A ferrous oxygenated form of cytochrome is characteristic of all cytochrome -type oxidases so far examined, but its participation in enzyme turnover is unclear. It is relatively stable, occurs in aerated cell suspensions and predominates during enzyme preparation. In this study, diode-array reflectance spectrophotometry was used to assess the redox poise and oxygenation of cytochrome , in the aerobic diazotroph . Mutants either lacking or overproducing the cytochrome oxidase were used to confirm the reliability of the optical configuration. Changes in absorbance attributed to cytochromes and were followed as the O supply was altered either in suspensions of harvested cells or during steady-state growth. In washed cell suspensions, three states of cytochrome , which differed in absorbance characteristics, were seen: (1) an oxygenated form that absorbs at 650 nm, (2) a form which has little absorbance at either 650 or 630 nm and (3) the reduced form that absorbs at 630 nm. The transition between states 2 and 3, but not 1 and 2, correlated with the changes in the redox states of cytochromes and . The dissolved O concentration at which this transition occurred coincided approximately with the apparent O affinity for the oxidase (approx. 5 μM). During steady-state growth, the cytochromes were partially reduced and the oxygenated form of cytochrome was undetected. These measurements support the view that an oxygenated form of cytochrome (absorbing at 650 nm) in the one-electron-reduced cytochrome -type oxidase does not take part in enzyme turnover.


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