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

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 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 ligandsthere 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 <δ> of the heme methyl groups with the g max values.

Study of the Individual Cytochrome b5 and Cytochrome b5 Reductase Domains of Ncb5or Reveals a Unique Heme Pocket and a Possible Role of the CS Domain

“…The dihedral angle between the planes of the His ligands in Ncb5or-b 5 is close to the maximum value of 90°, accounting for its very high g max value in EPR spectra, and is by far the largest among structurally characterized members of the cytochrome b 5 superfamily (supplemental Table 2). In fact, it is similar to the largest interplanar angles previously reported for natural heme proteins: heme b L in the cytochrome bc 1 complex of the mitochondrial respiratory chain (50,52,63) and proximal heme b in quinol: fumarate reductase (64,65). These and all other previously reported bis-histidine ligated b-hemes exhibiting HALS spectra are known or expected to be located in a membrane-embedded region of its protein, with the heme ensconced in a four-helix bundle exhibiting a left-handed twist (51).…”

“…2, top). This kind of spectrum, referred to as a highly anisotropic low spin (HALS) or "large g max " spectrum (49, 50), is relatively rare for low spin bis-histidine-ligated heme proteins and indicative of iron in a tetragonally distorted environment (51,52). HALS EPR spectra are defined as those in which the value of g z (g max ) is Ն3.2 and arise when the dihedral angle between the imidazolyl planes is Ͼ57° (48).…”

Functional Characterization of the FoxE Iron Oxidoreductase from the Photoferrotroph Rhodobacter ferrooxidans SW2

“…2). The spectrum suggests that both hemes are low-spin, rhombic, and hexacoordinated and agrees with typical His-His or His-Met coordinated hemes (19). The amino acid sequence of FoxE deduced from the processed gene shows histidines at positions 70, 147, 168, 183, 224, and 242 and methionines at positions 116, 194, and 230 that could serve as distal ligands for the hemes.…”

Analysis of Oligomerization Properties of Heme a Synthase Provides Insights into Its Function in Eukaryotes

“…To differentiate between heme binding and these alternative possibilities, we converted each conserved histidine to cysteine (or in a separate experiment to tyrosine). Cysteine and tyrosine residues can be potential heme axial ligands but are unlikely to provide an effective substitute for the histidine imidazole ring in catalysis (47). All eight variants (H169C, H245C, H368C, H431C, H169Y, H245Y, H368Y, and H431Y) were stably expressed in cox15⌬ cells (data not shown), but surprisingly, none of these histidine to cysteine or histidine to tyrosine variants were able to rescue the heme a biosynthetic defect of the cox15⌬ strain (Fig.…”