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We report the mechanistic investigation of catalytic H2 evolution from formic acid in water using a formate-bridged dinuclear Ru complex as a formate hydrogen lyase model. The mechanistic study is based on isotope-labeling experiments involving hydrogen isotope exchange reaction. [ABSTRACT FROM AUTHOR]

Membrane-bound [NiFe]-hydrogenase from Hydrogenophaga sp. AH-24 was purified to homogeneity. The molecular weight was estimated as 100±10 kDa, consisting of two different subunits (62 and 37 kDa). The optimal pH values for H2 oxidation and evolution were 8.0 and 4.0, respectively, and the activity ratio (H2 oxidation/H2 evolution) was 1.61 × 102 at pH 7.0. The optimal temperature was 75 °C. The enzyme was quite stable under air atmosphere (the half-life of activity was c. 48 h at 4 °C), which should be important to function in the aerobic habitat of the strain. The enzyme showed high thermal stability under anaerobic conditions, which retained full activity for over 5 h at 50 °C. The activity increased up to 2.5-fold during incubation at 50 °C under H2. Using methylene blue as an electron acceptor, the kinetic constants of the purified membrane-bound homogenase (MBH) were Vmax=336 U mg−1, kcat=560 s−1, and kcat/ Km=2.24 × 107 M−1 s−1. The MBH exhibited prominent electron paramagnetic resonance signals originating from [3Fe–4S]+ and [4Fe–4S]+ clusters. On the other hand, signals originating from Ni of the active center were very weak, as observed in other oxygen-stable hydrogenases from aerobic H2-oxidizing bacteria. This is the first report of catalytic and biochemical characterization of the respiratory MBH from Hydrogenophaga. [ABSTRACT FROM AUTHOR]

A hydrogen-oxidizing bacterium strain AH-24 was isolated, which was classified in the genus Hydrogenophaga, based on the 16S rRNA gene sequence. The isolate possessed a typical yellow pigment of Hydrogenophaga species. Its closest relative was Hydrogenophaga pseudoflava, but the assimilation profile of sugar compounds resembled that of no species of Hydrogenophaga. The optimum temperature and pH for autotrophic growth were, respectively, 33–35 °C and 7.0. Most hydrogenase activity (benzyl viologen reducing activity) was localized in the membrane fraction (MF), but NAD(P)-reducing hydrogenase activity was detected in neither the membrane nor the soluble fractions. Cytochromes b561 and c551 were present in MF; both were reduced when hydrogen was supplied to the oxidized MF, suggesting involvement in respiratory H2 oxidation as electron carriers. Cytochrome b561 was inferred to function as the redox partner of the membrane-bound hydrogenase. [ABSTRACT FROM AUTHOR]

We report the decomposition of formic acid to hydrogen and carbon dioxide, catalysed by a NiRu complex originally developed as a [NiFe]hydrogenase model. This is the first example of H2 evolution, catalysed by a [NiFe]hydrogenase model, which does not require additional energy. [ABSTRACT FROM AUTHOR]

Herein, we report the first crystal structure of a monomeric p-semiquinonato d-block complex and its reactivity toward dioxygen, closely associated with a biological system of an oxygen evolving centre of photosystem II. [ABSTRACT FROM AUTHOR]