BIOELECTROCATALYSIS OF NAD+ REDUCTION

A thermokinetic approach to the catalysis of the electrochemical reduction of NAD+ by the hydrogenase of Alcaligenes eutrophus H16 is presented. The influence of temperature is weak, with an activation energy of 18 kJ/mol, whereas classical NAD+ hydrogenation with gaseous hydrogen shows an activation energy of 34 kJ/mol. The influence of the electric field on the thermal degradation of the enzyme is emphasized: the hydrogenase is clearly activated by the electric field and is shielded against thermal degradation. In contrast, in the absence of an electric field the thermal degradation of the hydrogenase follows rapid first-order kinetics with a high activation energy of the order of 70 kJ/mole. The catalysis of the bioelectrochemical reduction of NAD+ follows a quite different path to that of the hydrogenation reaction. In practice, the temperature is no longer an essential parameter to be considered in the integration of the NAD+ bioelectrochemical reduction within an enzymatic synthesis process. Adsorption phenomena and specific area of the electrode should be preferentially taken into account. The synthesis-regeneration coupling is tested in a thin-layer cell and gives a high NAD turnover number value of 448 cycles/h.