Binding of Zn(II) ions to α-lactalbumin

Abstract

The binding of Zn(II) ions to human and bovine α-lactalbumin has been studied by fluorescence, scanning microcalorimetry, and proteolytic digestion. The intrinsic tryptophan fluorescence spectrum of Ca(II)-loaded α-lactalbumin is insensitive to Zn(II) binding to the strong cation binding sites (Zn:protein ratios up to 20), yet the thermal denaturation transition, as detected by intrinsic fluorescence, is shifted toward lower temperatures. On the other hand, low concentrations of Zn(II) ([Zn]:[protein]5 M⁻¹) increases its susceptibility to tryptic and chymotryptic digestion, slightly decreases its affinity for the fluorescent probe, bis-ANS, and alters its interactions with UDP-galactose. Zn(II) binding to aggregated forms of α-lactalbumin increases its affinity to bis-ANS.