The Rate-Limiting Step in O2 Reduction by Cytochrome ba3 from Thermus thermophilus

Abstract Cytochrome ba 3 ( ba 3 ) of Thermus thermophilus ( T . thermophilus ) is a member of the heme–copper oxidase family, which has a binuclear catalytic center comprised of a heme (heme a 3 ) and a copper (Cu B ). The heme–copper oxidases generally catalyze the four electron reduction of molecular oxygen in a sequence involving several intermediates. We have investigated the reaction of the fully reduced ba 3 with O 2 using stopped-flow techniques. Transient visible absorption spectra indicated that a fraction of the enzyme decayed to the oxidized state within the dead time (~ 1 ms) of the stopped-flow instrument, while the remaining amount was in a reduced state that decayed slowly ( k  = 400 s − 1 ) to the oxidized state without accumulation of detectable intermediates. Furthermore, no accumulation of intermediate species at 1 ms was detected in time resolved resonance Raman measurements of the reaction. These findings suggest that O 2 binds rapidly to heme a 3 in one fraction of the enzyme and progresses to the oxidized state. In the other fraction of the enzyme, O 2 binds transiently to a trap, likely Cu B , prior to its migration to heme a 3 for the oxidative reaction, highlighting the critical role of Cu B in regulating the oxygen reaction kinetics in the oxidase superfamily. This article is part of a Special Issue entitled: Respiratory Oxidases.