Time-resolved OH → EH transition of the aberrant ba3 oxidase from Thermus thermophilus

The kinetics of single-electron injection into the oxidized nonrelaxed state (OH → EH transition) of the aberrant ba3 cytochrome oxidase from Thermus thermophilus, noted for its lowered efficiency of proton pumping, was investigated by time-resolved optical spectroscopy. Two main phases of intraprotein electron transfer were resolved. The first component (τ ∼ 17 μs) reflects oxidation of CuA and reduction of the heme groups (low-spin heme b and high-spin heme a3 in a ratio close to 50:50). The subsequent component (τ ∼ 420 μs) includes reoxidation of both hemes by CuB. This is in significant contrast to the OH → EH transition of the aa3-type cytochrome oxidase from Paracoccus denitrificans, where the fastest phase is exclusively due to transient reduction of the low-spin heme a, without electron equilibration with the binuclear center. On the other hand, the one-electron reduction of the relaxed O state in ba3 oxidase was similar to that in aa3 oxidase and only included rapid electron transfer from CuA to the low-spin heme b. This indicates a functional difference between the relaxed O and the pulsed OH forms also in the ba3 oxidase from T. thermophilus.