Researchers determined the structure of the catalytic active state of [NiFe] hydrogenase
Hydrogen is known as an ultimate clean energy source and is thus discussed as a future sustainable energy carrier. In nature the enzyme 'Hydrogenase' catalyzes the reversible conversion of molecular hydrogen into protons and electrons. The active site of the [NiFe] hydrogenases is composed of abundant transition metals, Ni and Fe. The activity of such hydrogenases is comparable to that of precious metal (Pt) catalysts. Therefore, hydrogenases are in the focus of energy research worldwide because of their interesting prospects in biotechnology and in serving as natural models for biomimetic catalysts in hydrogen production and conversion.
For hydrogenases it is mandatory to see the hydrogens since hydrogen is the product (or educt) of the catalyzed reaction. We prepared 57Fe-labeled [NiFe] hydrogenase under an inert gas/hydrogen atmosphere and obtained a specific, essentially pure catalytic active state (Ni-R). Until now, there has been no spectroscopic method that can tell us how hydrogen is bound to this key intermediate. The current work addresses one of the fundamental issues about the catalytic cycle of [NiFe] hydrogenase. We have used a relatively new synchrotron-based technique, "Nuclear Resonance Vibrational Spectroscopy (NRVS)", to demonstrate that the hydrogen is bound as a bridging hydride between Ni and Fe. We have combined the information from NRVS with density functional theory (DFT) calculations that further constrain the structure of the Ni-R intermediate. We find that only a low-spin Ni(II) bridged by hydride to low-spin Fe(II) is consistent with the DFT calculations and the NRVS data. This will be of great interest to chemists striving to create synthetic models that have similar catalytic properties.
The project was funded by the Max Planck Society, the German Research Foundation (Deutsche Forschungsgemeinschaft) as part of the Cluster of Excellence RESOLV (EXC 1069), EU/Energy Network project SOLAR-H2 (FP7 contract 212508). This work was also supported by the DOE Office of Bio-logical and Environmental Research, NIH grant GM-65440, DOE grant DEFG02-90ER14146, and the DFG-funded 'Unifying Concepts in Catalysis' (UniCat) initiative.