Transferrin: internal homology in the amino acid sequence

A computational study of the open and closed forms of the N-lobe human serum transferrin apoprotein

Human serum transferrin tightly binds ferric ions in the blood stream but is able to release them in cells by a process involving receptor-mediated endocytosis and decrease in pH. Iron binding and release are accompanied by a large conformation change. In this study, we investigate theoretically the open and closed forms of the N-lobe human serum transferrin apoprotein by performing pKa calculations and molecular dynamics and free-energy simulations. In agreement with the hypothesis based on the x-ray crystal structures, our calculations show that there is a shift in the pKa values of the lysines forming the dilysine trigger when the conformation changes. We argue, however, that simple electrostatic repulsion between the lysines is not sufficient to trigger domain opening and, instead, propose an alternative explanation for the dilysine-trigger effect. Analysis of the molecular dynamics and free-energy results indicate that the open form is more mobile than the closed form and is much more stable at pH 5.3, in large part due to entropic effects. Despite a lower free energy, the dynamics simulation of the open form shows that it is flexible enough to sample conformations that are consistent with iron binding.

Human lactotransferrin: amino acid sequence and structural comparisons with other transferrins

The complete amino acid sequence (703 amino acid residues) of human lactotransferrin has been determined. The location of the disulfide bridges has also been investigated. Computer analysis established internal homology of the two domains (residues 1-338 and residues 339-703). Each domain contains a single iron-binding site and a single glycosylation site (asparagine residues 137 and 490) located in homologous positions. Prediction of the secondary structure of the two homologous moieties of human lactotransferrin has also been performed. The present results allowed a series of comparisons to be made with human serum transferrin and hen ovotransferrin.

The complete amino acid sequence of human serum transferrin

The complete amino acid sequence of human serum transferrin has been determined by aligning the structures of the 10 CNBr fragments. The order of these fragments in the polypeptide chain is deduced from the structures of peptides overlapping methionine residues and other evidence. Human transferrin contains 678 amino acid residues and--including the two asparagine-linked glycans--has an overall molecular weight of 79,550. The polypeptide chain contains two homologous domains consisting of residues 1-336 and 337-678, in which 40% of the residues are identical when aligned by inserting gaps at appropriate positions. Disulfide bond arrangements indicate that there are seven residues between the last half-cystine in the first domain and the first half-cystine in the second domain and therefore, a maximum of seven residues in the region of polypeptide between the two domains. Transferrin--which contains two Fe-binding sites--has clearly evolved by the contiguous duplication of the structural gene for an ancestral protein that had a single Fe-binding site and contained approximately 340 amino acid residues. The two domains show some interesting differences including the presence of both N-linked glycan moieties in the COOH-terminal domain at positions 413 and 610 and the presence of more disulfide bonds in the COOH-terminal domain (11 compared to 8). The locations of residues that may function in Fe-binding are discussed.

Organization and sequence studies of the 17-piece chicken conalbumin gene

The conalbumin gene has been cloned and shown to consist of at least 17 exons approximately 60-200 base pairs long. The DNA sequence upstream from the region coding for the 5' end of the mRNA shows similarities with sequences present in homologous positions in other genes. High and low frequency repetitive sequences are found both upstream from the conalbumin gene and within one intron.

Structural studies on rabbit transferrin: isolation and characterization of the glycopeptides

The structure of rabbit transferrin was investigated with regard to number, size, and composition of the heteropolysaccharide units and their relative location on the polypeptide chain. The composition and molecular weight of the Pronase glycopeptides revealed that rabbit transferrin contains two heteropolysaccharide units, each composed of 2 sialic acid residues, 2 galactose residues, 3 mannose residues, and 4-N-acetylglucosamine residues. The composition and molecular weight of the tryptic glycopeptides further substantiated the existence of two identical heteropolysaccharide units and revealed that both units have identical amino acid residues in the immediate vicinity of the carbohydrate attachment sites to the polypeptide chain, suggesting a sequence homology surrounding the two glycosylation sites. Characterization of the cyanogen bromide fragments from rabbit transferrin indicated that both heteropolysaccharide units are located within a single polypeptide fragment representing approximately one-third of the molecule.

Iron-binding fragments from the N-terminal and C-terminal regions of human lactoferrin

Digestion of lactoferrin with pepsin at pH3.0 gave an iron-binding half-molecule that represents the C-terminal part of the native protein. Tryptic or chymotryptic digestion of 30%-iron-saturated lactoferrin yielded the N- and C-terminal half molecules, which could be separated by DEAE-Sephadex chromatography. The N- and C-terminal fragments did not show any immunological cross-reaction. The carbohydrate of lactoferrin was distributed equally between the two fragments.

Characterization of monoferric fragments obtained by tryptic cleavage of bovine transferrin

1. The electrophoretically fast (F) and slow (S) fragments obtained by tryptic cleavage of bovine iron-saturated transferrin differed in carbohydrate content and peptide 'maps'. 2. A fragment capable of binding one Fe3+ ion per molecule was isolated after brief tryptic digestion of bovine apotransferrin and shown closely to resemble the S fragment obtained from the iron-saturated protein. 3. Fragments F and S are probably derived from the N- and C-terminal halves of the transferrin molecule respectively. 4. Bovine transferrin could donate iron to rabbit reticulocytes, but the monoferric fragments possessed little iron-donating ability.

Transferrin: physiology and function in iron transport

Since its identification about 30 years ago transferrin has attracted the interest of many investigators concerned with its structure, metal-binding properties, genetic polymorphism, and especially its role in the transport of iron within the body. Transferrin's two iron-binding sites appear to be structurally identical with equivalent iron binding after addition of iron in vitro. However, since the experiments of Fletcher and Huehns, the functional homogeneity of transferrin-bound iron has been questioned. Understanding of the precise mechanism of iron release from transferrin to receptor sites on reticulocytes and other tissues active in iron exchange is incomplete. Considerable evidence has been assembled from rats to support the Fletcher-Huehns hypothesis of selective release of A-site iron to erythrocyte precursors and placenta while B-site iron is delivered to hepatocytes. Results of experiments in rabbit and human systems remain controversial. Reasons for this controversy include: variations in the technique of iron addition to transferrin with the possibility of non-specific binding; variations in reticulocytes used in preparing selectively labelled transferrin and in its biological assay; and artifacts introduced in mixed-species experiments. Until methods are more refined and the transferrin-iron receptor interaction is better understood, the controversy about transferrins's iron transport function will persist.