Primary and predicted secondary structures of the caseins in relation to their biological functions

In free solution, the caseins behave as non-compact and largely flexible molecules with a high proportion of residues accessible to solvent. Historically, they have been described as random coil-type proteins with only a nutritional function. Nevertheless, secondary structure prediction algorithms indicate that many parts of the (unphosphorylated, unglycosylated) polypeptide chains can form regular structures. In particular, a recurrent motif of the Ca2+-sensitive caseins in man, rat, mouse, guinea pig and ruminant species is an alpha-helix--loop--alpha-helix conformation in which the loop region typically contains a cluster of sites of phosphorylation. The biological function of the caseins is considered and it is suggested that the potential or actual conformations of the group of Ca2+-sensitive caseins are suited to the function of modulating the precipitation of calcium phosphate from solution. Either they can act as sites for nucleation or they can bind rapidly to calcium phosphate nuclei as they form spontaneously from supersaturated solution.