Interaction with Al and Zn induces structure formation and aggregation in natively unfolded caseins

Caseins are phosphoproteins that form the principal protein component of milk, their chief function being the transport of inorganic calcium and phosphate to the neonates. The four major members of the casein family are alpha(s1)-, alpha(s2)- (together referred to as alpha(s)-casein), beta- and kappa-casein, each having a characteristic high negative net charge as well as high hydrophobicity and preferring extended conformational states in solution. We have investigated the influence of the polyvalent metal cations Zn(II) and Al(III) on the structure of bovine caseins, using fluorescence and circular dichroic (CD) spectroscopy and light scattering. Changes in Trp and ANS fluorescence parameters (blue shifts of the emission maxima and enhancement of fluorescence intensity) and in the far-UV CD spectra of the caseins caused by the presence of both metals suggest that conformational changes are induced in them by low concentrations (20-40 microM) of the metal cations. These changes lead to formation of solvent-accessible hydrophobic clusters or cavities that, in turn, cause self-association and precipitation of caseins at higher concentration of the metals. These conclusions are supported by increased binding of ThT to the caseins, as well as enhancement of light scattering intensity, observed in presence of Al(III). The chaperonic property of alpha(s)-casein, which enables it to inhibit thermal aggregation of alcohol dehydrogenase, is shown to be partially destroyed by Zn(II)-induced structural alterations, due possibly to loss of flexibility of the natively unfolded casein chains.