Determination of the orientation of the axial ligands and of the magnetic properties of the haems in the tetrahaem ferricytochrome from Shewanella frigidimarina

The unambiguous assignment of the nuclear magnetic resonance (NMR) signals of the (alpha-substituents of the haems in the tetrahaem cytochrome isolated from Shewanella frigidimarina NCIMB400, was made using a combination of homonuclear and heteronuclear experiments. The paramagnetic C-13 shifts of the nuclei directly bound to the porphyrin of each haem group were analysed in the framework of a model for the haern electronic structure. The analysis yields g-tensors for each haem, which allowed the assignment of some electron paramagnetic resonance (EPR) signals to specific haems, and the orientation of the magnetic axes relative to each haem to be established. The orientation of the axial ligands of the haems was determined semi-empirically from the NMR data, and the structural results were compared with those of the homologous tetrahaern cytochrome from Shewanella oneidensis MR-1 showing significant similarities between the two proteins. (C) 2002 Published by Elsevier Science B.V. on behalf of the Federation of European Biochemical Societies.