Cytochrome a 3 Structure in Carbon Monoxide-Bound Cytochrome Oxidase

The iron-carbon monoxide stretching mode and the iron-carbon-oxygen bending mode in carbon monoxide-bound cytochrome oxidase have been assigned at 520 and 578 cm -1 , respectively. The frequencies, widths, and intensities of these modes show that the Fe-C-O grouping in carbon monoxide-cytochrome a 3 is linear but tilted from the normal to the heme plane; that the iron-histidine bond in both five- and six-coordinate cytochrome a 3 is strained; and that the carbon monoxide and the proximal histidine each have characteristic, well-defined orientations in all molecules. These data can account for the binding affinities of carbon monoxide and dioxygen under physiological conditions.