Difference spectra at 77 K of intact K12 grown under oxygen-limited conditions revealed the presence of cytochrome . In the presence of CO, the band of the reduced form, observed in both the α and γ regions of the spectrum, was decreased. Dualwavelength scanning spectrophotometry at sub-zero temperatures revealed a flash-dissociable CO-binding pigment with a broad band around 595 nm, identified as cytochrome Photolysis in the presence of O revealed no such band in difference spectra where the reference spectrum was that of the CO-liganded form, a result consistent with the binding of O to cytochrome . Repeated cycles of photolysis and recombination of the cytochrome with CO were demonstrated at -46 °C. The apparent energy of activation for the reaction with CO was 10.9 kcal mol (45.6 kJ mol). The results are discussed in relation to previous assumptions and results regarding ligand binding to cytochrome and the function of this cytochrome in bacterial respiration.


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