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Infrared characterization of nitric oxide bonding to bovine heart cytochrome c oxidase and myoglobin.

The stable binding of nitric oxide to both Cu+B and Fe2+a3 in cytochrome c oxidase is shown in infrared spectra. The N-O stretch band for Fe2+a3-NO at 1610 cm-1 is similar to the band for MbNO at 1612 cm-1. The Cu+B-NO band is at 1700 cm-1. Thus, electron donation from metal to NO is greater with Fe2+a3 than Cu+B. However, the affinity for NO is only slightly greater at Fe2+a3 than at Cu+B. In contrast CO binds stably only to Fe2+a3. Infrared spectra of oxidase, myoglobin and hemoglobin NO and CO complexes are consistent with only one stable protein structure at the ligand binding site of nitrosyls and with dynamic multiple protein conformers at carbonyl sites. The binding of NO to both Cu+B and Fe2+a3 of cytochrome c oxidase can contribute to NO cytotoxicity and to the catalysis of NO reduction to N2O and makes possible the effective use of infrared spectroscopy in investigations of Cu as well as Fe at the binuclear O2 reduction sites of oxidases.