Ligand iron binding

Iron is, as part of several proteins, such as hemoglobin, essential for vertebrates. The element is not available as ion but mostly as the protein ligands transferrin (transport), lactoferrin (milk), and ferritin (storage), and cytochromes (electron transport) (Alexander 1994). Toxicity due to excessive iron absorption caused by genetic abnormalities exists. For the determination of serum Fe a spectrophoto-metric reference procedure exists. Urine Fe can be determined by graphite furnace (GF)-AAS, and tissue iron by GF-AAS and SS-AAS (Alexander 1994 Herber 1994a). Total Iron Binding Capacity is determined by fuUy saturated transferrin with Fe(III), but is nowadays mostly replaced by immunochemical determination of transferrin and ferritin. 

The iron-binding sites have been characterized by crystallographic studies on several transferrins, and in Figure 5.7 (Plate 7) that of the N-lobe of human lactoferrin is presented. The 3+ charge on the ferric ion is matched by the three anionic ligands Asp-63, Tyr-95 and Tyr-188 (the fourth, His-249, is neutral), while the charge on the carbonate anion is almost matched by the positive charge on Arg-124 and the... 

Several binding sites for Tb3+ or Cd2+ ions have been identified in the interior of the apoferritin protein shell, some of which may be iron-binding sites (Harrison et ai, 1989 Granier et ah, 1998). In HoSF and HoLF, two sites were identified on the inner surface of the B helix at the subunit dimer interface (Figure 6.15, Plate 11) which bind two Cd2+ ions. One involves Glu-57 and Glu-60 as ligands and the other Glu-61 and Glu-64 (Granier et al., 1998). In H-chain ferritins the first pair of Glu-57 and Glu-60 are both replaced by His and only a single Tb3+ is found bound to Glu-61 and Glu-64 (Lawson et al, 1991). 

It seems clear that complexes 28 and 29 both enter cancer cells by transferrin-mediation. Tumor cells are known to have a high density of transferrin receptors, and this provides a route for the uptake of ruthenium (175). In normal blood plasma, transferrin is only one-third saturated with Fe(III) and therefore vacant sites are available for Ru(III) binding. Baker et al. have shown by X-ray crystallography that complex 29 binds to His-253 of apolactoferrin, one of the Fe(III) ligands in the iron binding cleft of the N-lobe, with displacement of a chloride ligand (176). Ruthenium(III) is well known to have a high affinity for solvent-exposed His side chains of proteins (177). Complex... 

In addition to the amino acid side chains mentioned above, a number of other low molecular weight ligands are found in metalloproteins. These include cyanide and carbon monoxide, which we will describe later in this chapter. Here we consider carbonate and phosphate anions in the context of the super family of iron-binding proteins, the transferrins. 

Fig. 2.12 Examples of non-linear Arrhenius (or Eyring) plots (a) 1u(A oh)7 " ) vs T for the base hydrolysis of trans-Co(en)2ClJ. Curvature may result when k, k2 and A// , not equalling A// in the conjugate-base mechanism (Sec. 4.3.4). Reprinted with permission from C. Blakeley and M. L. Tobe, J. Chem. Soc. Dalton Trans. 1775 (1987). (b) nk vs T for iron removal from C- and N-terminal monoferric transferrin (lower and upper scales respectively). Transferrin contains two iron binding sites = 35 A apart. Either of the two sites, designated C- and N-terminal, can be exclusively labelled by Fe(lll) ions and these may be removed by a strong ligand such as a catechol (see Sec. 4.11). Reprinted with permission from S. A. Kretschmar and K. N. Raymond, J. Amer. Chem. Soc. 108, 6212 (1986). (1986) American Chemical Society.

The energetics of peptide-porphyrin interactions and peptide ligand-metal binding have also been observed in another self-assembly system constructed by Huffman et al. (125). Using monomeric helices binding to iron(III) coproporphyrin I, a fourfold symmetric tetracarboxylate porphyrin, these authors demonstrate a correlation between the hydropho-bicity of the peptide and the affinity for heme as well as the reduction potential of the encapsulated ferric ion, as shown in Fig. 12. These data clearly demonstrate that heme macrocycle-peptide hydrophobic interactions are important for both the stability of ferric heme proteins and the resultant electrochemistry. 

In this case the ferric iron site seems to activate the substrate. Our generalization of this model is shown in Fig. 7. The iron binds the substrate and could serve, as a Lewis acid, to facilitate ketolization of either the bidentate or monodentate substrate. The hydroxide (water) ligand may be more significant than simply a placeholder ligand to be displaced by substrate. Since both substrate hydroxyls must lose their... 

Detailed pictures of the iron-binding sites in transferrins have been provided by the crystal structures of lactoferrin (Anderson et ai, 1987, 1989 Baker etai, 1987) and serum transferrin (Bailey etal., 1988). Each structure is organized into two lobes of similar structure (the amino- and carboxy-terminal lobes) that exhibit internal sequence homology. Each lobe, in turn, is organized into two domains separated by a cleft (Fig. 3 and 10). The domains have similar folding patterns of the a//3 type. One iron site is present in each lobe, which occupies equivalent positions in the interdomain cleft. The same sets of residues serve as iron ligands to the two sites two tyrosines, one histidine, and one aspartate. Additional extra density completes the octahedral coordination of the iron and presumably corresponds to an anion and/or bound water. The iron sites are buried about 10 A below the protein surface and are inaccessible to solvent. 

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