Hydrogenases are highly active enzymes for converting hydrogen. They use abundant metals like iron and nickel in their active site. However, the most active type of hydrogenase, [FeFe] hydrogenase, is extremely oxygen-sensitive. Interestingly, some of these enzymes can be purified in an oxygen-stable inactive state called H_inact, which can be reactivated by hydrogen. So far, the structure and mechanism of formation of H_inact had remained unknown.

Serena DeBeer, James A. Birrell, Max Planck Institute for Chemical Energy Conversion, Mülheim an der Ruhr, Germany, Ingrid Span, University of Düsseldorf, Germany, and colleagues have studied this H_inact state using X-ray crystallography, a wide range of spectroscopic methods, i.e., X‐ray absorption spectroscopy (XAS), nuclear resonance vibrational spectroscopy (NRVS), resonance Raman (RR) spectroscopy, and infrared (IR) spectroscopy, as well as quantum-chemical calculations. The team used the [FeFe] hydrogenase from Desulfovibrio desulfuricans, a type of sulfate-reducing bacteria.

The researchers found a sulfur ligand in the H_inact state at the site where oxygen would attack (pictured). This prevents oxygen from binding and protects the enzyme from damage. This insight might allow researchers to use [FeFe] hydrogenases in biotechnological applications and provide design principles for developing further O₂-stable bio-inspired molecular catalysts.

• Caught in the H_inact: Crystal Structure and Spectroscopy Reveal a Sulfur Bound to the Active Site of an O₂‐stable State of [FeFe] Hydrogenase,
  Patricia Rodríguez-Maciá, Lisa Galle, Ragnar Bjornsson, Christian Lorent, Ingo Zebger, Yoshitaka Yoda, Stephen Cramer, Serena DeBeer, Ingrid Span, James Birrell,
  Angew. Chem. Int. Ed. 2020.
  https://doi.org/10.1002/anie.202005208