Ichimura K, Kihara H, Yamamura T, Satake K
Department of Chemistry, Faculty of Science, University of Tokyo.
J Biochem. 1989 Jul;106(1):50-4. doi: 10.1093/oxfordjournals.jbchem.a122818.
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.
鸡卵转铁蛋白是一种铁结合蛋白，有两个金属结合位点（氨基（N）端和羧基（C）端位点）。它能结合铜（II）、铝（III）、钴（II）以及其他金属，还有铁（III）。在本研究中，对N端和C端位点在结合铝（III）、钴（II）和铽（III）时的选择性和协同性进行了研究。根据金属对位点的偏好，将金属分为两组。钴（II）和铝（III）更倾向于结合N端位点而非C端位点，而铽（III）更倾向于结合C端位点。在结合铁（III）时，当另一个位点已被占据时，铁（III）的结合常数会变大。因此，呈现出正协同性。在本研究中，研究了钴（II）、铽（III）和铝（III）对N端和C端位点的结合协同性。在结合钴（II）和铽（III）时，未观察到协同性，这与铜（II）的情况相同[山村，T.等人（1985年），载于《铁储存与运输蛋白》（斯皮克，G.，蒙特雷伊尔，J.，克里顿，R.R.，&马祖里尔，J.编），第53 - 56页，爱思唯尔科学出版社，荷兰阿姆斯特丹]。相反，在结合铝（III）时观察到了负协同性。基于山村等人提出的模型[山村，T.等人（1985年），同上]，估算了结合常数的比值KC/KN和堆积系数Kst。铽（III）离子的KC/KN为2.2±0.4，钴（II）离子的为0.5±0.1，铝（III）离子的为0.12±0.02。Kst（在非协同情况下=1），铽（III）离子的为0.98±0.02，钴（II）离子的为1.03±0.02，铝（III）离子的为0.55±0.22。
