Nickel‐[iron‐sulfur]‐selenium‐containing hydrogenases from Desulfovibrio baculatus (DSM 1743)

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Abstract
The hydrogenase from Desulfovibrio baculatus (DSM 1743) was purified from each of three different fractions: soluble periplasmic (wash), soluble cytoplasmic (cell disruption) and membrane‐bound (detergent solubilization). Plasma‐emission metal analysis detected in all three fractions the presence of iron plus nickel and selenium in equimolecular amounts. These hydrogenases were shown to be composed of two non‐identical subunits and were distinct with respect to their spectroscopic properties. The EPR spectra of the native (as isolated) enzymes showed very weak isotropic signals centered around g∼ 2.0 when observed at low temperature (below 20 K). The periplasmic and membrane‐bound enzymes also presented additional EPR signals, observable up to 77 K, with g greater than 2.0 and assigned to nickel(III). The periplasmic hydrogenase exhibited EPR features at 2.20, 2.06 and 2.0. The signals observed in the membrane‐bound preparations could be decomposed into two sets with g at 2.34, 2.16 and ∼ 2.0 (component I) and at 2.33, 2.24, and ∼ 2.0 (component II). In the reduced state, after exposure to an H2 atmosphere, all the hydrogenase fractions gave identical EPR spectra.EPR studies, performed at different temperatures and microwave powers, and in samples partially and fully reduced (under hydrogen or dithionite), allowed the identification of two different iron‐sulfur centers: center I (2.03, 1.89 and 1.86) detectable below 10 K, and center II (2.06, 1.95 and 1.88) which was easily saturated at low temperatures. Additional EPR signals due to transient nickel species were detected with g greater than 2.0, and a rhombic EPR signal at 77 K developed at g 2.20, 2.16 and 2.0. This EPR signal is reminiscent of the Ni‐signal C (g at 2.19, 2.14 and 2.02) observed in intermediate redox states of the well characterized Desulfovibrio gigas hydrogenase (Teixeira et al. (1985) J. Biol. Chem. 260, 8942]. During the course of a redox titration at pH 7.6 using H2 gas as reductant, this signal attained a maximal intensity around ‐320 mV. Low‐temperature studies of samples at redox states where this rhombic signal develops (10 K or lower) revealed the presence of a fast‐relaxing complex EPR signal with g at 2.25, 2.22, 2.15, 2.12, 2.10 and broad components at higher field. The soluble hydrogenase fractions did not show a time‐dependent activation but the membrane‐bound form required such a step in order to express full activity. This indicates that the redox state of the isolated enzyme is important for the full expression of enzymatic activity. The catalytic properties were also followed by the proton‐deuterium exchange reaction. The isolated hydrogenases produced H2/HD ratios higher than those observed for non‐selenium‐containing hydrogenases.

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