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The Reaction of Cytochrome o in Escherichia coli K12 with Oxygen. Evidence for a Spectrally and Kinetically Distinct Cytochrome o in Cells from Oxygen-limited Cultures
Abstract
Intact cells harvested from O2-limited batch cultures of Escherichia coli K12 contained high levels of the CO-binding cytochromes d, o and a 1. In photodissociation difference spectra (i.e. photolysed minus reduced + CO), a peak at 436 nm and a trough at 415 nm have been assigned to an o-type cytochrome, and not cytochrome d, by photolysis with white light and an He-Ne laser. The reaction of reduced cytochrome o 436 with O2 at sub-zero temperatures involved O2 binding to give intermediate(s) with spectral characteristics similar to those of the reduced oxidase-CO complex. The reaction with O2 at successively higher temperatures (range − 98 to − 59 °C) was accompanied by the formation of a trough (with reference to the CO-liganded state) at 436 nm which eventually shifted to 432 nm, indicative of the oxidized form. The apparent energy of activation at low temperatures was 446; kJ mol−1 (107; kcal mol−1). There was a linear relationship between the rate of formation of the oxygen compound and the O2 concentration up to about 0·5; mm. The second-order constant for this reaction was 109; m−1 s−1 at -100C, at least 10-fold greater than for the reaction of cytochrome o 432 with O2 in cells from vigorously aerated cultures. The reaction of both types of cytochrome o with O2 was not readily reversible in the light or in the dark and was further distinguished from the reaction with CO by the markedly lower velocity of the CO reaction. Comparisons are drawn between the reactions with O2 of cytochrome(s) o in E. coli from O2-sufficient and O2-limited cultures and of mitochondrial cytochrome a 3. It is proposed that, like the synthesis of cytochrome d, the formation of cytochrome o 436 represents an adaptation of the organism to reduced O2 availability.
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© Society for General Microbiology 1981