Cytochrome a₃ 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 03 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.