Analysis of the paramagnetic shifts of haem carbon resonances in bovine ferricytochrome b5.

Recently published chemical shifts for haem 13C nuclei in bovine ferricytochrome b5 (Lee KB, Kweon J, Park H (1995) Assignment of hyperfine-shifted heme carbon resonances in ferricytochrome b5. FEBS Lett. 367:77-80) are analysed in terms of haem molecular orbitals with perturbed D4h symmetry. Since a crystal structure of this protein is available, together with extensive 1H 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 susceptibility tensor. The results are compared with the orientation of the tensor and the geometry of the haem ligands. This comparison casts doubt on one of the 13C assignments and demonstrates that the asymmetry of the haem electronic structure is dominated by the influence of both of the His ligands. The 13C 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.