The distance between cytochromes a and a3 in the azide compound of bovine-heart cytochrome oxidase

Abstract

The electron-spin relaxation rates of the two species of cytochrome a 3+ 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 +3 3-azide gives rise to two distinct EPR resonances, depending upon the oxidation state of Cyt a. When Cyt a is ferrous, Cyt a 3+ 3-azide has g = 2.88, 2.19 and 1.64; upon oxidation of Cyt a, the a 3+ 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 a 3 could be measured as the difference in microwave field saturation parameter H 1 2 between the g = 2.77 and g = 2.88 species. For each signal the spin-lattice relaxation time T 1 was determined from H 1 2 using the transverse relaxation time T 2. The value of T 2 at 10 K was extrapolated from a plot of line-width vs. temperature at higher temperature. The dipolar contribution to T 1 was related to the Cyt a-Cyt a 3 spin-spin distance utilizing available information on the relative orientation of Cyt a 3-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 g x and g z components of the Cyt a 3-azide g-tensor, the angle between the g z components of the Cyt a and Cyt a 3 g-tensors was determined to be between 0 and 18°, and the Cyt a-Cyt a 3 spin-spin distance was found to be 19 ± 8 A ̊ .