A new spatial structure for the axial methionine observed in cytochrome c5 from Pseudomonas mendocina: Correlations with the electronic structure of heme c

Abstract

Cytochrome c 5 from Pseudomonas mendocina has been isolated and the coordination geometry at the heme iron was investigated by 1H nuclear magnetic resonance and circular dichroism spectroscopy. Individual assignments were obtained for heme c and the axial ligands. From studies of nuclear Overhauser enhancements the axial histidine imidazole ring orientation relative to the heme group was found to coincide with that of other c-type cytochromes. In contrast, a new structure was observed for the axial methionine. This include S chirality at the iron-bound sulfur atom, but compared to cytochromes c-551 from Pseudomonads and Rhodopseudomonas gelatinosa, which also contain S-chiral methionine, the spatial arrangement of the γ-and β-methylene groups and the α carbon of methionine is markedly different. Analysis of the electron spin density for the hypothesis that the electronic structure of heme c is primarily governed by the orientation of the sp 3 lone-pair orbital of the axial sulfur atom with respect to the heme plane.