Carbonate transferrin

Other non-haem proteins, distinct from the above iron-sulfur proteins are involved in the roles of iron transport and storage. Iron is absorbed as Fe" in the human duodenum and passes into the blood as the Fe protein, transferrin, The Fe is in a distorted octahedral environment consisting of 1 x N, 3x0 and a chelating carbonate ion which... 

The determination of the structure of the iron transporter, ferric-binding, protein (hFBP)t from Haemophilus influenzae (Bruns et ah, 1997) at 0.16 nm resolution shows that it is a member of the transferrin superfamily, which includes both the transferrins and a number of periplasmic binding proteins (PBP). The PBPs transport a wide variety of nutrients, including sugars, amino acids and ions, across the periplasm from the outer to the inner (plasma) membrane in bacteria (see Chapter 3). Iron binding by transferrins (see below) requires concomitant binding of a carbonate anion, which is located at the N-terminus of a helix. This corresponds to the site at which the anions are specifically bound in the bacterial periplasmic sulfate- and... 

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... 

In addition to the amino acid side chains mentioned above, a number of other low molecular weight ligands are found in metalloproteins. These include cyanide and carbon monoxide, which we will describe later in this chapter. Here we consider carbonate and phosphate anions in the context of the super family of iron-binding proteins, the transferrins. 

Figure 3.2 Ribbon diagram of the C-lobe of human transferrin with the two domains shown in different colours (cyan for Cl and green for C2). The inset shows the four protein ligand residues together with the arginine residue which stabilizes binding of the synergistic carbonate ion (both in magenta). (Reprinted with permission from Mason et al., 2005. Copyright (2005) American Chemical Society.)...

It is conceivable that iron could be stored in the form of a complex such as transferrin or even hemoglobin, and in lower organisms ferrichrome apparently serves this purpose. Such storage is wasteful, however, and higher animals have evolved a simpler method of storing iron as ferritin. If iron(lll) nitrate is allowed to hydrolyze in a solution made slightly basic by the hydrogen carbonate ion (HCOJ-), it spontaneously forms spheres of FeOOH" of about 7000 pm in diameter. The core of a ferritin particle is similar and contains up to 4500 iron atoms and apparently some... 

Milk transferrin (lactoferrin51a b c also found in leukocytes), hen egg transferrin (ovotransferrin),52 52a and rabbit and human serum transferrin54 543 all have similar structures. Each Fe3+ is bonded to oxygen anions from two tyrosine side chains, an aspartate carboxy-late, an imidazole group, and the bound carbonate ion (Fig. 16-2B). 

Abbreviations BCA II bovine carbonic anhydrase II SOD superoxide dismutase TRN transferrin AP alkaline phosphatase Pj inorganic phosphate PDO phthalate dioxygenase E = empty . 

The importance of carbon dioxide in the formation of the iron complexes of the human serum transferrin was shown early by Fiala and Burk (46) and Schade et al. 117). Warner and Weber (133), by an indirect but exquisite method, proved the participation of CO2 in the formation of the complex, and also that it was in the form of bicarbonate or carbonate. This was done through experiments in which the iron complex was formed in the absence of CO2, or by the addition of small amounts, of gaseous CO2, and noting the red color of the complex was formed only very slowly. However, when the enzyme carbonic anhydrase was added, together with the CO2, the red color of the complex appeared rapidly. In addition the same workers showed by the use of C1402 that one mole of CO2 was bound per mole of iron bound. 

Although the binding of bicarbonate or carbonate in the iron complex of serum transferrin and chicken ovotransferrin has been established unquestionably, no such vigorous proof has been presented for the copper complexes. As will be discussed below, this is an important omission, because it is difficult to approximate the proposed formula for the iron and copper complexes without proof of the presence of bicarbonate or carbonate in the copper complex. In addition, there appear to be no investigations on the question of whether the carbonate or bicarbonate is required for the initial binding of iron, or for the formation of the colored complex. 

Early experimental evidence in support of the hypothesis that an attack on the anion is at the heart of the iron-exchange mechanism (53) was soon corroborated by work from several laboratories (54, 55, 88). Replacing carbonate with oxalate at the specific anion-binding site of transferrin results in a relatively stable ternary Fe(III)-transferrin-oxalate complex. Over the time course of many hours or even days the oxalate complex slowly reverts to the physiologic Fe(III)-transferrin-carbonate form, but since in vitro studies seldom require more than an hour or two, the biologic properties of the oxalate complex can be tested. 

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