Serum transferrins structure

All transferrins characterized so far consist of a single polypeptide chain of 670-700 amino acid residues. The lactoferrin and serum transferrin structure analyses show that the folding (polypeptide chain conformation) is the same in both proteins and, given their sequence homology, can be assumed to hold for all proteins of the transferrin family. 

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... 

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. 

Ser Se isotopic substitution, 38 105-107 Serum albumin, 46 470 Serum transferrins, 41 390 biological role, 41 391-392 half-molecules, 41 396 recombinant, 41 453 structure, 41 397... 

Fig. 4.5.8 Examples for structures and molecular masses of 2-aminobenzamide (2-AB)-labelled oligosaccharide moieties derived from serum transferrin. Values below the oligosaccharide structures indicate the expected masses (in Da) by matrix-assisted laser desorption ionisation - time of flight analysis.

FIG. 34.—Molecular Model of Diantennary Glycan of Human-serum Transferrin. [A, Y Conformation14 B, T Conformation. 4 Numbers correspond to the numbering used in Table II (see fundamental structure).]... 

Studies of the optical rotatory dispersion of the transferrins and their metal complexes have not only shown important differences between the two forms of the proteins but also have afforded penetrating insight into the possible groups involved in chelation of the metal and in the structure of the complex (128, 129). These authors studied both chicken ovotransferrin and human serum transferrin and obtained essentially identical results with both proteins. Metal-free ovotransferrin had a plain negative rotatory dispersion between 300 and 675 mjj. with a specific rotation,... 

Like the studies with optical rotatory dispersion, studies with electron spin (paramagnetic) resonance not only have revealed important differences among the metal-free transferrins and their metal complexes but also have given insight into the nature of the binding sites and the structure of the complexes. Aasa et al. (1) reported on the iron and copper complexes of human serum transferrin and chicken ovotransferrin while Windie et al. 137) reported on human serum transferrin, human lacto-transferrin, chicken ovotransferrin, quail ovotransferrin, and turkey ovotransferrin. 

Azari and Feeney (7) suggested that human serum transferrin and chicken ovotransferrin underwent structural changes on chelation of metal ions which stabilized the molecule to denaturation and proteolysis. This later was extended (8) to include stabilization to chemical cleavage or modification. A similar interpretation also was made by Glazer and McKenzie (55) who further suggested that the iron complex might provide two crosslinks between widely separated sections of the peptide chains. 

Since that time many more sequences have become available through the advent of recombinant DNA technology and the deduction of amino acid sequences from the base sequences of cloned DNA. At the present time, the primary structures (amino acid sequences) of 14 proteins of the transferrin family have been established. These include seven serum transferrins, from human 10, 36), pig (37), horse 38), rabbit 39), toad Xenopus laevis) 40), sphinx moth (M. sexta) 13), and cockroach Blaberus discoidalis 4) chicken 34, 35) and duck 41) ovotransfer-rins four lactoferrins, from human (11, 42), mouse 43), pig 44) and cattle 45, 46) and the human tumor cell melanotransferrin 47). All of these sequences are available from sequence databases such as EMBL and SWISSPROT. 

The first crystallographic studies on transferrins date back more than 20 years (58), and crystals of various transferrins have since been reported. These include the diferric forms of rabbit (59) and human (60) serum transferrins, hen (61) and duck (62) ovotransferrins, human (63) and bovine (64) lactoferrins, and the apo- (iron free) forms of human lactoferrin (65) and duck ovotransferrin (62). In spite of all this activity, the crystals in many cases have proved difficult to handle, and the X-ray analyses quite challenging. A low-resolution analysis of rabbit serum transferrin in 1979 demonstrated the bilobal nature of the molecule (66), but it was not until 1987, with the publication of the structure of human lactoferrin (67), that full details of a transferrin... 

Complementing the structural studies of the intact transferrins, a number of fragments have also been crystallized, including proteolytic N-terminal half-molecules of rabbit serum transferrin (69) and chicken ovotransferrin (70), recombinant N-terminal half-molecules of human lactoferrin (71) and human serum transferrin (72), and a quarter-molecule fragment of duck ovotransferrin (73). All of these have now led to high-resolution structures (74-77). 

The most detailed description of a complete transferrin molecule is that of human lactoferrin, at 2.8-A resolution (78), and most of the data in the following sections come from this work and from refinement of the same structure at 2.1-A resolution (79). As would be expected from the high level of sequence similarity, the three-dimensional structure of rabbit serum transferrin (68), although at lower resolution (3.3 A), is completely consistent with that of lactoferrin the differences are at the level of individual amino acid changes, together with some differences in lobe and domain orientations. These are discussed below (Section III.B.l). 

Coordinates for human lactoferrin, in both diferric (78) and apo- (80) forms, for diferric rabbit serum transferrin (68), and for the three fragment structures, the proteolytic N-lobe of rabbit serum transferrin (74), the recombinant N-lobe of human lactoferrin (75) and the duck ovotransferrin quarter-molecule (76), all in their iron-bound forms, can be obtained from the Brookhaven Protein Data Bank (Brookhaven National Laboratory, Upton, New York). 

The two lobes are joined by a short connecting peptide, which is the only covalent link between them. This peptide varies between different transferrins, both in length (7 to 14 residues—see Section III.C) and in conformation in lactoferrin, it is 12 residues long and forms a three-turn helix, whereas in serum transferrin it is 14 residues long and has a much less regular structure. 

One final structure which merits comment is that of a monoferric form of human serum transferrin. In this structure, determined at 3.0 A resolution 110) the N-lobe is metal-free and has an opening of —50° relative to the iron-bound N-lobe of rabbit transferrin the opening thus... 

RNA secondary structure plays a role in the regulation of iron metabolism in eukaryotes. Iron is an essential nutrient, required for the synthesis of hemoglobin, cytochromes, and many other proteins. However, excess iron can be quite harmful because, untamed by a suitable protein environment, iron can initiate a range of free-radical reactions that damage proteins, lipids, and nucleic acids. Animals have evolved sophisticated systems for the accumulation of iron in times of scarcity and for the safe storage of excess iron for later use. Key proteins include transferrin, a transport protein that carries iron in the serum, transferrin receptor, a membrane protein that binds iron-loaded transferrin and initiates its entry into cells, and ferritin, an impressively efficient iron-storage protein found primarily in the liver and kidneys. Twenty-four ferritin polypeptides form a nearly spherical shell that encloses as many as 2400 iron atoms, a ratio of one iron atom per amino acid (Figure 31.37). 

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