Structural Determinants of the Catalytic Ni a -L Intermediate of [NiFe]-Hydrogenase.

[NiFe]-hydrogenases catalyze the reversible cleavage of H 2 into two protons and two electrons at the inorganic heterobimetallic NiFe center of the enzyme. Their catalytic cycle involves at least four intermediates, some of which are still under debate. While the core reaction, including H 2 /H - binding, takes place at the inorganic cofactor, a major challenge lies in identifying those amino acid residues that contribute to the reactivity and how they stabilize (short-lived) intermediate states. Using cryogenic infrared and electron paramagnetic resonance spectroscopy on the regulatory [NiFe]-hydrogenase from Cupriavidus necator , a model enzyme for the analysis of catalytic intermediates, we deciphered the structural basis of the hitherto elusive Ni a -L intermediates. We unveiled the protonation states of a proton-accepting glutamate and a Ni-bound cysteine residue in the Ni a -L1, Ni a -L2, and the hydride-binding Ni a -C intermediates as well as previously unknown conformational changes of amino acid residues in proximity of the bimetallic active site. As such, this study unravels the complexity of the Ni a -L intermediate and reveals the importance of the protein scaffold in fine-tuning proton and electron dynamics in [NiFe]-hydrogenase.