Effect of protein charge manipulation on αs1-casein-enriched protein self-assembly

Abstract

Enzymatic dephosphorylation of casein removes phosphate groups from serine residues and reduces the negative charge. In contrast, succinylation caps the ε-amino group of lysine residues and increases the negative charge on the caseins. The effect of these modifications on the self-association of α_s1-casein was studied using analytical ultracentrifugation. Dephosphorylation and succinylation have contrasting effects on α_s1-casein self-assembly. Native α_s1-casein was a mixture of dimers, trimers and higher order oligomers under these experimental conditions. Increasing the level of succinylation dissociated oligomeric α_s1-casein resulting in an increase in the monomeric protein. In contrast, dephosphorylated samples formed larger assemblies compared to the native α_s1-casein. Experiments performed at protein concentrations of 1, 2 or 3 mg mL⁻¹ provided consistent results indicating that across this concentration range there is no major difference in the assembly formation. This study demonstrated the utility of analytical ultracentrifugation to understand casein assembly, which will underpin the understanding of proteins structures in dairy foods.