Metals binding, transferrins, spectroscopic

The four metal-binding amino acid residues (2 Tyr, 1 Asp, 1 His) are present in both N- and C-sites of all transferrins so far sequenced, apart from melanotransferrin and the insect proteins (Table III). The same is true of the anion-binding Arg and Thr residues, and the residues at the N-terminus of the anion-binding helix are also strongly conserved. Superposition of the 81 common atoms of these residues, plus metal and anion, shows that their rms deviation in the N- and C-sites of diferric human lactoferrin is only 0.3 A. This close structural similarity is reflected in their spectroscopic properties. Where these have been compared, with the physiological Fe3+ and C032- ions bound, they are so similar as to be virtually identical (107, 56, 199). Nevertheless, there are a number of factors that can potentially lead to inequivalence in properties ... 

Finally, thallium has been used as an NMR probe of transferrin through the binding of 205T13+ (146). Although thallium is bound in both metal sites, they are distinguishable spectroscopically perhaps because the presence of a larger cation (radius of Tl3+, 0.89 A) accentuates the differences between the two sites (see also Section IV.D). 

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). 

The spectrophotometric technique exploits the fact that when it binds to transferrin, AF+ replaces hydroxyl protons from two tyrosines, thus causing a change in the UV region of the spectrum (Fig. 10). Titration of the spectral change as a function of [AP+] at constant transferrin concentration allows the binding stoichiometry and stability constant to be measured. This approach has been used to study the interaction of a large number of metals [e.g., Nd +, Sm , Zn +, and Ga 61, 62,136)] with transferrin, including AP+ (33, 43, 136, 138) some of these metals would otherwise be spectroscopically silent. Such... 

While several models of Cr(III) pharmacokinetics in mammals have been developed (111, 430, 502), little is known about Cr(III) speciation in organisms. Transferrin, a major transport protein for many metal ions (503), is likely to be the main Cr(III) carrier in blood (504). Although iron transport is a primary function of transferrin, the strength of its binding to Cr(HI) is comparable to that for Fe(III) (45, 505) and Cr -transferrin complexes [unlike for the complexes of Mn(II), Cu(n), or Zn(II)] mimic those of Fe(III) in interactions with cell transferrin receptors (506). Preliminary results of X-ray absorption spectroscopic studies suggest that Cr(lll) may occupy the same sites in the transferrin protein as Fe(lll) (507). 

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