Structural Analysis of the Environment of Calcium Ions in Crystalline and Amorphous Calcium Phosphates by X-Ray Absorption Spectroscopy and a Hypothesis Concerning the Biological Function of the Casein Micelle

Determination of the structural environment around calcium ions, using X-ray absorption spectroscopy, in crystalline calcium phosphates is described. The spherically averaged short-range order in an amorphous calcium phosphate can be quantitatively determined using, as a model, the environment of calcium ions in the crystalline calcium phosphate which is closest to it in chemical composition. Structural studies, together with other evidence, suggest that the colloidal calcium phosphate in milk comprises small clusters of Ca2+ and HPO42- ions in a fairly disordered state. The ion clusters are distributed throughout the micelle and are probably linked through phosphoseryl residues, to the Ca2+-sensitive caseins. Calculations indicate that probably about five polypeptide chains interact with a typical ion cluster. The small size of the ion clusters, their amorphous nature and the relative conformational flexibility of the casein proteins suggests a possible biological function for caseins and the casein micelle in the control of calcium phosphate precipitation. During calcium transport by the secretory cells of the mammary epithelium, calcium phosphate is precipitated in the Golgi vesicles. It is suggested that the caseins trap the growth of the precipitate at the nucleation stage, effectively preventing a potentially disastrous. first-order phase transition.