Structural Insights into Phosphite Dehydrogenase Variants Favoring a Non-natural Redox Cofactor

Implementation of a non-natural cofactor alternative to the ubiquitous redox cofactor nicotinamide adenosine dinucleotide (NAD) is of great scientific and biotechnological interest. Several redox enzymes have been engineered to favor nicotinamide cytosine dinucleotide (NCD), a smaller-sized NAD analogue. However, molecular interactions involving NAD analogues remain elusive, preventing us from devising more enzymes to accept those analogues. Here we took a semirational approach to evolve phosphite dehydrogenase (Pdh) and identified variants with substantially improved NCD preference. These mutants are valuable components for regeneration of reduced NCD by using phosphite as the electron donor. We then collected X-ray crystal structures of three Pdh variants and their NCD-complexes to delineate molecular basis for NCD binding. It was found that the incorporation of amino acid residues with large side chains enclosing the NAD-binding pocket led to compacted environment favoring NCD over NAD, and additional ...