The microbial metabolism of C1 compounds. The stoicheiometry of respiration-driven proton translocation in Pseudomonas AM1 and in a mutant lacking cytochrome c

Abstract

The role of cytochrome c in Pseudomonas AM1 is clarified 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 .fwdarw. H+/O ratio (protons translocated out of the bacteria per atom of O2 consumed during respiration) was about 4 and, except when respiration was markedly affected, this ratio was similar in mutant and wild-type bacteria. The .fwdarw. H+/O ratios were unaltered when the usual oxidase (cytochrome a3) was inhibited by 300 .mu.M-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 2 proton-translocating segments operating during the oxidation of NADH. During oxidation of formaldehyde or methylamine only 1 pair of protons is translocated. Proton translocation could not be measured with methanol as substrate, because its oxidation was inhibited (90-95%) by 5 mM-KSCN. It is tentatively proposed that the electron-transport chain for NADH oxidation in Pseudomonas AM1 is arranged such that the NADH-ubiquinone oxido-reductase forms 1 proton-translocating segment and the 2nd 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.