Near-Infrared Time-Resolved Optical Absorption Studies of the Reaction of Fully Reduced Cytochrome c Oxidase with Dioxygen

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Abstract
Electron transfer during the reaction of fully reduced bovine heart cytochrome oxidase with dioxygen has been studied at 24 °C in the near-infrared region following photolysis of the fully reduced CO-bound complex. The transient spectral changes and kinetics were followed on microsecond to millisecond time scales at nine different wavelengths between 597 and 935 nm and were analyzed using singular value decomposition and global exponential fitting. Four apparent lifetimes, 14 μs, 40 μs, 86 μs, and 1.1 ms, were resolved. The near-infrared spectra of the intermediates are extracted on the basis of a previously proposed mechanism [Sucheta et al. (1998) Biochemistry37, 17905−17914] and compared to model spectra of the postulated intermediates. The data provide a comprehensive picture of the spectral contributions of the different redox centers in their respective oxidation or ligation states in the near-infrared region and strongly support that CuA is partially (2/3), but not fully, oxidized in the 3-electron-reduced ferryl intermediate.