Generation of protonic potential by the bd-type quinol oxidase of Azotobacter vinelandii

AbstractInside-out subcellular vesicles of Azotobacter vinelandii are found to produce ΔpH and ΔΨ (interior acidic and positive) when oxidising malate or menadiol. These effects are inherent in both Cyd+Cyo− (lacking the o-type oxidase) and Cyd+Cyo− (lacking the bd-type oxidase) strains. They appear to be myxothiazol-sensitive in the Cyd−Cyo+ strain but not in the Cyd+Cyo− strain. The H+/e− ratio for the terminal part of respiratory chain of a bd-type oxidase overproducing strain is established as being close to 1. It is also shown that NADH oxidation by the vesicles from the Cyd−Cyo+ strain is sensitive to low concentrations of myxothiazol and antimycin A whereas that of the Cyd+Cyo− strain is resistant to these Q-cycle inhibitors. It is concluded that (i) the bd-type oxidase of A. vinelandii is competent in generating a protonic potential but its efficiency is lower than that of the o-type oxidase and (iii) Q-cycle does operate in the o-type cytochrome oxidase terminated branch of the A. vinelandii respiratory chain and does not in the bd-type quinol oxidase terminated branch. These relationships are discussed in the context of the respiratory protection function of the bd-type oxidase in A. vinelandii