5.B.11.  The One Electron Transmembrane Transfer Complex (TmcABCD) Family  

Pereira et al. 2006 reported that three membrane-bound redox complexes are present in Desulfovibrio spp., whose genes are not found in the genomes of other sulfate reducers such as Desulfotalea psycrophila and Archaeoglobus fulgidus. These complexes contain a periplasmic cytochrome c subunit of the cytochrome c₃ family, and their presence in these organisms correlates with the presence of a pool of periplasmic cytochromes c₃, also absent in the two other sulfate reducers mentioned above.  Pereira et al. 2006 isolated and characterized the first of such complexes, Tmc from D. vulgaris Hildenborough, which is associated with the tetraheme type II cytochrome c₃. The isolated Tmc complex contains four dissimilar subunits, including the small TpIIc₃ (TmcA), an integral membrane cytochrome b (TmcC), and two cytoplasmic proteins, an iron-sulfur protein (TmcB) and a tryptophan-rich protein (TmcD). Spectroscopic studies indicated the presence of eight hemes c and two hemes b in the complex, pointing to an A₂BCD composition (TmcA₂BCD). EPR analysis reveals the presence of a [4Fe4S]³⁺ center and up to three other iron-sulfur centers in the cytoplasmic subunits. Nearly full reduction of the redox centers in the Tmc complex could be obtained upon incubation with hydrogenase/TpIc(3), supporting the role of this complex in transmembrane transfer of electrons to sulfate in the cytoplasm, resulting from periplasmic oxidation of hydrogen concomitant with the reduction of sulfate (Pereira et al. 2006).