The effect of sulfur compounds on H2 evolution/consumption reactions, mediated by various hydrogenases, in the purple sulfur bacterium, Thiocapsa roseopersicina

The influence of reduced sulfur compounds ( including stored S-0) on H-2 evolution/ consumption reactions in the purple sulfur bacterium, Thiocapsa roseopersicina BBS, was studied using mutants containing only one of the three known [ NiFe] hydrogenase enzymes: Hox, Hup or Hyn. The observed eVects depended on the kind of hydrogenase involved. The mutant harbouring Hox hydrogenase was able to use S2O32-, SO3 (2-), S2- and S-0 as electron donors for light- dependent H-2 production. Dark H-2 evolution from organic substrates via Hox hydrogenase was inhibited by S-0. Under light conditions, endogenous H-2 uptake by Hox or Hup hydrogenases was suppressed by S compounds. CO2- dependent H-2 uptake by Hox hydrogenase in the light required the additional presence of S compounds, unlike the Hup- mediated process. Dark H-2 consumption via Hyn hydrogenase was connected to utilization of S-0 as an electron acceptor and resulted in the accumulation of H2S. In wild type BBS, with high levels of stored S-0, dark H-2 production from organic substrates was significantly lower, but H2S accumulation signiWcantly higher, than in the mutant GB1121( Hox(+)). There is a possibility that H-2 produced via Hox hydrogenase is consumed by Hyn hydrogenase to reduce S-0.