Conformational changes induced by zinc and terbium binding to native bovine alpha-lactalbumin and calcium-free alpha-lactalbumin.

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.