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 iron-sulfur 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.