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Review: studies of ferric heme proteins with highly anisotropic/highly axial low spin (S = 1/2) electron paramagnetic resonance signals with bis-histidine and histidine-methionine axial iron coordination.

Biopolymers (2009-06-19)
Giorgio Zoppellaro, Kara L Bren, Amy A Ensign, Espen Harbitz, Ravinder Kaur, Hans-Petter Hersleth, Ulf Ryde, Lars Hederstedt, K Kristoffer Andersson
ABSTRACT

Six-coordinated heme groups are involved in a large variety of electron transfer reactions because of their ability to exist in both the ferrous (Fe(2+)) and ferric (Fe(3+)) state without any large differences in structure. Our studies on hemes coordinated by two histidines (bis-His) and hemes coordinated by histidine and methionine (His-Met) will be reviewed. In both of these coordination environments, the heme core can exhibit ferric low spin (electron paramagnetic resonance EPR) signals with large g(max) values (also called Type I, highly anisotropic low spin, or highly axial low spin, HALS species) as well as rhombic EPR (Type II) signals. In bis-His coordinated hemes rhombic and HALS envelopes are related to the orientation of the His groups with respect to each other such that (i) parallel His planes results in a rhombic signal and (ii) perpendicular His planes results in a HALS signal. Correlation between the structure of the heme and its ligands for heme with His-Met axial ligation and ligand-field parameters, as derived from a large series of cytochrome c variants, show, however, that for such a combination of axial ligands there is no clear-cut difference between the large g(max) and the "small g-anisotropy" cases as a result of the relative Met-His arrangements. Nonetheless, a new linear correlation links the average shift delta of the heme methyl groups with the g(max) values.

MATERIALS
Product Number
Brand
Product Description

Sigma-Aldrich
L-Histidine, ReagentPlus®, ≥99% (TLC)
Sigma-Aldrich
L-Histidine, suitable for cell culture, meets EP, USP testing specifications, from non-animal source
SAFC
L-Histidine
Sigma-Aldrich
L-Histidine, BioUltra, ≥99.5% (NT)
Supelco
L-Histidine, Pharmaceutical Secondary Standard; Certified Reference Material
Histidine, European Pharmacopoeia (EP) Reference Standard
Supelco
L-Histidine, certified reference material, TraceCERT®