Biochemical and biophysical studies on cytochrome aa3. IV. Some properties of oxygenated cytochrome aa3

Abstract

1. Our data support the idea that the so-called ‘oxygenated’ cytochrome aa 3 is no oxygenated or peroxidic compound, but a different conformational state of the oxidized enzyme. 2. The conversion of oxygenated to oxidized cytochrome c oxidase obeys first-order kinetics ( k = 5 · 10 −4 sec −1) and has an activation energy of 11.5 kcal/mole. 3. Under conditions where the oxidized enzyme is rapidly reduced by NADH plus phenazine methosulphate, the oxygenated form does not react at all. 4. By means of an anaerobic titration technique the oxygenated form, like the oxidized enzyme, was shown to consume 4 electron equivalents per molecule (or 2 per haem) for complete reduction. 5. The addition under anaerobic conditions of only one electron per mole oxidized cytochrome aa 3, followed by aeration, results in nearly quantitative formation of the oxygenated compound. 6. Aeration experiments with partly and fully reduced cytochrome aa 3 indicate that only the latter is immediately oxidized by oxygen. In both cases the final product is the oxygenated compound.