Aerobic and Anaerobic Electron Transport Chains in Anoxygenic Phototrophic Bacteria

Abstract

SummaryAerobic and anaerobic electron transport chains of facultative phototrophs have been of increasing interest because of their diverse organization of redox carriers and their adaptive regulatory mechanisms of gene expression. During the last decade, studies on the biochemistry of bacterial redox complexes such as NADH-deh and bc1 from Rhodobacter species, and Cyt c-oxidases of aa3 type from Rb. sphaeroides and Chloroflexus aurantiacus, have revealed the presence of fewer subunits than corresponding eukaryotic enzymes. This evidence has provided new insights into the biochemical evolution of respiration and also useful indications on structure/function relationships. Recent advances in studying the aerobic and anaerobic respiratory pathways of facultative phototrophs have taken advantage of modern molecular genetics. In particular, the role of soluble cytochrome c2, until recent years considered to be essential for electron transport in the two closely related species Rb, capsulatus and Rb. sphaeroides, has been better defined. Indeed, it is now clear that two different classes of alternative electron carriers (soluble Cyt iso-c2 and membrane-bound Cyt cy) can operate between the membrane-bound redox complexes instead of, or along with, the Cyt c2. The presence of multiple electron carriers between redox complexes suggests that Cyt cy-like components might be more widely spread among those photosynthetic bacteria where photooxidizable soluble c-type hemes are not readily detected, e.g. Cf. aurantiacus. The outstanding metabolic versatility of Rb. capsulatus made also possible the use of mutants defective in redox carriers of aerobic respiration for the analysis of anaerobic electron transport pathways. Thus, if the role of Cyt c2 in anaerobic light-driven electron flow has partially been reshuffled, Cyt c2 seems to play a key role in the dark anaerobic pathways leading to NO2 and N2O reduction. The use of Cyt c-deficient mutants also demonstrated that the ubiquinol/Cyt c oxidoreductase is not required for growth with DMSO or TMAO as electron acceptors. These dark anaerobic processes, however, cannot sustain a ‘consistent’ cell growth in the presence of non fermentable substrates; thus, they must be regarded as advantageous metabolic systems facilitating anaerobic growth in the dark and/or in the light.