Metals site, transferrins

Like the studies with optical rotatory dispersion, studies with electron spin (paramagnetic) resonance not only have revealed important differences among the metal-free transferrins and their metal complexes but also have given insight into the nature of the binding sites and the structure of the complexes. Aasa et al. (1) reported on the iron and copper complexes of human serum transferrin and chicken ovotransferrin while Windie et al. 137) reported on human serum transferrin, human lacto-transferrin, chicken ovotransferrin, quail ovotransferrin, and turkey ovotransferrin. 

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). 

The metal ions that bind to transferrins are very diverse in size and in their coordination preferences, raising the question of what degree of induced fit there is in the metal sites. When the protein domains close over the bound metal ion (Section III.B.5), does the metal simply fit into a prepared site or are the details of the binding site moulded to some degree by the stereochemical preferences of the metal This question is of great importance for correlating structure with spectroscopy. 

Spectroscopic studies have consistently demonstrated the existence of multiple conformational states for the metal sites in transferrins, especially when using metal ions other than Fe3+ and anions other than C032. The differences are not necessarily related to intrinsic geometrical differences between the two sites in each molecule, but also reflect changes dependent on pH, the nature of the synergistic anion, or salt effects. 

Spin echo measurements on metal complexes have been reviewed extensively by Mims and Peisach [267]. One use of this technique has been to study anion binding at paramagnetic metal centers. When cupric ion is substituted for ferric ion in transferrin, a pattern corresponding to the weak coupling for in C-doped bicarbonate indicates that bicarbonate is bound directly to the metal ion [269]. Thus, a modulation pattern detected by spin echo can confirm that adducts with a weakly coupled nuclear spin are bound directly to the metal site. 

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