α-lactalbumin possesses a distinct zinc binding site

It has been proposed that the binding of Zn²⁺ to α-lactalbumin switches the conformation to one akin to a state intermediate in the folding of the protein. However, the high resolution x-ray crystal structure of human α-lactalbumin-Zn²⁺ complex at 1.7-Å resolution (pH 7.6) does not reveal any significant change in conformation from the native state. The Zn²⁺ ion binds specifically in the "cleft" of α-lactalbumin (the region which forms the active site of the homologous protein lysozyme). This may suggest a possible role for Zn²⁺ binding in lactose synthase complex. The coordination of the Zn²⁺ ion involves a symmetry-related molecule in the crystal, the crystal contacts being stabilized by a SO₄^2- ion bound at the interface between three molecules.