Stepwise assembly of the active site of [NiFe]-hydrogenase.

[NiFe]-hydrogenases are biotechnologically relevant enzymes catalyzing the reversible splitting of H into 2e and 2H under ambient conditions. Catalysis takes place at the heterobimetallic NiFe(CN)(CO) center, whose multistep biosynthesis involves careful handling of two transition metals as well as potentially harmful CO and CN molecules. Here, we investigated the sequential assembly of the [NiFe] cofactor, previously based on primarily indirect evidence, using four different purified maturation intermediates of the catalytic subunit, HoxG, of the O-tolerant membrane-bound hydrogenase from Cupriavidus necator. These included the cofactor-free apo-HoxG, a nickel-free version carrying only the Fe(CN)(CO) fragment, a precursor that contained all cofactor components but remained redox inactive and the fully mature HoxG. Through biochemical analyses combined with comprehensive spectroscopic investigation using infrared, electronic paramagnetic resonance, Mössbauer, X-ray absorption and nuclear resonance vibrational spectroscopies, we obtained detailed insight into the sophisticated maturation process of [NiFe]-hydrogenase.