ИСТИНА

Catalytic cycle of cytochrome c oxidase (COX) comprises two sequential chemical reactions: an oxidase phase that involves 2 electron reduction of the molecular oxygen to bound peroxide, and a peroxidase phase in which the bound peroxide converts the oxidized enzyme to Compound I (a34+=O2-, R•) which is then reduced in 2 single electron steps, first to Compound II (a34+=O2-) and then to the resting oxidized state (a33+-OH-). The peroxidase phase is much more exergonic and provides the major part of free energy for proton pumping by COX. An operative model of COX electrogenic mechanism is considered including the following elements. 1. Proton translocation in the F>O (and perhaps P>F) transition takes place well after electron transfer from heme a to heme a3 and therefore implies protein relaxation mechanism to be involved in proton pumping. The proton pumping step, and presumably protein relaxation, is specifically inhibited by Zn2+. 3. The protonic K-channel is required for proton uptake in the eu-oxidase half reaction. Two protons are delivered for charge compensation when heme a3 and CuB are reduced and 2 more protons are taken up for water formation during heterolytic cleavage of the O-O bond in the iron-peroxy complex of heme a3. 4. Proton-assisted heterolytic cleavage of the O-O bond in the transient bound peroxide state results in formation of a strong oxene ligand at ferryl heme a3 which pulls the heme iron in the distal direction and triggers a conformational transition of COX from the eu-oxidase conformation (K-channel open) into the peroxidase conformation (D-channel open). 5.The D-channel is required for the uptake of the 4 protons pumped during the peroxidase phase of the reaction.