The Light-reversible Binding of Carbon Monoxide to Cytochrome a 1 in Escherichia coli K12

Difference spectra at 77 K of intact Escherichia coli K12 grown under oxygen-limited conditions revealed the presence of cytochrome a 1. 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 a 1 Photolysis in the presence of O2 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 O2 to cytochrome a 1. 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-1 (45.6 kJ mol-1). The results are discussed in relation to previous assumptions and results regarding ligand binding to cytochrome a 1 and the function of this cytochrome in bacterial respiration.