Orientation of the axial ligands and magnetic properties of the hemes in the triheme ferricytochrome PpcA from G. sulfurreducens determined by paramagnetic NMR

The geometry of the axial ligands of the hemes in the triheme cytochrome PpcA from Geobacter sulfurreducens was determined in solution for the ferric form using the unambiguous assignment of the NMR signals of the α-substituents of the hemes. The paramagnetic 13C shifts of the hemes can be used to define the heme electronic structure, the geometry of the axial ligands, and the magnetic susceptibility tensor. The latter establishes the magnitude and geometrical dependence of the pseudocontact shifts, which are crucial to warrant reliable structural constraints for a detailed structural characterization of this paramagnetic protein in solution.