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 E. coli K12 contained high levels of the CO-binding cytochromes d, o and a1. In photodissociation difference spectra (i.e., photolysed minus reduced + CO), a peak at 436 nm and a trough at 415 nm were 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 o436 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.degree. 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-1 (10.7 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 2nd-order constant for this reaction was 10.9 M-1 s-1 at -100.degree. C, at least 10-fold greater than for the reaction of cytochrome o432 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 a3. As does the synthesis of cytochrome d, the formation of cytochrome o436 may represent an adaptation of the organism to reduced O2 availability.