Resolution of two compound C-type intermediates in the reaction with oxygen of mixed-valence state membrane-bound cytochrome oxidase

The reaction of mixed-valence state membrane-bound cytochrome oxidase with oxygen has been studied by difference spectroscopy with reference to the unliganded state and by the low temperature technique of Chance and coworkers. Three intermediates, compound A2 and two compound C-type components denoted C606 and C610, have been resolved in time and wavelength in the alpha region. Their optical properties are defined in the visible range. Compound A2 disappearance and compound C606 formation exhibit first-order kinetics with identical rate constants: 2.4 . 10(-3) s-1 at -94 degrees C. Compound A2 has its alpha band maximum at 590 nm and shares an isosbestic point at 595 nm with the C606 species. The alpha band of this intermediate peaks at 606 nm. Compound C610 is the real end point of the reaction and its alpha band maximum appears at 610 nm. Compound C606 is interpreted as resulting from the transfer of one electron from heme alpha 3 copper to oxygen and compound C610 as expressing a molecular reorganization due to the effect of the temperature. Structural requirements for the location of CuB in the active site are discussed. It is concluded that the three observed compounds are the only intermediates formed in the reaction between oxygen and mixed-valence state membrane-bound cytochrome oxidase.