Hydroxy-bridged Active Site States of [NiFe]-Hydrogenase Unraveled by Cryogenic Vibrational Spectroscopy and DFT Computations

The catalytic mechanism of H₂ conversion by [NiFe]-hydrogenase is subject of extensive research. Apart from at least four reaction intermediates of H₂/H⁺ cycling, there is also a number of resting states, which are formed under oxidizing conditions. While not directly involved in the catalytic cycle, knowledge of their molecular structure and reactivity is important, because these states usually accumulate in the course of hydrogenase purification, and they may also play a role in vivo during hydrogenase maturation. Here, we applied low-temperature infrared (cryo-IR) and nuclear resonance vibrational spectroscopy (NRVS) to the isolated catalytic subunit, HoxC, of the heterodimeric regulatory [NiFe]-hydrogenase (RH) from Ralstonia eutropha. Cryo-IR spectroscopy revealed that the HoxC protein can be enriched in almost pure redox states suitable for NRVS investigation. NRVS analysis of the hydrogenase catalytic center is usually hampered by strong spectral contributions of the FeS clusters of the small, electron-transferring subunit. Therefore, our approach to investigate the FeS cluster-free, ⁵⁷Fe labeled HoxC granted an unprecedented view onto the active site modes, including those obscured by FeS cluster-derived bands. Rationalized by density functional theory (DFT) calculations, our data allow the structural description of two hydroxy-containing resting states. Our work highlights the relevance of cryogenic vibrational spectroscopy and DFT to elucidate the structure of barely defined redox states of the [NiFe]-hydrogenase active site.