Hydrogenases in sulfate-reducing bacteria function as chromium reductase

The ability of sulfate-reducing bacteria (SRB) to reduce chromate VI has been studied for possible application to the decontamination of polluted environments. Metal reduction can be achieved both chemically, by H₂S produced by the bacteria, and enzymatically, by polyhemic cytochromes c₃. We demonstrate that, in addition to low potential polyheme c-type cytochromes, the ability to reduce chromate is widespread among Fe], NiFe], and NiFeSe] hydrogenases isolated from SRB of the genera Desulfovibrio and Desulfomicrobium. Among them, the Fe] hydrogenase from Desulfovibrio vulgaris strain Hildenborough reduces Cr(VI) with the highest rate. Both Fe] and NiFeSe] enzymes exhibit the same K_m towards Cr(VI), suggesting that Cr(VI) reduction rates are directly correlated with hydrogen consumption rates. Electron paramagnetic resonance spectroscopy enabled us to probe the oxidation by Cr(VI) of the various metal centers in both NiFe] and Fe] hydrogenases. These experiments showed that Cr(VI) is reduced to paramagnetic Cr(III), and revealed inhibition of the enzyme at high Cr(VI) concentrations. The significant decrease of both hydrogenase and Cr(VI)-reductase activities in a mutant lacking Fe] hydrogenase demonstrated the involvement of this enzyme in Cr(VI) reduction in vivo. Experiments with 3Fe-4S] ferredoxin from Desulfovibrio gigas demonstrated that the low redox Fe-S] (non-heme iron) clusters are involved in the mechanism of metal reduction by hydrogenases.