| Abstract: | The reaction of H2O2 with reduced cytochrome c oxidase was investigated with rapid-scan/stopped-flow techniques. The results show that the oxidation rate of cytochrome a3 was dependent upon the peroxide concentration (k = 2 X 10(4) M-1 X s-1). Cytochrome a and CuA were oxidised with a maximal rate of approx. 20 s-1, indicating that the rate of internal electron transfer was much slower with H2O2 as the electron acceptor than with O2 (k greater than or equal to 700 s-1). Although other explanations are possible, this result strongly suggests that in the catalytic cycle with oxygen as a substrate the internal electron-transfer rate is enhanced by the formation of a peroxo-intermediate at the cytochrome a3-CuB site. It is shown that H2O2 took up two electrons per molecule. The reaction of H2O2 with oxidised cytochrome c oxidase was also studied. It is shown that pulsed oxidase readily reacted with H2O2 (k approximately 700 M-1 X s-1). Peroxide binding is followed by an H2O2-independent conformational change (k = 0.9 s-1). Resting oxidase partially bound H2O2 with a rate similar to that of pulsed oxidase; after H2O2 binding the resting enzyme was converted into the pulsed conformation in a peroxide-independent step (k = 0.2 s-1). Within 5 min, 55% of the resting enzyme reacted in a slower process. We conclude from the results that oxygenated cytochrome c oxidase probably is an enzyme-peroxide complex. |
