Phosphoprotein particles: calcium and inorganic phosphate binding structures.

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Phosphoprotein particles were isolated in their native state from the physiological fluid of the estuarine clam Rangia cuneata , and the characteristics of the mineral ion-protein complex which constitutes the native particle were investigated by using mineral ion binding and mineral ion exchange techniques. The particles are aspartic acid rich, highly phosphorylated proteins containing calcium, magnesium, and inorganic phosphate ions and covalently cross-linked via histidinoalanine residues. Twenty-nine percent of the amino acid residues are phosphorylated. In their native state, the particles contain a protected pool of calcium and inorganic phosphate ions and an exchangeable pool of calcium and magnesium ions. The Ca/PO4 ratio in the protected pool is about 2.5. The number of binding sites for the protected mineral is unknown, but on the average, the native particles contain about 0.2 inorganic phosphate ion per organic phosphate residue. There is 1.0 exchangeable metal ion binding site per organic phosphate residue, and there is probably a phosphoserine residue at each site. These sites bind calcium with an apparent binding constant (KCa) of 4 X 10(3) M-1 at 50% saturation under physiological conditions, and KCa/ KMg is about 1.6. In vivo, about 85% of the exchangeable sites are occupied. The total number of calcium ion binding sites (N) in the phosphoprotein particles is related to the number of organic phosphate residues (Po) and the number of bound inorganic phosphate ions (Pi) by the equation N = Po + 2. 5Pi . The phosphoprotein particles probably serve as both the transporter and reserve source of calcium ions for shell development.