The distance between cytochromes a and a3 in the azide compound of bovine-heart cytochrome oxidase |
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Authors: | Gay Goodman and John S Leigh Jr |
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Institution: | Department of Biochemistry and Biophysics, School of Medicine G4, University of Pennsylvania, Philadelphia, PA, U.S.A. |
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Abstract: | The electron-spin relaxation rates of the two species of cytochrome a3+3-azide found in the azide compound of bovine-heart cytochrome oxidase were measured by progressive microwave saturation at T = 10 K. It has been shown previously that Cyt a+33-azide gives rise to two distinct EPR resonances, depending upon the oxidation state of Cyt a. When Cyt a is ferrous, Cyt a3+3-azide has g = 2.88, 2.19 and 1.64; upon oxidation of Cyt a, the a3+3-azide g-values become g = 2.77, 2.18, and 1.74 (Goodman, G. (1984) J. Biol. Chem. 259, 15094–15099). The relaxation effect of Cyt a on Cyt a3 could be measured as the difference in microwave field saturation parameter H1/2 between the g = 2.77 and g = 2.88 species. For each signal the spin-lattice relaxation time T1 was determined from H1/2 using the transverse relaxation time T2. The value of T2 at 10 K was extrapolated from a plot of line-width vs. temperature at higher temperature. The dipolar contribution to T1 was related to the Cyt a-Cyt a3 spin-spin distance utilizing available information on the relative orientation of Cyt a3-azide and Cyt a (Erecinska, M., Wilson, D.F. and Blasie, J.K. (1979) Biochim. Biophys. Acta 545, 352–364). By taking into account the relaxation parameters for both gx and gz components of the Cyt a3-azide g-tensor, the angle between the gz components of the Cyt a and Cyt a3g-tensors was determined to be between 0 and 18°, and the Cyt a-Cyt a3 spin-spin distance was found to be 19 ± 8 Å. |
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Keywords: | Cytochrome oxidase Cytochrome a Cytochrome a3 Spin-spin distance ESR (Bovine heart) |
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