Metal-ion binding and the molecular conformational properties of alpha lactalbumin.

Mammary galactosyltransferase and alpha lactalbumin are the two protein components of lactose synthase which catalyze the transfer of galactose from UDP-gal to glucose in the presence of divalent cations. Recent studies suggest that alpha lactalbumin may have a broader function in modifying cell surface carbohydrates in cell-cell interactions and cell differentiation. Since the discovery that alpha lactalbumin, like galactosyltransferase, is a metalloprotein, there has been a great deal of interest in the metal-binding properties of this protein and how these relate to the metal-ion requirements of the lactose synthase reaction. The recent availability of an X-ray crystal structure of alpha lactalbumin has provided further impetus for establishing the molecular determinants of its biological activity. This review is directed toward critically examining and integrating our present knowledge of the properties of this protein, particularly the relationship between metal-ion binding and conformational state, and how these might relate to its biological function.