Discovery and characterization of NADH oxidases for selective sustainable synthesis of 5-hydroxymethylfuran carboxylic acid.

Efficient regeneration of NAD⁺ remains a significant challenge for oxidative biotransformations. In order to identify enzymes with higher activity and stability, a panel of NADH oxidases (Nox) was investigated in the regeneration of nicotinamide cofactors for the oxidation of hydroxymethyl furfural (HMF) to 5-hydroxymethyl-2-furancarboxylic acid (HMFCA). We present novel Nox that exhibit remarkable catalytic activities, elevated thermal and pH stabilities, and higher intrinsic flavin loadings, thus eliminating the need for external flavin addition. The kinetic analysis of the NADH oxidases indicates that AdNox, GdNox, CmNox, and LvNox exhibit V_max values of 86 U/mg, 50 U/mg, 4.3 U/mg, and 23 U/mg, respectively. When these NADH oxidases were applied in a HMF oxidation reaction, LvNox demonstrated the highest HMFCA yield of 97 % in the presence of 0.1 mM NAD and 10 mM HMF. In contrast to previously reported NADH oxidases from the same family, these NADH oxidases naturally accept NADPH as a substrate. Rapid kinetics experiments identified the oxidative reaction as the rate-limiting step of the reaction. NADH oxidases achieved high atom economy, a high reaction mass efficiency and a low process mass intensity. The findings contribute significantly to the field of biocatalysis and offer potential avenues for more environmentally friendly cofactor regeneration in chemical synthesis.