Composition and structure of micellar calcium phosphate

Abstract

Micellar calcium phosphate has the chemical composition and physicochemical properties that are consistent with it being a complex of the phosphate centres of casein with an acidic amorphous calcium phosphate. Similar acidic amorphous calcium phosphates have been prepared in the laboratory and for these, as well as for micellar calcium phosphate, the most appropriate crystalline model compound from which the short-range structure may be derived is brushite, CaHPO 4 .2H 2 O. The predicted secondary structures around sites of phosphorylation in the Ca 2+ -sensitive caseins often comprise an a-hclix-loop-a-helix motif with the sites of phosphorylation in the loop region. This motif may be important in linking the colloidal calcium phosphate with casein in native casein micelles.In this paper, the term colloidal calcium phosphate (CCP) refers to the salt formed from small ions (mainly Ca 2+ , Pj, Mg 2+ and citrate) which is thought to exist in the native bovine casein micelle (Holt, 1985). Colloidal Ca or P 4 in milk can be clearly defined operationally by a dialysis equilibrium experiment as the difference between total and diffusible Ca or Pjj however, some colloidal Ca may not be associated completely with the calcium phosphate (the so-called caseinate Ca) making it impossible to determine the precise composition of the CCP. The term micellar calcium phosphate (MCP) is used here to refer to the complex of CCP with casein. It reflects a view of the complex in which the phosphate moieties of phosphoseryl residues and possibly other functional groups from the protein are incorporated in the CCP structure. Nevertheless, the definition of MCP is no more satisfactory than that of CCP since the boundary of the complex is a matter of choice. A practical definition of MCP is: that complex of casein phosphopeptides and small ions which is recovered following non-specific proteolytic digestion of casein micelles, carried out under conditions where the CCP does not dissolve 1986).Two different views of the nature of MCP have been put forward. In one (McGann et al. 1983;Holt, 1982) the CCP is regarded as a basic tricalcium phosphate (Ca/P = 1-5) and the interaction with phosphopeptides ignored. The hypothesis of Schmidt (1982) that the CCP comprises Ca 9 (PO 4 ) 6 ion clusters linked through Ca bridges to

Invited review: Caseins and the casein micelle: Their biological functions, structures, and behavior in foods

“…There is a close similarity in salt and peptide composition, solubility, size, and structure between casein-calcium phosphate nanoclusters prepared in the laboratory with casein phosphopeptides and the sequestered ACP in native bovine casein micelles, strongly suggesting that most caseins are cemented into the micelle by links through their PC to an acidic ACP (Holt et al, 1989). The successful ab initio calculation of the salt partition in bovine milk confirms the correctness of this assumption (Holt, 2004).…”

“…It predicts successfully, among other things, the partition of milk salts and the fraction of each of the caseins that is bound through one or more of its PC to the sequestered ACP. It also resolves the apparent paradox that the sequestered ACP is a basic salt in composition but an acidic salt according to its solubility behavior (Holt et al, 1989;Holt, 2004). An explicit expression for the equilibrium radius of the sequestered ACP nanoclusters has also been derived (Holt et al, 2009).…”

Structural studies of hydrated samples of amorphous calcium phosphate and phosphoprotein nanoclusters

“…Previous studies have shown (Holt 2004, Holt et al 1989) that the composition of micellar calcium phosphate is consistent with that of an acidic ACP. High-resolution electron microscopy (Lyster et al 1984) of dried samples of micellar calcium phosphate showed that longer-range order did not extend over distances more than ~1.5 nm.…”

Évolution des caractéristiques physico-chimiques et des paramètres de coagulation du lait de brebis collecté dans la région de Roquefort

“…Une partie est liée directement aux groupements phosphoséryls des caséines, l'autre est présente sous forme de phosphate de calcium et assure la cohésion de la micelle. La nature chimique de ce phosphate de calcium étant encore discutée (eg Holt et al, 1989;Van Dijk, 1990), nous avons retenu l'hypothèse d'un phosphate de calcium colloïdal tricalcique sous forme amorphe (Schmidt, 1982) L'évolution des pourcentages de calcium et de phosphore inorganique solubles semble traduire un transfert progressif de constituants de la phase soluble vers la phase colloïdale au fur et à mesure de l'avancement de la période de production. On peut émettre l'hypothèse que l'augmentation de la teneur en caséine du lait, alors que les concentrations en calcium et phosphore inorganique restent relativement stables, entraîne un certain déficit minéral des micelles qui se traduit par un déplace-ment des équilibres salins au profit de la phase colloïdale.…”