Figure 5.

Proposed scheme for a cycle of photolysis and recombination of CO to the heme-copper cytochrome oxidase caa₃ from T. thermophilus (A2 family) in the two-electron-reduced mixed-valence (MV) and fully reduced (R⁴) states. In the R⁴ state, photodissociation of CO from the ferrous heme a₃ iron (Fe²⁺_a3) is followed by its rebinding to the heme with τ of 14 ms. In the MV state, in addition to the CO rebinding, there is a transient back-flow of an electron from the binuclear center to the input redox center (heme a, Cu_A, and cytochrome c) with τ of 0.04 ms. Intermediate binding of CO to Cu_B is not shown. The scheme is based on time-resolved spectroscopic studies [26,135].