CO and CN- syntheses by [FeFe]-hydrogenase maturase HydG are catalytically differentiated events

The synthesis and assembly of the active site [FeFe] unit of [FeFe]-hydrogenases require at least three maturases. The radical S-adenosyl-L-methionine HydG, the best characterized of these proteins, is responsible for the synthesis of the hydrogenase CO and CN- ligands from tyrosine-derived dehydroglycine (DHG). We speculated that CN-and the CO precursor(-):CO2H may be generated through an elimination reaction. We tested this hypothesis with both wild type and HydG variants defective in second ironsulfur cluster coordination by measuring the in vitro production of CO, CN-, and -:CO2H-derived formate. We indeed observed formate production under these conditions. We conclude that HydG is a multifunctional enzyme that produces DHG, CN-, and CO at three well-differentiated catalytic sites. We also speculate that homocysteine, cysteine, or a related ligand could be involved in Fe(CO)(x)(CN)(y) transfer to the HydF carrier/scaffold.