Yoshiki Higuchi

[NiFe] hydrogenases catalyze the reversible cleavage of molecular hydrogen into protons and electrons. Here, we have studied the impact of temperature and illumination on an oxygen‐tolerant and thermostable [NiFe] hydrogenase by IR and EPR spectroscopy. Equilibrium mixtures of two catalytic [NiFe] states, Nia‐C and Nia‐SR’’, were found to drastically change with temperature, indicating a thermal exchange of electrons between the [NiFe] active site and iron‐sulfur clusters of the enzyme. In addition, IR and EPR experiments performed under illumination revealed an unusual photochemical response of the enzyme. Nia‐SR’’, a fully reduced hydride intermediate of the catalytic cycle, was found to be reversibly photoconverted into another catalytic state, Nia‐L. In contrast to the well‐known photolysis of the more oxidized hydride intermediate Nia‐C, photoconversion of Nia‐SR’’ into Nia‐L is an active‐site redox reaction that involves light‐driven electron transfer towards the enzyme’s iron‐sulfur clusters. Omitting the ground‐state intermediate Nia‐C, this direct interconversion of these two states represents a potential photochemical shortcut of the catalytic cycle that integrates multiple redox sites of the enzyme. In total, our findings reveal the non‐local redistribution of electrons via thermal and photochemical reaction channels and the potential of accelerating or controlling [NiFe] hydrogenases by light.

Formate dehydrogenases (FDHs) catalyze the two-electron oxidation of formate to carbon dioxide. FDHs can be divided into several groups depending on their subunit composition and active-site metal ions. Metal-dependent (Mo- or W-containing) FDHs from prokaryotic organisms belong to the superfamily of molybdenum enzymes and are members of the dimethylsulfoxide reductase family. In this short review, recent progress in the structural analysis of FDHs together with their potential biotechnological applications are summarized.

We report the first neutron structure of [NiFe]-hydrogenase in its oxidized state. This study leads to new insights into the oxidized active site and visualization of the protons characteristic of the oxidized enzyme.