Lactoferrin lanthanide binding

For lactoferrin, which binds metal ions more strongly than transferrin (166), all the lanthanide ions, from Yb3+ to La3+, bind with two metal ions per molecule (147), with the log K2 values ranging from 6.4... 

Differences between the two sites become more pronounced for metal ions other than Fe " and anions other than COa . The differences are most pronounced for larger metal ions such as lanthanides. For transferrin some of the larger lanthanides appear to bind in only one of the two sites (Section IV.B.3), and for lactoferrin, although binding occurs in both sites, the second metal ion binds much more weakly, as shown by the curvature of the UV difference titration graph (Fig. 18) the biphasic release of Ce from lactoferrin contrasts with that of Fe (Fig. 28). Even metal ions of the first transition series, of similar size to Fe "", enhance the differences between the two sites. When Cr is bound to either transferrin 134) or lactoferrin (154), different EPR signals are seen for the two sites, and one Cr " ion is much more readily displaced by Fe than the other. Likewise, the EPR spectra of VO " -substituted transferrin indicate different metal configurations in the two sites (207), as do NMR studies of Co -substituted ovotransfer-rin (139). In these cases one metal ion is also released much more readily than the other as the pH is lowered. 

Transferrin and lactoferrin belong to a family of Fe cation buffer and carrier proteins. X-ray crystallography has shown that both proteins include two distorted octahedral Fe + sites, each of which is coordinated with one aspartate, one histidine, and two tyrosinate residues as well as one exogenous carbonate. Luminescent lanthanides such as Tb + and Eu + cations can be incorporated along with Ee cations into these binding sites to provide water-soluble luminescent lanthanide complexes in sophisticated three-dimensional cages. 

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