Krypton Derivatization of an O 2 ‐Tolerant Membrane‐Bound [NiFe] Hydrogenase Reveals a Hydrophobic Tunnel Network for Gas Transport

[NiFe] hydrogenases are metalloenzymes catalyzing the reversible heterolytic cleavage of hydrogen into protons and electrons. Gas tunnels make the deeply buried active site accessible to substrates and inhibitors. Understanding the architecture and function of the tunnels is pivotal to modulating the feature of O 2 tolerance in a subgroup of these [NiFe] hydrogenases, as they are interesting for developments in renewable energy technologies. Here we describe the crystal structure of the O 2 ‐tolerant membrane‐bound [NiFe] hydrogenase of Ralstonia eutropha (ReMBH), using krypton‐pressurized crystals. The positions of the krypton atoms allow a comprehensive description of the tunnel network within the enzyme. A detailed overview of tunnel sizes, lengths, and routes is presented from tunnel calculations. A comparison of the ReMBH tunnel characteristics with crystal structures of other O 2 ‐tolerant and O 2 ‐sensitive [NiFe] hydrogenases revealed considerable differences in tunnel size and quantity between the two groups, which might be related to the striking feature of O 2 tolerance.