Human serotransferrin: structure and function

A pmr study of the effects of pH and anion and metal ion binding of the histidyl residues of ovotransferrin

High resolution proton magnetic resonance studies of ovotransferrin show clear resolution of four groups of C(2)-H histidyl resonances to low field of the major aromatic envelope. Titrations of the protein in the absence and presence of synergistic anions, oxalic acid, malonic acid, and 2,6-dipicolinic acid, and anions plus metal ions reveal that six histidines are involved in the binding sites. These histidines, three in each binding site, are near to one another. In each binding site one histidine is involved in binding to anions and two are involved in binding to metal ions.

Ferric ion sequestering agents: kinetics of iron release from ferritin to catechoylamides

The removal of ferric ion from the iron storage protein ferritin to synthetic catechoylamide sequestering agents has been studied using visible spectroscopy at 487 nm. One ligand which has been investigated in detail is N,N',N' ',-tris(2,3-dihydroxy-5-sulfobenzoyl)-1,5,10-triazadecane (3,4-LICAMS), which octahedrally coordinates the metal ion via six phenolic oxygens. For some related catechoylamide chelates, the percentage of iron removed after 6 h has been determined. These ligands incorporate various modifications, either on the catechol moiety or on the backbone structure of the ligand. Mobilization of iron by the catechoylamide ligands alone results in very slow exchange, and virtually no iron removal after 6 h. In contrast, addition of ascorbic acid to the reaction mixture facilitates iron exchange, with the release of 7% of the available iron in the same time span. Variation of the initial rate with ascorbic acid concentration results in Michaelis-Menten kinetics with Km = 1.7 . 10(-3) M and a maximal rate of 1.28 . 10(-7) M . min-1. The ascorbic acid-mediated rate was not affected by changing the catechoylamide ligand concentration, and was only slightly affected by variation of the ligand employed. These data are consistent with a multistep process which includes diffusion of a reductant into the ferritin inner core, reduction and possible chelation of the ferrous ion, diffusion out of the protein shell, and subsequent iron exchange with the catechoylamide molecule.

Antimicrobial properties of iron-binding proteins

The transferrins are iron-binding proteins with molecular weights of around 80,000, which interact with a maximum of two ferric atoms per each protein molecule. The best known transferrins are the serotransferrins from animal sera, lactoferrins from milk, and conalbumin from egg-white. The iron-deficient transferrins will inhibit the growth of certain bacteria and fungi by making iron unavailable for bacterial metabolism. Such activity is abolished if the transferrin is saturated with iron. Many organisms can produce small molecular-weight iron-binding compounds called siderophores that can successfully utilize the iron sequestered by the transferrins. Such organisms are very virulent. Overwhelming evidence is now available to indicate that the transferrins play an important role in mammalian host-defense mechanisms. Thus, iron injections into animals infected with virulent bacteria result in increased death rates, and parenteral iron administration to human infants predisposes them to fatal septicemia. On the other hand, in cases of systemic infection, the organism responds by lowering its total serum iron, so as to make the serotransferrin present less saturated with iron. This phenomenon is called nutritional immunity. The iron apparently moves into the storage tissues from the circulation, and furthermore, it is withheld from circulation by the reticuloendothelial system. Laboratory results in such cases indicate low total serum iron levels and high unsaturated iron-binding activity values, thus increasing the bacteriostatic effects of the serotransferrins. Increased lactoferrin levels are observed in the milks of mastitic cattle.

Characterization of transferrin metal-binding sites by diffusion-enhanced energy transfer

The distance from the protein surface to ferric or manganic ions in the two specific metal-binding sites of human serum transferrin has been estimated by measuring energy transfer from freely diffusing terbium chelaters in aqueous solution to transferrin-bound metal ions. In addition, both monoferric forms of the protein were studied, as well as the diferric complex formed by using oxalate instead of (bi)carbonate as the auxiliary anion in binding of iron(III) to transferrin. Second-order rate constants for energy transfer between electrically neutral terbium(III)--N-(2-hydroxy-ethyl)ethylenediaminetriacetate and the FeA, FeB, and Fe2 forms of transferrin were 0.9 X 10(5) M-1 S-1, 1.4 X 10(5) M-1 S-1, and 2.6 X 10(5) M-1 S-1, respectively (based on iron concentraton). For the Fe2 species, substitution of oxalate for (bi)carbonate has the effect of decreasing the accessibility of both electrically neutral and negatively charged terbium chelates to the protein-bound iron chromophores. Theoretical considerations of the effect of acceptor location in the protein on energy transfer suggest that the iron chromophores are not on the surface of the protein but are less than 1.7 nm below the surface. The use of diterbium transferrin as energy donor to a small cobalt chelate in solution or to diferric transferrin corroborates these results.

The kinetics of iron release from human transferrin by EDTA. Effect of salts and detergents

The kinetics of iron release from diferric human transferrin by EDTA at pH 7.4 and 37 degrees C has been studied. Equations have been derived which give the expected absorbance-time curves when the two sites are kinetically homogeneous or heterogeneous and these have been compared to the experimental results. In the absence of added salts or detergents,the two sites are kinetically similar but not identical with respect to iron release. However, the two sites are kinetically homogeneous in 0.02 M laurylpyridinium chloride and 2 M LiCl. In NaClO4 (greater than 0.50 M) there are two parallel pathways for iron release; 80% of the total iron being released heterogeneously and 20% apparently homogeneously. For the simple electrolyte ions studied, the rate of iron release from diferric-transferrin increases in accordance with the lyotropic series: SO4 2- less than HCO-2 less than Cl- less than ClO4- and Na+ less than Li+. Sodium dodecyl sulphate (0.02 M) and LiCl (2.0 M) increase the rate of ion release by 0.1 M EDTA by 32- and 27-fold, respectively. The results show that conformational states are attainable from which iron can be released either homogeneously or heterogeneously and from which iron release is facilitated. It is proposed that the structure of the transferrin binding site on the cell membrane and/or the presence of another molecule may promote these conformers in vivo.

Binding of xylenol orange to transferrin. Demonstration of metal-anion linkage

The iron transport protein, transferrin, binds two metal ions and, concomitantly, two carboxylate anions. The metal ion indicators, xylenol orange and semi-xylenol orange are carboxylate anions which exhibit a characteristic visible spectrum when attached to a metal. We prepared the ternary complexes VO2+-transferrin-xylenol orange and VO2+-transferrin-semi-xylenol orange. The EPR spectra show that the vanadyl ion is attached to the protein and the visible spectra show that the xylenol orange or semi-xylenol orange is attached to the metal. The implication is that in other metal-transferrin-anion complexes, the anion is directly attached to the metal.