One cofactor, many enzymes: mechanistic and functional insights into nickel-pincer nucleotide-dependent biochemistry

Nickel-pincer nucleotide (NPN) cofactors represent a groundbreaking class of organometallic cofactors in which a nickel ion is embedded within a tridentate pincer ligand derived from a modified nucleotide. Initially discovered in lactate racemase (LarA), NPN cofactors have since been identified in three distinct enzyme (super)families: LarA homologs (racemases/epimerases), ornithine cyclodeaminases, and aldo-keto reductases. These enzymes catalyse diverse reactions from stereochemical inversion to oxidative deamination and redox disproportionation, unified by a proton-coupled hydride transfer mechanism. The broad distribution of NPN biosynthetic genes suggested that NPN-dependent enzymes are very widespread among prokaryotes, with bioinformatic analyses predicting novel superfamilies. However, key challenges persist: the biosynthetic pathway remains inefficient for potential industrial applications, and the functional roles of many enzymes are still unknown. This review synthesises structural, mechanistic, and functional insights into NPN-dependent biochemistry, highlighting emerging opportunities for enzyme engineering, metabolic pathway discovery, and sustainable biocatalysis.