Tetraheme cytochrome c-554 from Nitrosomonas europaea. Heme-heme interactions and ligand binding

K. K. Andersson, J. D. Lipscomb, M. Valentine, E. Münck, A. B. Hooper

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Abstract

Cytochrome c-554 functions in the ammonia oxidizing system of Nitrosomonas europaea. We have investigated its molecular and ligand binding properties and studied the protein with optical, EPR, and Mossbauer spectroscopies in the pH range from 2 to 13. Amino acid, heme, and metal analyses show that the protein has m(r) = 25,000 and that it contains four c-type hemes per molecule. Optical spectra reveal that the heme ligand structures are sensitive to the pH of the medium and that the hemes can bind small molecules such as CN-, CO, and NO under certain conditions. According to the Mossbauer and EPR studies of the ferric protein, the hemes are predominantly (75%) high spin at pH 2 and low spin (≃100%) above pH 10. At neutral pH, Mossbauer data show that 75% of the heme is low spin and that the remainder is high spin. The EPR data, however, do not reveal any signals attributable to typical high spin or low spin species. Rather, a very complex and unusual spectrum with a main feature at g = 3.3 is observed at X-band, this feature shifts to approximately g = 3 at S-band. The EPR and Mossbauer data show clearly that the hemes are magnetically interacting, by dipolar and exchange interactions. At pH 2, the EPR spectra reveal resonances at g = 6 and 2. The Mossbauer spectra prove that all hemes are magnetically coupled at this pH. Coupling is also borne out by the observation of a half-field EPR resonance near g = 12.

Original languageEnglish (US)
Pages (from-to)1126-1138
Number of pages13
JournalJournal of Biological Chemistry
Volume261
Issue number3
StatePublished - Jul 9 1986

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