Membrane-bound oxygen reductases of the anaerobic sulfate-reducing Desulfovibrio vulgaris Hildenborough: roles in oxygen defence and electron link with periplasmic hydrogen oxidation

Cytoplasmic membranes of the strictly anaerobic sulfate-reducing bacterium Desulfovibrio vulgaris Hildenborough contain two terminal oxygen reductases, a bd quinol oxidase and a cc(b/o)o(3) cytochrome oxidase (Cox). Viability assays pointed out that single Delta bd, Delta cox and double Delta bd Delta cox deletion mutant strains were more sensitive to oxygen exposure than the WT strain, showing the involvement of these oxygen reductases in the detoxification of oxygen. The Delta cox strain was slightly more sensitive than the Delta bd strain, pointing to the importance of the cc(b/o)o(3) cytochrome oxidase in oxygen protection. Decreased O-2 reduction rates were measured in mutant cells and membranes using lactate, NADH, ubiquinol and menadiol as substrates. The affinity for oxygen measured with the bd quinol oxidase (K-m, 300 nM) was higher than that of the cc(b/o)o(3) cytochrome oxidase (K-m, 620 nM). The total membrane activity of the bd quinol oxidase was higher than that of the cytochrome oxidase activity in line with the higher expression of the bd oxidase genes. In addition, analysis of the Delta bd Delta cox mutant strain indicated the presence of at least one O-2-scavenging membrane-bound system able to reduce O-2 with menaquinol as electron donor with an O-2 affinity that was two orders of magnitude lower than that of the bd quinol oxidase. The lower O-2 reductase activity in mutant cells with hydrogen as electron donor and the use of specific inhibitors indicated an electron transfer link between periplasmic H-2 oxidation and membrane-bound oxygen reduction via the menaquinol pool. This linkage is crucial in defence of the strictly anaerobic bacterium Desulfovibrio against oxygen stress.