Active site assembly of [NiFe]-hydrogenase scrutinized on the basis of purified maturation intermediates

doi:10.26434/chemrxiv-2022-jvtgw
[NiFe]-hydrogenases are biotechnologically relevant enzymes catalyzing the reversible splitting of H$_2$ into 2 e$^- $ and 2 H$^+$ under ambient conditions. Catalysis takes place at the heterobimetallic NiFe(CN)$_2$(CO) center, whose multistep biosynthesis involves careful handling of two transition metals as well as potentially harmful CO and CN$^–$ molecules. Herein, 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$_2$-tolerant membrane-bound hydrogenase from Cupriavidus necator. These included the cofactor-free apo-HoxG, a nickel-free version carrying only the Fe(CN)$_2$(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.