Figure 1. Anaerobic metabolism in helminths requires Rhodoquinone (RQ).

(a) Electron flow in the Electron Transport Chain (ETC) under aerobic conditions. Electrons enter the ETC either via Complex I or via a number of Quinone-coupled Dehydrogenases (QDH; cyan). These complexes transfer electrons to Ubiquinone (red circle ‘UQ’) which shuttles them to Complex III. They exit the ETC at Complex IV where they are transferred to oxygen as the terminal electron acceptor. Proton pumping is coupled to electron transport and is carried out by Complexes I, III and IV. Electron flow is shown in red and proton pumping in green. (b) Electron flow in the Electron Transport Chain (ETC) under anaerobic conditions. Electrons still enter the ETC at Complex I which transfers electrons to RQ (cyan circle ‘RQ’). RQ shuttles electrons to the QDHs which now operate as reductases, allowing electrons to exit the ETC and onto a diverse set of terminal electron acceptors. Complex I is the sole proton pump in this truncated ETC. (c) Schematic of Complex II activity under aerobic and anaerobic conditions. Under aerobic conditions, Complex II acts as a succinate dehydrogenase, transferring electrons from succinate onto UQ. Under anaerobic conditions, Complex II operates in the reverse direction acting as a fumarate reductase, accepting electrons from RQ and transferring them to succinate as the terminal electron sink. (d) Structure of UQ and RQ. The critical amine group differing between UQ and RQ is highlighted; the polyprenyl tail is shown schematically as a green wavy line.