Studies on partially reduced mammalian cytochrome oxidase. Reactions with carbon monoxide and oxygen

摘要

A number of methods were used to prepare a species of mammalian cytochrome oxidase (EC 1.9.3.1, ferrocytochrome c–oxygen oxidoreductase) in which only cytochrome a3 is reduced and in combination with CO. The kinetics of CO binding by cytochrome a3²⁺ in this species is significantly different from that exhibited by cytochrome a3²⁺ in the fully reduced enzyme. The second-order rate constant for combination was 5×10⁴m⁻¹·s⁻¹ and the `off' constant was 3×10<sup>−2</sup>s<sup>−1</sup>. The kinetic difference spectra cytochrome a3<sup>2+</sup>–cytochrome a3<sup>2+</sup>–CO reveal further differences between the mixed-valence and the fully reduced enzyme. The reaction between cytochrome a3<sup>2+</sup> and oxygen in the mixed-valence species was followed in flow–flash experiments and reveals a fast, oxygen-dependent (8×10<sup>7</sup>m<sup>−1</sup>·s<sup>−1</sup> at low oxygen) rate followed by a slow process, whose rate is independent of oxygen but whose amplitude is dependent on [O2]. The fast oxygen-dependent reaction yields as the first product the so-called `oxygenated' enzyme. We conclude from these experiments that the ligand-binding behaviour of cytochrome a3 depends on the redox state of its partners, a fact which represents clear evidence for site–site interaction in this enzyme. The fact that oxygen reacts rapidly with this enzyme species in which only one component, namely cytochrome a3, is reduced represents clear and unequivocal evidence that this is indeed the O2-binding site in cytochrome oxidase and may indicate that reduction of oxygen can proceed via single electron steps.