ANALYSIS OF THE PARAMAGNETIC SHIFTS OF HEME CARBON RESONANCES IN BOVINE FERRICYTOCHROME B(5)

Recently published chemical shifts for haem C-13 nuclei in bovine ferr icytochrome b(5) (Lee KB, Kweon J, Park H (1995) Assignment of hyperfi ne-shifted heme carbon resonances in ferricytochrome b(5). FEBS Lett. 367:77-80) are analysed in terms of haem molecular orbitals with pertu rbed D-4h symmetry. Since a crystal structure of this protein is avail able, together with extensive H-1 assignments both in the oxidised and reduced forms, the paramagnetic shifts can be separated into dipolar and Fermi contact contributions by using an empirical magnetic suscept ibility tenser. The results are compared with the orientation of the t ensor and the geometry of the haem ligands. This comparison casts doub t on one of the C-13 assignments and demonstrates that the asymmetry o f the haem electronic structure is dominated by the influence of both of the His ligands. The C-13 chemical shifts of two haem methyl groups in the minor form of the protein, in which the haem is approximately rotated by 180 degrees about its 5CH-15CH axis, are also evaluated.