Lactoferrin iron ligands

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. 

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... 

The large size of transferrins (670-700 residues), with the consequent difficulties of chemical sequencing, meant that it was not until 1982 that the first amino acid sequences, those of human serum transferrin 10) and chicken ovotransferrin 34,35), were established. These were closely followed by that of human lactoferrin 11). The twofold internal repeat in each sequence (see below) was immediately apparent, and comparison of all three sequences then identified conserved tyrosines and histidines that were potential ligands for iron (11). 

Pig. 27. Possible sites for the binding of secondary, nonsynergistic anions, which may perturb the iron site and modulate release. Residue 121 is the essential arginine. Residues 210 and 301 are located behind the iron site, near the Tyr ligands and the hinge region. Numbering is as for the N-lobe of lactoferrin. The identities of these residues, which vary in the two lobes and between different transferrins, can be seen in Table III. 

In the case of infants, the ligand may very well be the lactoferrin which transports the ferric ion either to the cell mucosal binding protein or into the mucosal cell directly. Inside the mucosal cell the iron is bound to either transferrin or ferritin for storage. 

To meet these metabolic requirements, nature has developed protein ligands which maintain iron in a soluble, bioavailable form while preventing it from catalyzing reactions that produce harmful oxidative products. These iron-binding proteins are transferrin, lactoferrin, and ferritin. 

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