Electron paramagnetic resonance studies of the soluble cu, protein from the cytochrome ba(3) of Thermus thermophilus

The electron paramagnetic resonance (EPR) spectrum of the binuclear Cu-A center in the water-soluble subunit II fragment from cytochrome ba(3) of Thermus thermophilus was recorded at 3.93, 9.45, and 34.03 GHz, and the EPR parameters were determined by computer simulations. The frequency and M(l) dependence of the linewidth was discussed in terms of g strain superimposed on a correlation between the A and g values, The g values were found to be g(x) = 1.996, g(y) = 2.011, g(z) = 2.187, and the two Cu ions contribute nearly equally to the hyperfine structure, with \A(x)\ approximate to 15 G, \A(y)\ = 29 G, and \A(z)\ = 28.5 G (Cu-65). Theoretical CNDO/S calculations, based on the x-ray structure of the Paracoccus denitrificans enzyme, yield a singly occupied antibonding orbital in which each Cu is pi*-bonded to one S and sigma*-bonded to the other, In contrast to the equal spin distribution suggested by the EPR simulations, the calculated contributions from the Cu ions differ by a factor of 2. However, only small changes in the ligand geometry are needed to reproduce the experimental results.