Protein bond, iron

Some detailed comparisons of the protein environments around the HiPIP and Fd clusters have been made.769,770 It is noteworthy that the HiPIP cluster is more deeply buried (about 4.5 A) than is the case for the clusters in the other iron-sulfur proteins. All iron-sulfur proteins for which structural data are available, with the exception of the three-iron protein from Azotobacter vinelandii, have hydrogen bonding between the cysteine sulfur in the iron-sulfur cluster and the backbone peptide link. It appears that there is an approximate correlation between the number of NH S hydrogen bonds in the environment of a cluster and its redox potential. In HiPIP, these hydrogen bonds become more linear and shorten on reduction of the cluster. It is possible, therefore, that the oxidation states of the cluster may be controlled by the geometries of the hydrogen bonds.770... 

In many of the haemoproteins we shall be discussing, the protoporphyrin IX group is held to the polypeptide chain only by hydrogen bonding, Van der Wools forces and iron-protein bonds. In several other cases, notably in cytochrome-c and its related compounds, the haem is covalently linked to the protein via substituents at the pyrrole carbon atoms. Cyto-chrome-c can be regarded as an iron protoporphyrin IX group with the addition of a protein cysteine side-chain across the vinyl double bonds giving two thio-ether links (Fig. 3). 

Fe(TPP)(l-MeIm)(NO)] reveal that the Fe motion is primarily perpendicular to the plane of the porphyrin, as expected for stretching of the Fe-NO bond. We attributed the large frequency decrease relative to the approximately 530 cm Fe-NO stretching frequency measured for five-coordinate NO derivatives of heme proteins and iron porphyrins to a significant weakening of the Fe-NO bond in the presence of the histidine hgand. ... 

Fe motion along the two in-plane directions constitutes two thirds of the VDOS, and vibrations of the in-plane bonds to the pyrrole nitrogens appear to dominate the VDOS reported for heme proteins and iron porphyrins. NRVS data on deoxyMb allowed the first identification of the Fe Npyr vibrations for a heme protein. The appearance of two resolved features at 251 and 267 cm for deoxyMb (Figure 7) indicates that deviations from fourfold symmetry are sufficient to remove the nominal equivalence between the two in-plane directions. Comparison of experimental and computational results on iron porphyrins indicates additional structure that is not resolved experimentally. The in-plane vibrations... 

Figure 4 Overlay of the structures of RdCp and the corresponding part of the V44A protein. The iron atom and residues 6-8 and 42-44 are shown. The modeled NH protons of residues 44 which are involved in the 44NH—S42 hydrogen bonds are labelled x in RdCp and y in V44A. The bond is 0.4(1) A shorter in the V44A mutant protein than in RdCp.

Free poiphyrin motions could be considerably altered in the presence of a metal ion, such as the iron, which covalently links the porphyrin to the protein. Linkages between heme and protein involve iron and the porphyrin part of the heme. Iron is linked to an imidazole group of the proximal histidine, His-93. The other heme protein bonds involve, on the heme side, the propionic acid groups, the vinyl groups and the porphyrin as a whole. The two heme propionates help to stabilize the heme by making hydrogen bonds to the side chains of the distal histidine (His-64) and an arginine. 

If linkages of the porphyrin part of the heme have a great importance in the iron-protein bond for stabilizing the heme protein complex, the presence of iron which links covalently the porphyrin to the protein could alter considerably the free porphyrin motion. 

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