Time-resolved single-turnover of caa₃ oxidase from Thermus thermophilus. Fifth electron of the fully reduced enzyme converts O_H into E_H state

The oxidative part of the catalytic cycle of the caa₃-type cytochrome c oxidase from Thermus thermophilus was followed by time-resolved optical spectroscopy. Rate constants, chemical nature and the spectral properties of the catalytic cycle intermediates (Compounds A, P, F) reproduce generally the features typical for the aa₃-type oxidases with some distinctive peculiarities caused by the presence of an additional 5-th redox-center-a heme center of the covalently bound cytochrome c. Compound A was formed with significantly smaller yield compared to aa₃ oxidases in general and to ba₃ oxidase from the same organism. Two electrons, equilibrated between three input redox-centers: heme a, Cu_A and heme c are transferred in a single transition to the binuclear center during reduction of the compound F, converting the binuclear center through the highly reactive O_H state into the final product of the reaction-E _H (one-electron reduced) state of the catalytic site. In contrast to previous works on the caa₃-type enzymes, we concluded that the finally produced E_H state of caa₃ oxidase is characterized by the localization of the fifth electron in the binuclear center, similar to the O_H → E_H transition of the aa₃-type oxidases. So, the fully-reduced caa₃ oxidase is competent in rapid electron transfer from the input redox-centers into the catalytic heme-copper site.