A switch in the electron transfer from heme a to binuclear centre of cytochrome c oxidase

New experimental evidence that a switch controls the reduction of the heme a₃－Cu_B binuclear centre has been observed in the N₂-dried thin film of purified cytochrome oxidase. When immersing the enzyme film into the acid phosphate buffer with extremely low concentration of dithionite, a spectrum was given to show a reduction of heme a with no electrons resting on Cu_A. By increasing dithionite, electrons could be accumulated gradually on Cu_A, but the binuclear centre still remains in the oxidized state. When the accumulation of electrons on Cu_A and/or heme a exceeded a threshold, a turnover of reduction of the binuclear centre and oxidation of heme a occurred abruptly. This switch-like action is pH-dependent.