Determination of Haem Electronic Structure in His‐Met Cytochromes c by ¹³C‐NMR

Abstract

The assignment of 13C resonances of nuclei a to the haem in horse ferricytochrome c is completed and the Fenni contact shifts are evaluated at 30°C and 50°C using empirical magnetic susceptibility tensors to correct for dipolar interactions. The Fermi contact shifts are fitted to a model of molecular orbitals of e, symmetry, which are subject to a rhombic perturbation. A similar analysis is performed using published data for Pseudomonas aeruginosa cytochrome c551. The relationship between the orientation of the effective g tensor and that of the rhombic perturbation in these proteins is shown to agree with theoretical predictions. A comparison between the orientation of the rhombic perturbations and the crystal structures of horse cytochrome c and I? aeruginosa cytochrome cS5, reveals that the orientation of the histidine and methionine axial ligands dominates the rhombic perturbation and that the two ligands have approximately equal influence. The magnitude of the perturbation shows that the orientation of the axial ligands has little effect on the haem redox potential. However, the relationship that is established between the magnetic susceptibility tensor, the partially filled haem molecular orbitals, and the orientation of the haem ligands offers a new source of precise structural information.

Hyperfine Correlation Spectroscopy and Electron Spin Echo Envelope Modulation Spectroscopy Study of the Two Coexisting Forms of the Hemeprotein Cytochrome c6 from Anabaena Pcc7119

“…In His-Met hemeproteins such as Cyt c 6 , the actual form of the heme center has to be controlled by the orientation of the axial ligands as well, although the influence of each residue could be different. For some cytochromes c where coexistence of two forms was revealed from EPR measurements, NMR studies suggested that a change in the conformation of Met ligand would cause switching from one EPR form to another (25,26). The process may be similar in our case, although we have no data supporting this hypothesis.…”