Negative cooperativity of chicken ovotransferrin on Al(III)-binding

Chicken ovotransferrin, an iron binding protein, has two metal binding sites (amino (N) and carboxy (C) terminal sites). It binds Cu(II), Al(III), Co(II), and other metals, as well as Fe(III). In this study, the selectivity and cooperativity of the N and C sites on Al(III), Co(II), and Tb(III) binding were investigated. Metals were classified into two groups according to their site preference. Co(II) and Al(III) bound to the N site more preferably than to the C site, whereas Tb(III) bound to the C site more preferably. On Fe(III) binding, the binding constant of Fe(III) becomes larger when the other site is already occupied. Thus, positive cooperativity is seen. In the present study, the binding cooperativities of Co(II), Tb(III), and Al(III) as to the N and C sites were investigated. On Co(II) and Tb(III) binding, no cooperativity was observed, as in the case of Cu(II) [Yamamura, T. et al. (1985) in Proteins of Iron Storage and Transport (Spik, G., Montreuil, J., Crichton, R.R., & Mazurier, J., eds.) pp. 53-56, Elsevier Science Publ. B.V., Amsterdam]. In contrast, negative cooperativity was observed on Al(III) binding. Based on a model proposed by Yamamura et al. [Yamamura, T. et al. (1985) ibid.], the ratio of the binding constants, KC/KN, and the stacking coefficient, Kst, were estimated. KC/KN is 2.2 +/- 0.4 for the Tb(III) ion, 0.5 +/- 0.1 for the Co(II) ion, and 0.12 +/- 0.02 for the Al(III) ion. Kst (= 1 in a non-cooperative case) is 0.98 +/- 0.02 for the Tb(III) ion, 1.03 +/- 0.02 for the Co(II) ion, and 0.55 +/- 0.22 for the Al(III) ion.