Effects of extrinsic imidazole ligation on the molecular and electronic structure of cytochrome c

Although imidazole ligand binding to cytochrome c is not directly related t o its physiological function, it has the potential to provide valuable info rmation on the molecular and electronic structure of the protein. The solut ion structure of the imidazole adduct of oxidized horse heart cytochrome c (Im-cyt c) has been determined through 2D NMR spectroscopy. The Im-cyt c, 8 mM in 1.2 M imidazole solution at pH 5.7 and 313 K, provided altogether 25 42 NOEs (1901 meaningful NOEs) and 194 pseudocontact shifts. The 35 conform ers of the family show the RMSD values to the average structure of 0.063 +/ -0.007 nm for the backbone and 0.107 +/-0.007 nm for all heavy atoms, respe ctively. The characterization of Im-cyt c is discussed in detail both in te rms of structure and electronic properties. The replacement of the axial li gand Met80 with the exogenous imidazole ligand induces significant conforma tion changes in both backbone and side chains of the residues located in th e distal axial ligand regions. The imidazole ligand binds essentially paral lel to the imidazole of the proximal histidine, the two planes forming an a ngle of 8 +/-7 degrees. The electron delocalization on the heme moiety and the magnetic susceptibility tensor are consistent with these structural fea tures.