Cyanide and Carbon Monoxide Ligand Formation in Hydrogenase Biosynthesis

Carbon monoxide (CO) and cyanide (CN-) can act as potent inhibitors of enzymes necessary for primary biochemical processes, however they also play important roles in biological systems. Well-studied cases include CN- biosynthesis in plants to act as a defense against herbivores and pathogens, CN- biosynthesis in certain species of bacteria to remove excess glycine, and CO biosynthesis by microbes for energy metabolism in the Wood-Ljungdahl pathway. The utilization of CO and CN- as essential metal ligands in biology is even more limited, with the only known examples being at the active sites of hydrogenase enzymes. This class of enzymes catalyzes the reversible oxidation of hydrogen, a reaction that in biology appears to be entirely dependent on the presence of CO and/or CN- ligands. To date, synthetic mimics of hydrogenase active sites have not reproduced hydrogen production rates observed in some hydrogenases; it is thus of considerable interest to understand how biology has solved the intriguing problem of biosynthesizing efficient hydrogen catalysts. Of the hydrogenase enzymes discussed herein, recent advances in the [FeFe]-hydrogenase family has provided important insights into the synthesis of the CO and CN- ligands for its active site (H-cluster). Biosynthesis of the complex [FeFe]-hydrogenase active site requires only three ironsulfur cluster-containing maturation proteins, where two act as radical S-adenosylmethionine (AdoMet) enzymes (HydE and HydG) and the other as a GTPase (HydF). In this review, biological CO and CN- genesis mechanisms will be assessed with specific focus on [FeFe]-hydrogenase maturation.