Structural characterization of the PCO/O2PCO/O2 compound of cytochrome c oxidase

The structural properties of a key transient oxygen intermediate of cytochrome c oxidase, PR, remain an enigma, although inferences have been drawn from its equilibrium analogues, PCO/O2PCO/O2, PH and PM. With resonance Raman spectroscopy, an oxygen isotope-sensitive band at 806 cm−1 was observed in PCO/O2PCO/O2 produced by adding CO and O2 to the resting enzyme. The vibrational band shifted to 771 cm−1 upon isotopic substitution of 16O2 with 18O2. The same modes at 806 and 771 cm−1 were present simultaneously when the mixed isotope, 18O16O, was employed, indicating that in PCO/O2PCO/O2 the O–O bond is cleaved, resulting in a Fe4+O2− structure. This result unifies the nature of the three equilibrium analogues of the PR intermediate.