Transferrin carbonate complex, iron

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. 

Fig. 29. A structural model of the steps involved in the in vitro uptake of iron by transferrins, shown for one lobe. ( ) Iron (A) carbonate Y, Tyr ligands H, His ligand D, Asp ligand (O) chelate ligands. The positive charge at the anion site is due to the helix N-terminus and the Arg side chain. (Note that this is for the case in which Fe is added as a chelate complex.)...

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... 

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. 

The storage of iron in humans and other mammals has been dealt with in the previous section. Only a small fraction of the body s inventory of iron is in transit at any moment. The transport of iron from storage sites in cellular ferritin or hemosiderin occurs via the serum-transport protein transferrin. The transferrins are a class of proteins that are bilobal, with each lobe reversibly (and essentially independently) binding ferric ion. This complexation of the metal cation occurs via prior complexation of a synergistic anion that in vivo is bicarbonate (or carbonate). Serum transferrin is a monomeric glycoprotein of molecular weight 80 kDa. The crystal structure of the related protein, lactoferrin, has been reported, and recently the structure of a mammalian transferrin" has been deduced. 

Figure 14 Iron release from transferrin. Iron is coordinated through four protein residues (D63, H249, Y95, and Y188) and the synergistic bidentate carbonate anion. The lobe continually samples the open and closed conformations. Partial loss of the carbonate, aspartate, and histidine are the first steps in the process. Coordination of the anion or chelator (A/C) forms a quaternary complex between the protein, iron, carbonate and the chelator/anion. Decay of the quaternary complex yields apo transferrin.

The importance of coordination in the biochemistry of essential metallic elements may be illustrated by numerous examples of metal complexes of which the following are representative the iron complex hemoglobin and numerous enzymes containing the heme and related structures such as catalases, peroxidases and cytochromes and the iron-containing proteins ferritin, transferrin, and hemosiderin the zinc complexes zinc-insulin, carbonic anhydrase and the carboxypeptidases the cobalt complex vitamin B12 the copper complex, ceruloplasmin the molybdenum-containing enzymes, xanthine oxidase, and nitrate reductase DNA-metal ion complexes. 

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