Role of calcium phosphate nanoclusters in the control of calcification

Calcium phosphate nanoclusters are equilibrium particles of defined chemical composition in which a core of amorphous calcium phosphate is sequestered within a shell of casein phosphopeptides. Sequence analyses and a structure prediction method were applied to secreted phosphoproteins of known importance in controlling calcification, and eight noncasein phosphoproteins were identified as containing one or more subsequences capable of forming nanoclusters. Small-angle X-ray scattering was used to confirm that a plasmin phosphopeptide of one of the identified proteins, osteopontin, formed a novel type of calcium phosphate nanocluster in which the radius of the amorphous calcium phosphate core was four times larger than is typical of casein nanoclusters. A thermodynamic treatment of nanocluster formation identified the factors of importance in determining the equilibrium size of the core, and showed how a nanocluster solution could be thermodynamically stable yet supersaturated with respect to the mineral phase of bones and teeth. It is suggested that the ability of some secreted phosphoproteins to form nanoclusters is physiologically important for the control or inhibition of calcification in soft and mineralized tissues, the extracellular matrix and a wide range of biofluids, including milk and blood.