Studies of the heme components of cytochrome c oxidase by EPR spectroscopy

Cytochrome c oxidase was studied by electron paramagnetic resonance (EPR) spectroscopy at different states of oxidation in the presence or absence of specific ligands. The EPR signals observed are described, and an attempt is made to quantitatively evaluate and assign them to components of cytochrome c oxidase. According to this only cytochrome a and part of the copper were detected by EPR in the oxidized form of the enzyme. The reduced form was devoid of significant signals. At intermediate states of oxidation, cytochrome a3 was manifest in its ferric form by a signal typical of high-spin ferric iron. This indicates that in the oxidized form of the enzyme, ferricytochrome a3 interacted with another component, viz. another molecule of ferricytochrome a3 or the EPR-undetectable fraction of the copper, so as to result in a decreased paramagnetism of the system. Strong ligands such as cyanide or azide prevent the intermediate appearance of cytochrome a3 in the high-spin state; in the presence of azide, a low-spin ferric intermediate was detected. The changes in absorbance at 605 mμ and of the EPR signals during complete titrations of the enzyme with NADH and phenazine methosulfate or dithionite have been measured. Whereas the absorbance at 605 mμ measured at room temperature appeared to decrease linearly with added reductant, the EPR-detectable copper was not reduced initially except in the presence of cyanide. Cytochrome a was reduced in a linear fashion, while the EPR-detectable form of ferricytochrome a3 accumulated and was reduced after cytochrome a. In the presence of azide, an EPR-detectable low-spin form of ferricytochrome a3 accumulated, which was not readily reducible. The high-spin form of ferricytochrome a3 was also seen on rapid (> 10 msec) reduction of cytochrome c oxidase by ferricytochrome c or dithionite; it was not seen (> 10 msec) on rapid reoxidation of fully reduced cytochrome c oxidase. These findings support the concept of the separate existence of cytochromes a and a3, and indicate that cytochrome a and the EPR-undetectable copper are titrated first, at which point ferricytochrome a3 becomes detectable by EPR. The EPR-detectable copper and cytochrome a3 are titrated thereafter.