Biochemical, genetic and genomic characterization of anaerobic electron transport pathways in sulphate-reducing Delta proteobacteria

INTRODUCTION Sulphate-reducing bacteria (SRB) derive energy for growth by coupling the oxidation of hydrogen or organic compounds to the reduction of sulphate to sulphide. The bioenergetics and the global topology of energy-conserving reactions have already been discussed in Chapter 1. Understanding the bioenergetics of the coupling of hydrogen oxidation and sulphate reduction is simple, in principle. Four H2 are oxidized by periplasmic hydrogenases and the eight protons and electrons are transferred to the cytoplasm through ATP synthase and transmembrane-electron-transfer complexes for sulphate reduction. This produces approximately three adenosine triphosphates (ATPs), of which two are needed to activate sulphate. Hence a net yield of one ATP is produced per sulphate reduced. Energy conservation by coupling the reduction of sulphate to the incomplete oxidation of lactate is more complex because the primary oxidation reactions are now also cytoplasmic. Because these yield two ATPs by substrate level phosphorylation, the same number as required for the activation of sulphate, a net energetic benefit can only be obtained by hydrogen cycling as proposed by Odom and Peck (Odom and Peck, 1981), cycling of formate or CO (Heidelberg et al., 2004; Voordouw, 2002) or by electrogenic proton translocation associated with the electron transport chain for reduction of sulphate. The components that participate in these anaerobic electron transport pathways will be considered in detail here. Harry Peck and Jean LeGall, the pioneers of the biochemistry of SRB, contributed greatly by purifying and characterizing many of the redox proteins present in these organisms.