 x 10^-4 M. Rapid reaction studies show that 1) the reductive half reaction is essentially irreversible with a maximum rate of reduction of 180 s^-1; 2) the free reduced enzyme cannot be the species which is reoxidized during turnover since its reoxidation by oxygen (second order rate constant equal to 5.3 x 10² M^-1 S^-1) is too slow to be of relevance in catalysis; 3) reduced enzyme can bind a lignad rapidly and be reoxidized as a complex at a rate faster than that observed for the free reduced enzyme; 4) the rate of reoxidation of reduced enzyme by oxygen during turnover is dependent on both O₂ and D-aspartate concentrations (second order rate constant of reaction between O₂ and reduced enzyme-substrate complex equal to 6.2 x 10⁴ M^-1 s^-1); and 5) the rate-limiting step in catalysis occurs after reoxidation of the enzyme and before its reduction in the following turnover. A mechanism involving reduction of enzyme by substrate, dissociation of product from reduced enzyme, binding of a second molecule of substrate to the reduced enzyme, and reoxidation of the reduced enzyme-substrate complex is proposed for the enzyme-catalyzed oxidation of D-aspartate.