Transferrin diferric carbonate

Fig. 23. Visible absorption spectra for diferric transferrin complexes utilizing various synergistic anions, showing the variation in mal for the charge transfer band. Anions are 1, nitrilotriacetate 2, carbonate 3, salicylate 4, thioglycolate 5, glycine 6, glyoxy-late and 7, glycolate. From Schlabach and Bates (178), with permission.

Distinct differences are also seen when anions other than C032 are used. The crystal structure of oxalate-substituted diferric lactoferrin shows differences in the anion binding in the two sites in the C-site the oxalate is symmetric bidentate, whereas in the N-site it is asymmetric (193). When Cu2+ is the metal ion the oxalate binding differences become even more pronounced. Copper-transferrin binds oxalate only in its N-terminal site (91). Copper-lactoferrin and copper-ovotransfer-rin each bind two oxalate ions but binding occurs preferentially in the C-lobe (157,192). These different affinities mean that hybrid complexes can be prepared with oxalate in one site and carbonate in the other (92, 157, 192). The use of oxalate as synergistic anion gives rise to spectroscopically distinct sites for other metal ions also (171). 

Most mammalian cells take up iron transported in serum by transferrin, an 80 kD bilobal protein with two identical iron-binding sites, one in each half of the molecule. The coordination of the iron atom involves four protein ligands and a carbonate anion, as we described in Chapter 4, and the transferrin to cell cycle for delivering iron was outlined in Chapter 7. Iron release from the extremely stable diferric transferrrin bound to its receptor is facilitated by the acidic pH prevailing in the endosome and by the fact that the receptor imposes a conformation on the bound transferrin that is essentially that of the open apotransferrin form. 

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