The Light-reversible Binding of Carbon Monoxide to Cytochrome a1 in Escherichia coli K12

Difference spectra at 77 K of intact Escherichia coli K 12 grown under oxygen-limited conditions revealed the presence of cytochrome a1. In the presence of CO, the band of the reduced form, observed in both the alpha and gamma regions of the spectrum, was decreased. Dual-wavelength scanning spectrophotometry at sub-zero temperatures revealed a flash-dissociable CO-binding pigment with a broad band around 595 nm, identified as cytochrome alpha 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 a1. Repeated cycles of photolysis and recombination of the cytochrome with CO were demonstrated at --46 degrees 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 a1 and the function of this cytochrome in bacterial respiration.