The 650 nm Chromophore in Escherichia coli is an 'Oxy-' or Oxygenated Compound, Not the Oxidized Form of Cytochrome Oxidase d: An Hypothesis

Publisher Website
Google Scholar
Abstract
The form of cytochrome d in E. coli and Azotobacter vinelandii that shows an absorption maximum at 648-652 nm (cytochrome d650) is generally regarded as the oxidized form of this terminal oxidase. Membranes from E. coli grown under O2-limited conditions, when treated with fericyanide, do not reveal cytochrome d650, whereas a sharp symmetrical band at 652 nm results from the reaction of the reduced enzyme with O2 at room temperature or after flash photolysis of the CO-liganded form at -130.degree. C. EPR spectroscopy of cytochrome d650 trapped at -130.degree. C shows that its spectrum is indistinguishable from the CO-liganded form and does not reveal resonances of high spin ferric heme previously attributed to cytochrome d. An hypothesis is proposed in which cytochrome d650 is an early intermediate in the reaction of reduced cytochrome d with O2 and is not the fully-oxidized (ferric) species. An analogy between cytochrome d650 and oxyhemoglobin is presented and the hypothesis discussed in relation to earlier work, in which the indirect interconversions of reduced cytochrome d and d650 are explained by proposing the existence of an invisible form. This form could be the oxidized enzyme.