Iron Coordination Structure

Non-heme iron protein Fe Labile Sulfur Cysteine 

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The iron environment of adrenodoxin is an entity of the protein which is essential for catalysis. The iron coordination structure was investigated to find out whether the sulfur atoms of sulfhydryl groups and labile... 

IV. Models of the Iron Coordination Structure A. Chemical Nature of Labile Sulfur... 

For the purpose of determining the iron-coordination structure of nonheme iron proteins, reconstitution experiments from apoprotein and other constituents are an elegant approach which can inductively indicate the original iron structure. 

MCD Spectroscopy as a Probe of Heme Iron Coordination Structure... 

Fig. 2. Schematic representation of haem iron coordination structures in ferrous hexacoordinated (A) and pentacoordinated (B) states. L in A represents an exogenous ligand such as O2 or CO.

Tomita T, Gonzalez G, Chang AL, Ikeda-Saito M, Gilles-Gonzalez MA. A comparative resonance Raman analysis of heme-binding PAS domains heme-iron coordination structures of the BjFixL, AxPDEAl, EcDos, and MtDos proteins. Biochemistry 2002 41 4819 826. 

The crystallographic structure of rubredoxin from Clostridium pasteurianum at 2.5 A, a resolution sufficient to reveal the sequence of several of the bulky amino acid side chains, shows the iron coordinated to two pairs of cysteine residues located rather near the termini of the polypeptide chain (Fig. 1). A related rubredoxin, with a three times larger molecular weight, from Pseudomonas oleovorans is believed to bind iron in a similar fashion. This conclusion is based on physical probes, especially electron paramagnetic resonance spectroscopy, all of which indicate that the iron is in each case situated in a highly similar environment however, the proteins display some specificity in catalytic function. 

Fortunately, the structure of the fully reduced P. pantotrophus cytochrome cdi could be obtained, and this has provided important clues as to how the enzyme functions, as well as raising unexpected questions (12). First of all, the structure of the reduced enzyme reveals an nnprecedented switch of one axial ligand at the c heme from His to Met (Figs. 5a,b). The second important feature revealed by this structure was that the Tyr 25 had vacated the di heme iron coordination... 

Deoxvhemerythrin. Henrerythrin appears to retain its triply bridged crae structure in the deoxy form. A low (3.9 A) resoluticm difference electron density map of deoxyHr vs. metHr fi- n X-ray diffraction suggests that the iron atoms move slightly further apart in deoxyHr, but remain five and six coordinate, respectively (29). Confirmation of the iron coordination comes from near-IR absorption and circular dichroism spectra (40,46,47). Based on model high-spin ferrous complexes, the six coordinate iron is expected to give two of the three observed transitions near 10000 cm- while the five-coordinate iron accounts for the d-d transition at ca. 5000 cm-. ... 

Fig. 4. The structure of the cation of 4B, [Fe402(BICOH)2(BICO)2 (02CPh)4]showing the 40% probability thermal ellipsoids. Only the first atom of the phenyl rings is shown. BICOH stands for bis(N-methylimldazol-2-yl)carbinol. BICO is the alkoxide anion, (a) The Fe402 core iron coordination spheres and bridging benzoate groups. Continued on next page.

Fig. 6. The structure of the anion of 4D, [Fe402(OH)2(5-MeHXTA)2) . 5-MeHXTA stands for N,N -(2-hydroxy-5-methyl l,3-xylylene)bis(N-(carboxymethyl) glycine). Only the Iron coordination spheres are shown. (Reproduced from Ref. 10a. Copyright 1988 American Chemical Society.)...

All physical evidence points to a compact structure for the pol5mier. Its shape is spherical and its density, 3.0, is fairly high. Nevertheless, the tetrahedral coordination of the iron significantly lowers the coordination requirements for the oxygen from what they are in the ferric oxyhydroxides, which are similar in composition. In these materials, each oxygen is surrounded by four iron atoms on the average 43). Given the stoichiometry of the polymer, and the assumption that the waters are at the surface occup)dng one iron coordination site, then a coordi-... 

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