[NiFe] hydrogenases: structural and spectroscopic studies of the reaction mechanism

[NiFe] hydrogenases catalyze the reversible oxidation of dihydrogen. For this simple reaction the molecule has developed a complex catalytic mechanism, during which the enzyme passes through various redox states. The [NiFe] hydrogenase contains several metal centres, including the bimetallic Ni–Fe active site, iron–sulfur clusters and a Mg²⁺ ion. The Ni–Fe active site is located in the inner part of the protein molecule, therefore a number of pathways are involved in the catalytic reaction route. These consist of an electron transfer pathway, a proton transfer pathway and a gas-access channel. Over the last 10–15 years we have been investigating the crystal structures of the [NiFe] hydrogenase from Desulfovibrio vulgaris Miyazaki F, which is a sulfate-reducing anaerobic bacterium. So far the crystal structures of the oxidized, H₂-reduced and carbon monoxide inhibited states have been determined at high resolution and have revealed a rather unique structure of the hetero-bimetallic Ni–Fe active site. Furthermore, intensive spectroscopic studies have been performed on the enzyme. Based on the crystal structure, a water-soluble Ni–Ru complex has been synthesized as a functional model for the [NiFe] hydrogenases. The present review gives an overview of the catalytic reaction mechanism of the [NiFe] hydrogenases.