KINETICS OF THE CATALYSIS BY THE ALCALIGENES-EUTROPHUS H16 HYDROGENASE OF THE ELECTROCHEMICAL REDUCTION OF NAD+

The regeneration of the pyridine coenzyme NADH is the key step in numerous enzymatic synthesis processes. This work deals with the kinetics of the catalysis by the Atcaligenes eutrophus H16 hydrogenase of the electrochemical reduction of NAD+ on a platinum electrode. In a classical way the enzyme catalyses the homogeneous reduction of NAD+ by dissolved hydrogen. The kinetics of this reaction has been determined. It follows a Michaelis-Menten law with inhibition by the substrate. On the other hand, the kinetics of the electrochemical reduction is of zero order with respect to the NAD+ concentration and clearly shows an adsorption step. The control experiments verify that the species obtained by electrochemical reduction is the enzymatically active form of NADH. Comparison of the features of the two catalysis reactions shows that the electrochemical reduction is a heterogeneous process quite different from the classical homogeneous one, where the enzyme catalyses the electronic transfer from the platinum electrode to NAD+.