Model of Fe(II) oxidation in A. ferrooxidans ATCC 23270. The flow of electrons is shown from the oxidation of Fe+2 by Cyc2 to reduce oxygen via the aa3 complex (downhill pathway) or to reduce NAD+ via bc1/quinone pool/NADH complex (uphill pathway). The downhill pathway can consume protons entering via the ATPase complex helping to drive ATP synthesis or via the bc1/quinone pool/NADH complex that drives the flow of electrons in the uphill pathway. The switch point between the downhill and uphill flow is suggested to be at the level of rusticyanin (Rus). Abbreviations used can be found in the text.