[Oxygenated cytochrome bd from Escherichia coli could be transformed into an oxidized form by lipophilic electron...

The cytochrome bd complex as isolated from Escherichia coli under aerobic conditions is in a stable oxygenated form [formula: see text], characterized by an intense peak at 650 nm in an absolute absorption spectrum. The commonly used oxidants ferricyanide and persulfate have no effect on the oxygenated form, whereas the addition of lipophilic electron acceptors, such as tetrachlorobenzoquinone or ferricinium, results in the decay of the heme d oxy-complex and the enzyme transition into the fully oxidized form, [formula: see text]. Interaction of the oxygenated cytochrome bd complex with both tetrachlorobenzoquinone and ferricinium is suppressed by pentachlorophenol, an inhibitor of the enzyme ubiquinol oxidase activity. It is suggested that redox centers of cytochrome bd reside in the hydrophobic environment which can prevent their interaction with the hydrophilic oxidants.