Figure 1

Experimental system and the studied reaction. (a) CytcO accommodates four redox-active centers: Cu_A, heme a and the catalytic site composed of heme a₃ and Cu_B, that upon reduction each bind one electron. The cyt. c-CytcO electrostatic complex accommodates in total five electrons in the reduced state. Two proton-uptake pathways, called D and K are indicated in the figure, but not discussed in the text. (b) Turnover of CytcO. One electron at a time is donated from cyt. c to CytcO. The initial state is the “relaxed” oxidized state. Consecutive addition of two electrons yields the two-electron reduced catalytic site which binds O₂ to form the peroxy state P_M. Reduction of P_M yields F, which is then reduced to form the “activated” oxidized state, O_H. This state is then reduced by cyt. c. (c) Reaction studied in this work. The oxidized cyt. c-CytcO complex is reduced by 5 electrons. Upon initiation of the reaction (with a laser flash, not shown), O₂ binds to the catalytic site and an electron is transferred from heme a to the catalytic site to form the peroxy state, P_R. This step is followed in time by formation of the ferryl state, F, which occurs over the same time scale as electron equilibration among Cu_A/cyt. c and heme a. Electron transfer from this equilibrium to the catalytic site leads to formation of state O_H. Over a longer time scale of ~0.1 s an electron equilibrates between the CytcO molecules via the bound cyt. c to eventually yield oxidized CytcO. Note that proton-transfer reactions are not indicated in the Figure.