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

T. H. Stevens, Gary W Brudvig, D. F. Bocian, S. I. Chan

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The addition of NO to oxidized cytochrome c oxidase (ferrocytochrome c:oxygen oxidoreductase, EC causes the appearance of a high-spin heme electron paramagnetic resonance (EPR) signal due to cytochrome a3. This suggests that NO coordinates to Cu+2a3 and breaks the antiferromagnetic couple by forming a cytochrome a+33-Cu+2a3-NO 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 one heme. The effect of N-3 on the cytochrome a+33-Cu+2a3-NO 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 a+23 and Cu+2a3 is proposed--i.e., cytochrome a+23-NO-Cu+2a3. The half-field transition observed at g = 4.34 in the EPR spectrum of this triplet species exhibits resolved copper hyperfine splittings with [A+2] = 0.020 cm-1, indicating that the Cu+2a3 in the cytochrome a+23-NO-Cu+2a3 complex is similar to a type 2 copper site.

Original languageEnglish
Pages (from-to)3320-3324
Number of pages5
JournalProceedings of the National Academy of Sciences of the United States of America
Issue number7
Publication statusPublished - Jul 1979


ASJC Scopus subject areas

  • General
  • Genetics

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