Lactoferrin (Lf), isolated from human colostrum, has been complexed with a variety of transition metal ions. In addition to binding two iron(III) or copper(II) ions it strongly and specifically binds two cobalt(III), chromium(III) and manganese(III) ions, in the presence of bicarbonate. Such strong, specific binding of copper(II) to lactoferrin will not occur in the absence of bicarbonate, instead only a weak interaction is observed. Lactoferrin has also been shown to weakly bind manganese(II), cobalt(II), nickel(II), zinc(II), cadmium(II), lead(II) and mercury(II), though manganese(II) will undergo aerial oxidation in the presence of the protein and bind as manganese(III). Those metal ion protein complexes have been examined using the techniques of fluorescence, electronic, e.s.r. and Resonance Raman spectroscopy. The close similarity between the spectra of the complexes Fe2Lf, Cu2Lf, Co2Lf, Mn2Lf and Cr2Lf with those of transferrin and ovotransferrin reinforce the postulate that the metal binding sites in all three proteins are similar. The sites in all three proteins are essentially rhombic in character and contain 2-3 tyrosyl residues and at least one histidyl residue. A distinct heterogeneity is evident from the e.s.r. spectrum of Cr2Lf and metal ion replacement studies indicate that chromium in one site is more labile than the other. In addition to the Specific sites lactoferrin appears to have a number of non-specific sites on the outside of the protein capable of weakly coordinating metal ions such as chromium(III) and copper(II). Differences evident in the non-specific sites available to copper when manganese(III) is bound to lactoferrin, to those available when iron(III), cobalt(III) and chromium(III) are bound, suggest that the stronger binding of manganese(III) results from it inducing a different conformational change in the protein to those other metals and one that favours the higher stability of its metal-protein complex. A series of small molecular weight complexes of iron(III) with bi-, tri-, tetra- and hexadentate ligands, containing phenolate groups were prepared and examined by electronic, e.s.r. and Mössbauer spectroscopy. Complexes of copper(II) with nitrogen bases and chloro- and bromo-substituted phenolates were prepared and examined by electronic and e.s.r. spectroscopy. Results from these studies would favour the involvement of three tyrosyl residues and two cis histidyl residues in the metal-binding sites of lacto-ferrin and from studies on the copper complexes it seems likely that one histidyl and one tyrosyl are axially coordinated to the metal. Evidence from a single crystal x-ray structure is presented which would favour the interaction of bicarbonate ion with iron(III) via a solvent (water) molecule.
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W. Ainscough, E., Brodie, A. M., & Plowman, J. E. (1979). The chromium, manganese, cobalt and copper complexes of human lactoferrin. Inorganica Chimica Acta, 33(C), 149-153.