This paper clarifies the role of cytochrome c in Pseudomonas AM1 by measuring the stoicheiometry of proton translocation driven by respiration of endogenous or added substrates in wild-type bacteria and in a mutant lacking cytochrome c (mutant PCT76). The maximum →H+/O ratio (protons translocated out of the bacteria per atom of oxygen consumed during respiration) was about 4 and, except when respiration was markedly affected, this ratio was similar in mutant and wild-type bacteria. The →H+/O ratios were unaltered when the usual oxidase (cytochrome a3) was inhibited by 300μm-KCN and respiration involved the single cytochrome b functioning as an alternative oxidase. Ratios measured in cells respiring endogenous substrate and in cells loaded with malate or 3-hydroxybutyrate suggest that there are two proton-translocating segments operating during the oxidation of NADH. By contrast, during oxidation of formaldehyde or methylamine only one pair of protons is translocated. Proton translocation could not be measured with methanol as substrate, because its oxidation was inhibited (90–95%) by 5mm-KSCN. It is tentatively proposed that the electron-transport chain for NADH oxidation in Pseudomonas AM1 is arranged such that the NADH–ubiquinone oxidoreductase forms one proton-translocating segment and the second segment consists of ubiquinone and cytochromes b and a/a3. The cytochrome c appears to be essential only for respiration and proton translocation from methanol (and possibly from methylamine); there is no conclusive evidence that cytochrome c ever mediates between cytochromes b and a/a3 in Pseudomonas AM1.

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