Biological electron transport goes the extra mile [Microbiology]
Microorganisms stand out by the creative ways in which they gain energy during respiration. Using versatile metabolic networks, respiratory microorganisms harvest electrons from organic or inorganic donor molecules and transfer them to specialized electron transport chains, which redirect their flow toward a terminal electron acceptor. Respiratory electrons move with a purpose: to generate a transmembrane ion gradient, most often of protons, that drives the synthesis of ATP by the ATP synthase complex. Thus, the electron carriers team up and transfer electrons in sequential steps to minimize energy losses to heat while simultaneously pumping protons, consuming them in the cytoplasm, and/or releasing them outside the membrane (Fig. 1). In PNAS, Bjerg et al. (1) report that the electron transport chain of some marine bacteria sequentially transfers electrons at several millimeters, something long believed to be unattainable in biological systems.
Electron transport chains for the cytoplasmic reduction of O2 and fumarate in E. coli ( Left ) and the extracellular reduction of Fe(III) oxide minerals in G. sulfurreducens ( Right ). Abbreviations: bo3, cytochrome bo3; Frd, fumarate reductase complex; IM, inner membrane; MQ, menaquinone; MQH2, menaquinol; NDH I, NADH dehydrogenase complex I; OM, outer membrane; Omc, outer membrane cytochrome; PccB/C, porin-cytochrome complex B or C; PpcA–E, periplasmic cytochromes A–E; PS, periplasmic space; Qox, one of two putative quinol oxidases (ImcH and CbcL); UQ, ubiquinone; UQH2, ubiquinol.
The traditional view of microbial electron transport chains is that of a set of membrane-bound electron carriers organized from high to low redox potentials to permit the spontaneous flow of electrons to the terminal electron acceptor (Fig. 1). The membrane carriers are not structurally linked so they can diffuse laterally in the membrane and collide with one another to promote the rapid exchange of electrons (2). The model bacterium Escherichia coli …
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