Kronman M J, Bratcher S C
J Biol Chem. 1984 Sep 10;259(17):10887-95.
Terbium at submillimolar concentrations appears to bind to the calcium site of apo bovine alpha-lactalbumin and stabilizes the N conformation (fluorescence criterion; Kronman, M. J., Sinha, S., and Brew, K. (1981) J. Biol. Chem. 256, 8582-8586). At millimolar concentrations however, it binds additionally to a low affinity site of both apo- and calcium-liganded protein, inducing a time-dependent conformational change to an "expanded A-like state." The pH dependence of the transformation implicates the alpha-amino group of glutamic acid 1 of the protein in the binding process. The Zn2+ concentration dependence of the fluorescence of the calcium-free protein indicates there to be two binding sites for this metal ion in agreement with the binding studies with Zn2+ (Bratcher, S.C., and Kronman, M. J. (1984) J. Biol. Chem. 259, 10875-10886). Binding of Zn2+ at submillimolar concentrations stabilizes the A conformation of the protein in contrast to what was observed with Tb3+ at comparable metal ion concentrations. Millimolar concentrations of Zn2+ induce a time-dependent conformational change in both calcium-free and calcium-liganded alpha-lactalbumin to produce an "expanded A-like state" comparable to that seen with terbium at similar concentrations. In contrast to the "expanded A state" induced by high concentrations of zinc or terbium, a "collapsed A state" results from binding of 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid buffer species, EDTA, and ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (Kronman, M. J., and Bratcher, S. C. (1983) J. Biol. Chem. 258, 5707-5709), lysine methyl ester, arginine methyl ester, and histidine and by increasing ionic strength of the medium. Since the Zn2+ form of the protein (A conformation) promotes lactose synthetase activity (Kronman et al. cited above), the conformational flexibility of alpha-lactalbumin is likely to be of importance in the formation of the catalytically active complex with galactosyltransferase.
亚毫摩尔浓度的铽似乎与脱辅基牛α-乳白蛋白的钙结合位点结合，并稳定N构象（荧光标准；Kronman, M. J., Sinha, S., and Brew, K. (1981) J. Biol. Chem. 256, 8582 - 8586）。然而，在毫摩尔浓度下，它还会额外结合到脱辅基蛋白和钙配体蛋白的一个低亲和力位点上，诱导出一种随时间变化的构象变化，转变为“扩展的A样状态”。这种转变对pH的依赖性表明，蛋白质中谷氨酸1的α-氨基参与了结合过程。无钙蛋白荧光对锌离子浓度的依赖性表明，这种金属离子有两个结合位点，这与锌离子结合研究结果一致（Bratcher, S.C., and Kronman, M. J. (1984) J. Biol. Chem. 259, 10875 - 10886）。与在类似金属离子浓度下观察到的铽离子情况相反，亚毫摩尔浓度的锌离子会稳定蛋白质的A构象。毫摩尔浓度的锌离子会在无钙和钙配体的α-乳白蛋白中诱导出一种随时间变化的构象变化，产生一种与类似浓度铽离子作用下所见的“扩展的A样状态”。与高浓度锌或铽诱导的“扩展的A状态”不同，4-(2-羟乙基)-1-哌嗪乙磺酸缓冲液成分、乙二胺四乙酸（EDTA）和乙二醇双(β-氨基乙基醚)-N,N,N',N'-四乙酸（Kronman, M. J., and Bratcher, S. C. (1983) J. Biol. Chem. 258, 5707 - 5709）、赖氨酸甲酯、精氨酸甲酯、组氨酸以及增加介质离子强度会导致“塌陷的A状态”。由于蛋白质的锌离子形式（A构象）能促进乳糖合成酶活性（Kronman等人，见上文引用），α-乳白蛋白的构象灵活性在与半乳糖基转移酶形成催化活性复合物的过程中可能很重要。
