Open AccessHeme axial methionine fluxionality in Hydrogenobacter thermophilus cytochrome c 552
Author(s) -
Linghao Zhong,
Xin Wen,
Terry M. Rabinowitz,
Brandy S. Russell,
Elizabeth F. Karan,
Kara L. Bren
Publication year - 2004
Publication title -
proceedings of the national academy of sciences of the united states of america
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 5.011
H-Index - 771
eISSN - 1091-6490
pISSN - 0027-8424
DOI - 10.1073/pnas.0402033101
Subject(s) - heme , chemistry , unpaired electron , stereochemistry , crystallography , hemeprotein , cytochrome c , histidine , methionine , cytochrome , amino acid , biochemistry , molecule , organic chemistry , mitochondrion , enzyme
The heme group in paramagnetic (S = 1/2) ferricytochromes c typically displays a markedly asymmetric distribution of unpaired electron spin density among the heme pyrrole beta substituents. This asymmetry is determined by the orientations of the heme axial ligands, histidine and methionine. One exception to this is ferricytochrome c(552) from Hydrogenobacter thermophilus, which has similar amounts of unpaired electron spin density at the beta substituents on all four heme pyrroles. Here, determination of the orientation of the magnetic axes and analysis of NMR line shapes for H. thermophilus ferricytochrome c(552) is performed. These data reveal that the unusual electronic structure for this protein is a result of fluxionality of the heme axial methionine. It is proposed that the ligand undergoes inversion at the pyramidal sulfur, and the rapid interconversion between two diastereomeric forms results in the unusual heme electronic structure. Thus a fluxional process for a metal-bound amino acid side chain has now been identified.