Transferrin model studies

Transferrin model compounds and 44 related iron(III) crystal structures were used to modify the AMBER force field for subsequent studies of ferric transferrin. Eneigy minimization was conducted both in vacuo and, more interestingly, with the generalized Bom/surface area (GB/SA) continuum treatment described in Chapter 2, Section 2.712201. 

As mentioned above. X-ray diffraction of transferrin furnishes little information on the 3D-structure of the glycans and the images we have today remain largely speculative since they result from molecular modelling studies. We have represented in Fig. 21 the 3D-structure, determined by molecular modelling on the basis of X-ray diffraction data of rabbit serotransferrin [276] and of human lactotransferrin [89,92]. In rabbit serotransferrin, the single glycan linked to the peptide chain is immobilized into only... 

Fluxes of iron from the plasma towards BM and other tissues can be quantified by ferrokinetic studies, using 59Fe and sophisticated computer models (Ricketts et ah, 1975 Ricketts and Cavill, 1978 Barosi et ah, 1978 Stefanelli et ah, 1980). Plasma iron turnover (PIT), erythroid iron turnover (EIT), non-erythroid iron turnover (NEIT), marrow iron turnover (MIT), and tissue iron turnover (TIT) could be calculated in many disorders of iron metabolism and in all kinds of anaemias. Iron is rapidly cleared from the plasma in iron deficiency and in haemolytic anaemias. If more iron is needed for erythropoiesis, more transferrin receptors (TfR) are expressed on erythroblasts, resulting in an increased flux of iron from intestinal mucosal cells towards the plasma. In haemolytic anaemias MPS, and subsequently hepatocytes, are overloaded. In hereditary haemochromatosis too much iron is absorbed by an intrinsic defect of gut mucosal cells. As this iron is not needed for erythropoiesis,... 

Kinetic studies were attempted at 4, 15, 25 and 37 °C, but the colloids tended to aggregate at all temperatures above 4°C. Four models were used to determine the binding mechanisms from the kinetic data. A detailed analysis of binding at 4 °C, was made. Models were set up involving one or two surface sites which also satisfied the overall kinetics but the analyses were not definitive. Although it was demonstrated that the cells were capable of endocytosis of fluorescence-conjugated transferrin, there was no evidence for the endocytosis of the cationic colloids. 

Once in the serum, aluminium can be transported bound to transferrin, and also to albumin and low-molecular ligands such as citrate. However, the transferrrin-aluminium complex will be able to enter cells via the transferrin-transferrin-receptor pathway (see Chapter 8). Within the acidic environment of the endosome, we assume that aluminium would be released from transferrin, but how it exits from this compartment remains unknown. Once in the cytosol of the cell, aluminium is unlikely to be readily incorporated into the iron storage protein ferritin, since this requires redox cycling between Fe2+ and Fe3+ (see Chapter 19). Studies of the subcellular distribution of aluminium in various cell lines and animal models have shown that the majority accumulates in the mitochondria, where it can interfere with calcium homeostasis. Once in the circulation, there seems little doubt that aluminium can cross the blood-brain barrier. 

Although there is some experimental evidence which points to a binding of iron ions by specific cytosolic proteins (see Cytosolic Iron Donor, below), these proteins, with the exception of transferrin, are available only in minute quantities, and the nature and extent of iron-protein interactions are poorly understood. Therefore, a number of nonprotein iron chelates have been studied as possible model donor complexes (Table I). Because of the high stability constants of, for example, the Fe(II)/Fe(III)-8-hydroxyquinoline and Fe(III)-ADP complexes (20), these iron-chelate complexes are unfavorable as iron donors, and in fact no energy-dependent uptake of iron has been detected using these complexes (21, 23). 

Fig. 26. Binding modes for anions other than carbonate. In (a) the mode of binding of oxalate to human lactoferrin, as determined crystallographically (192,193), is shown. In b is a generalized model for synergistic anion binding to transferrins, based on EPR studies (191) and the crystallographic results for oxalate. From Shongwe et al. (192), with permission.

Fig. 1 illustrates the general flowchart for sample preparation. Two proteins, carbonic anhydrase possessing a blocked N-termind and transferrin possessing a free N-terminal, were used as models in this study. Both proteins were prepared as follows. 

The visible spectra of the intradiol dioxygenases (Fig. 2) are characterized by a broad absorption band centered near 460 nm with molar extinction coefficients of 3000-4000 M-1 cm-1 16>34). The color disappears upon reduction of the ferric ion with dithionite and is regenerated upon exposure of the solution to oxygen. Resonance Raman studies on these enzymes15 35-38) have been reported by several laboratories (Table 4). These spectra are characterized by a set of four peaks at ca. 1605, 1505, 1270, and 1175 cm-1, which are assigned to ring vibrations of Fe(III) coordinated tyrosinate ligands. Similar spectra are obtained for the transferrins as well as for model iron-phenolate complexes (Table 4). A new class of iron proteins seems to... 

The synthesis and structure of bis-(2,4,6-trichlorophenolato)di-imidazolecopper(n) monohydrate is reported and claimed to be a possible model for copper binding in transferrins. The copper is at the centre of a tetragonally elongated octahedron two imidazole N and two phenolic O atoms occupy the corners of a plane and O and Cl atoms occupy axial positions. Copper(ii) and bismuth(iii) and lead(n) complexes of hydroxyethylenediaminetriacetic acid have been studied CuHL and CuL formation was suggested. ... 

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