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 β substituents. This asymmetry is determined by the orientations of the heme axial ligands, histidine and methionine. One exception to this is ferricytochrome c552 from Hydrogenobacter thermophilus, which has similar amounts of unpaired electron spin density at the β 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 c552 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.
|Original language||English (US)|
|Number of pages||6|
|Journal||Proceedings of the National Academy of Sciences of the United States of America|
|State||Published - Jun 8 2004|
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