Time-resolved O H → E H transition of the aberrant ba3 oxidase from Thermus thermophilus

Abstract

The kinetics of single-electron injection into the oxidized nonrelaxed state (O H → E H transition) of the aberrant ba 3 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 Cu A and reduction of the heme groups (low-spin heme b and high-spin heme a 3 in a ratio close to 50:50). The subsequent component ( τ ∼ 420 μs) includes reoxidation of both hemes by Cu B. This is in significant contrast to the O H → E H transition of the aa 3-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 ba 3 oxidase was similar to that in aa 3 oxidase and only included rapid electron transfer from Cu A to the low-spin heme b. This indicates a functional difference between the relaxed O and the pulsed O H forms also in the ba 3 oxidase from T. thermophilus.