Intact cells harvested from O-limited batch cultures of K12 contained high levels of the CO-binding cytochromes and . 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 -type cytochrome, and not cytochrome , by photolysis with white light and an He-Ne laser. The reaction of reduced cytochrome with O at sub-zero temperatures involved O binding to give intermediate(s) with spectral characteristics similar to those of the reduced oxidase-CO complex. The reaction with O 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 44·6; kJ mol (10·7; kcal mol). There was a linear relationship between the rate of formation of the oxygen compound and the O concentration up to about 0·5; mM. The second-order constant for this reaction was 10·9; m s at -100°C, at least 10-fold greater than for the reaction of cytochrome with O in cells from vigorously aerated cultures. The reaction of both types of cytochrome with O 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 O of cytochrome(s) in from O-sufficient and O-limited cultures and of mitochondrial cytochrome . It is proposed that, like the synthesis of cytochrome , the formation of cytochrome represents an adaptation of the organism to reduced O availability.


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