Abstract: Cytochrome P450 enzymes catalyze diverse oxidative transformations at the expense of reduced nicoti-namide adenine dinucleotide phosphate (NADPH), however, their applications remain limited largely because NADPH is cost-prohibitive for biocatalysis at scale yet tightly regulated in host cells. A highly challenging task for P450 catalysis has been to develop an alternative and biocompatible electron-donating system. Here we engineered P450 BM3 to favor reduced nicotinamide cytosine dinucleotide (NCDH) and created non-natural cofactor-dependent P450 catalysis. Two outstanding mutants were identified with over 640-fold NCDH preference improvement and good catalytic efficiencies of over 15,000 M^-1 s^-1 for the oxidation of the fatty acid probe 12-(para-nitrophenoxy)-dodecanoate. Molecular docking analysis indicated that these mutants bear a compacted cofactor entrance. Upon fus-ing with an NCD-dependent formate dehydrogenase, fused proteins functioned as NCDH-specific P450 catalysts by using formate as the electron donor. Importantly, these mutants and fusions catalyzed NCDH-dependent hydroxylation of fatty acids with similar chain length preference to those by natural P450 BM3 in the presence of NADPH and also similar regioselectivity for subterminal hydroxylation of lauric acid. As P450 BM3 and its variants are catalytically powerful to take diverse substrates and convey different reaction paths, our results offer an exciting opportunity to devise advanced cell factories that convey oxidative biocatalysis with an orthogonal reducing power supply system.

Key words: Biocatalysis, Cytochrome P450, Directed evolution, Fatty acid hydroxylation, Non-natural cofactor, Synthetic biology