Summary: The form of cytochrome in and that shows an absorption maximum at 648 to 652 nm (‘cytochrome ’) is generally regarded as the oxidized form of this terminal oxidase. Membranes from grown under oxygen-limited conditions, when treated with ferricyanide, do not reveal cytochrome , whereas a sharp symmetrical band at 652 nm results from the reaction of the reduced enzyme with O at either room temperature or after flash photolysis of the CO-liganded form at — 130 °C. Electron paramagnetic resonance spectroscopy of cytochrome trapped at — 130 °C shows that its spectrum is indistinguishable from the CO-liganded form and does not reveal resonances of high spin ferric haem previously attributed to cytochrome An hypothesis is proposed in which cytochrome is an early intermediate in the reaction of reduced cytochrome with oxygen and is not the fully-oxidized (ferric) species. An analogy between cytochrome and oxyhaemoglobin is presented and the hypothesis discussed in relation to earlier work, in which the indirect interconversions of reduced cytochrome and have been explained by proposing the existence of an ‘invisible’ form. It is suggested that this form could be the oxidized enzyme.


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