Structure of cytochrome a3-Cua3 couple in cytochrome c oxidase as revealed by nitric oxide binding studies.

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Abstract
The addition of NO to oxidized [beef heart] cytochrome c oxidase (ferrocytochrome c:oxygen oxidoreductase, EC 1.9.3.1) causes the appearance of a high-spin heme electron paramagnetic resonance (EPR) signal due to cytochrome a3. This suggests that NO coordinates to .**GRAPHIC**. and breaks the antiferromagnetic couple by forming a cytochrome .**GRAPHIC**. complex. The intensity of the high-spin cytochrome a3 signal depends on the method of preparation of the enzyme and maximally accounts for 58% of 1 heme. The effect of .**GRAPHIC**. on the cytochrome .**GRAPHIC**. complex is to reduce cytochrome a3 to the ferrous state, and this is followed by formation of a new complex that exhibits EPR signals characteristic of a triplet species. On the basis of optical and EPR results, a NO bridge between cytochrome .**GRAPHIC**. and .**GRAPHIC**. is proposed.sbd.i.e., cytochrome .**GRAPHIC**. The half-field transition observed at g = 4.34 in the EPR spectrum of this triplet species exhibits resolved copper hyperfine splittings with A.dblvert. = 0.020/cm, indicating that the .**GRAPHIC**. in the cytochrome .**GRAPHIC**. complex is similar to a type 2 copper site.