Group 13 metals, transferrin binding

The chemistry of the group 13 metals makes binding studies difficult, as careful control of pH and bicarbonate concentration is necessary to prevent formation of species such as Al(OH)4 and Ga(OH)4". Nevertheless, UV difference spectra have shown that Al3+, Ga3 +, In3+, and Tl3 all form transferrin complexes with two metal ions per molecule (126, 144-146). Ae values imply the ionization of two tyrosines per bound metal ion, as for other specifically bound metals. NMR studies using 13C-enriched bicarbonate show virtually identical spectra for Al3+ and Ga3+, implying equivalent metal-anion environments (99). 

Chemists have a variety of different interests in the transferrins. One interest concerns the chemistry of the binding of metal ions. No prosthetic or unusual chemical groupings are present in the proteins, and the binding is rather a property of the unique folding and juxtaposition of the normal chemical groupings in protein molecules (49). Another interest is in the mechanism of the release of iron by the serum transferrin to the tissues. The iron is so tightly bound that some investigators have attempted to invoke a special enzymatic mechanism for the dissociation in the tissues, but no evidence for this has been found. 

The transferrins are a group of specific metal-binding proteins, the best characterized of which are serotransferrin (present in blood plasma, milk, spinal fluid and semen), ovotransferrin (conalbumin present in avian and reptile egg white) and lactotransferrin (present in milk, pancreatic juice, tears and leucocytes). 

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