The high-resolution X-ray crystal structure of human α-lactalbumin (at 1.8 Å) in the presence of an elevated level of calcium reveals a new secondary calcium binding site, 7.9 Å away from the primary calcium binding site known in all α-lactalbumin structures so far. The new calcium binding site is different from the zinc and sulfate binding sites [Ren, J., et al. (1993) J. Biol. Chem. 268, 19292-19298] but shares common features with the manganese binding site as described by Gerkin [Gerkin, T. A. (1984) Biochemistry 23, 4688-4697]. The proximity of the manganese and calcium binding region and the location of the functional site on one side of the charged surface of the α-lactalbumin molecule suggest that these binding sites might play a role in the formation of the lactose synthase complex.
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