title: Ability of a beta-casein phosphopeptide to modulate the precipitation of calcium phosphate by forming amorphous dicalcium phosphate nanoclusters.
abstract: The ability of casein in the form of colloidal-sized casein micelles to modulate the phase separation of calcium phosphate during milk secretion is adapted to produce nanometre-sized particles of calcium phosphate stabilized by a casein phosphopeptide (nanoclusters). The nanoclusters were prepared from an undersaturated solution of salts and the peptide by raising the pH homogeneously from about 5.5 to 6.7 with urea plus urease. Chemical analysis and IR spectroscopy showed that they comprise an amorphous dicalcium phosophate bound to the phosphopeptide. Multinuclear NMR spectroscopy of the cluster solutions showed that the small ions and free peptide in the solution were in a state of dynamic exchange with the nanoclusters. The peptide is linked to the calcium phosphate through its sequence of phosphorylated residues, but, in a proportion of adsorbed conformational states, the termini retain the conformational freedom of the unbound peptide. The ability of casein to form nanoclusters in milk suggests a more general mechanism for avoiding pathological calcification and regulating calcium flow in tissues and biological fluids exposed to or containing high concentrations of calcium.
authors.affiliation: Hannah Research Institute, Ayr, Scotland, U.K.
authors.forename: C
authors.initials: C
authors.lastname: Holt
authors.forename: N M
authors.initials: NM
authors.lastname: Wahlgren
authors.forename: T
authors.initials: T
authors.lastname: Drakenberg
mesh_terms: D000327:Adsorption; D000595:Amino Acid Sequence; D002118:Calcium; D002130:Calcium Phosphates / Q000737:chemistry*; D002364:Caseins; D011232:Chemical Precipitation; D004492:Edetic Acid; D007700:Kinetics; D008274:Magnesium; D009682:Magnetic Resonance Spectroscopy; D008969:Molecular Sequence Data; D010446:Peptide Fragments / Q000737:chemistry; D010748:Phosphopeptides / Q000737:chemistry; D011485:Protein Binding; D011487:Protein Conformation; D013055:Spectrophotometry, Infrared; D013816:Thermodynamics; D014462:Ultrafiltration; D014508:Urea; D014510:Urease; D014961:X-Ray Diffraction
chemical_list: D002130:Calcium Phosphates; D002364:Caseins; D010446:Peptide Fragments; D010748:Phosphopeptides; D014508:Urea; D004492:Edetic Acid; D014510:Urease; D008274:Magnesium; D002118:Calcium