The role of Hox hydrogenase in the H2 metabolism of Thiocapsa roseopersicina

The purple sulfur phototrophic bacterium Thiocapsa roseopersicina BBS synthesizes at least three NiFe hydrogenases (Hox, Hup, Hyn). We characterized the physiological H2 consumption/evolution reactions in mutants having deletions of the structural genes of two hydrogenases in various combinations. This made possible the separation of the functionally distinct roles of the three hydrogenases. Data showed that Hox hydrogenase (unlike the Hup and Hyn hydrogenases) catalyzed the dark fermentative H2 evolution and the light-dependent H2 production in the presence of thiosulfate. Both Hox+ and Hup+ mutants demonstrated light-dependent H2 uptake stimulated by CO2 but only the Hup+ mutant was able to mediate O2-dependent H2 consumption in the dark. The ability of the Hox+ mutant to evolve or consume hydrogen was found to depend on a number of interplaying factors including both growth and reaction conditions (availability of glucose, sulfur compounds, CO2, H2, light). The study of the redox properties of Hox hydrogenase supported the reversibility of its action. Based on the results a scheme is suggested to describe the role of Hox hydrogenase in light-dependent and dark hydrogen metabolism in T. roseopersicina BBS.