Transferrin iron-binding sites

Figure 5.7 Structures of the alternative conformations of the iron-binding sites in the orthorhombic crystal form of the recombinant N-lobe of human transferrin. Reproduced with permission from MacGillivray et ah, 1998. Copyright (1998), American Chemical Society. 

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... 

A recently obtained high resolution structure of two crystal forms of the N-lobe of human serum transferrin (at 0.16 and 0.18 nm resolution) shows disorder at the iron-binding sites (MacGillivray et ah, 1998). Model building and refinement show... 

Under normal physiological conditions, iron is transported in serum by transferrin, an 80 kDa bilobal protein with two almost identical iron-binding sites, one in each half of the molecule. 

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.

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. 

The transferrins are proteins that bind and transport iron as peIII 16-U.8 They indude lactoferrin from milk, ovotransferrin from egg white, and serum transferrin from a range of organisms. Uteroferrin, considered in Section 62.1.5.5.2 on the purple acid phosphatases, is an iron-binding protein with phosphatase activity, that has been proposed to transport iron from maternal to foetal circulation.824 826 There are distinct differences between the iron-binding sites in uteroferrin and transferrin, and so uteroferrin will not be discussed in this section. 

The coordination of indazolium ligands, as present in KP1019, appears beneficial for transferrin binding, compared to analogous complexes with imidazole or triazole ligands [102], as these ligands are better accommodated into the iron binding sites [97]. 

Treating ovotransferrin and human serum transferrin with 170-400 molar excess of ethoxyformic anhydride resulted in complete ethoxy-formylation of histidines with complete loss in iron-binding activity (33). The binding of each iron (two iron-binding sites per protein molecule) protected two histidines from ethoxyformylation, and in both cases the proteins remained completely active. These results plus kinetic analyses of the inactivations indicated two essential histidines in each binding site. Ethoxyformic anhydride also may react with amino groups. 

There are several modes of protection from the activity of available iron or copper in vivo. (Antioxidant action is discussed in more detail in Chapter 4). Apotransferrin binds iron(III) for transport and delivery to cells. It is its capacity as an iron-binding protein which renders it also able to function as an antioxidant by making iron(III) unavailable for participation in iron-catalysed radical reactions. Only about 30% of the iron-binding sites on the transferrin in human plasma are normally occupied in vivo (transferrin concentration 1.2-2.0mg/ml). The copper-containing protein caeruloplasmin (0.2-0.4 mg/ml) is... 

Typically only 30% of the iron binding sites of transferrin are occupied. Thus it retains sufficient excess binding capacity to ensure that it is never completely... 

Figure 19. A model for the anion- and iron-binding sites of transferrin depicted assuming an interlocking-site hypothesis. The protein furnishes five ligands to the metal in the iron binding site three tyrosines and two histidines. The carbonate ion binds to an arginine in the anion-binding site and functions as a sixth ligand to the metal center. The carbonate forms a bridge between the metal- and the anion-binding sites in the active center (36).

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