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

Abstract

The structural properties of a key transient oxygen intermediate of cytochrome c oxidase, P_R, remain an enigma, although inferences have been drawn from its equilibrium analogues,P_CO/O2 P_H and P_M. With resonance Raman spectroscopy, an oxygen isotope‐sensitive band at 806 cm⁻¹ was observed in P_CO/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 P_CO/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 P_R intermediate.