High Capacity Calcium-Binding Proteins as Intermediate Calcium Carriers in Biological Mineralization
High-capacity calcium-binding macromolecules have been identified at mineralization fronts in heterodont bivalves, vertebrate tooth dentin and unicellular algae. They appear to be functionally similar to casein, the mineral ion carrier of milk. The bivalve phosphoprotein studied in this laboratory occurs as discrete high molecular weight (MW) particles in the hemolymph and extrapallial fluid and at the inner shell surface during mineral deposition. The phosphoprotein particles are characterized by a high content of aspartic acid, phosphoserine and histidine residues which comprise 80% or more of the peptide chain. The native particles contain a protected pool of calcium and phosphate ions and an exchangeable pool of calcium and magnesium ions. The phosphoprotein monomers are covalently cross-linked via histidinoalanine residues and ionically cross-linked via divalent cations into compact particles with a MW of about 50 million. High concentrations of phosphoprotein particles accumulate at the mineralization front during stimulated biomineralization, but neither inhibit mineral deposition nor influence the crystal habits. The particles are probably calcium ion transporters. However, the mechanism by which protein bound calcium is utilized to form calcium carbonate crystals is unknown.
The bivalve phosphoprotein particles share common characteristics with vertebrate tooth dentin and algal coccolithosomes which have been studied in other laboratories. All three are high-capacity calcium-binding macromolecules which have been localized at mineralization fronts. Both the bivalve phosphoprotein particles and dentin phosphophoryn are aspartic acid-rich, highly phosphosylated proteins which have a tendency to cross-link and fragment. Both coccolithosomes and the bivalve phosphoprotein occur as discrete high MW particles about 20–40 nm in diameter surrounding growing calcium carbonate crystals in vivo. It is postulated that intermediate calcium carriers in the form of high-capacity calcium-binding macromolecules may be a general phenomenon in the biological calcification of skeletal tissues.
KeywordsCalcium Phosphate Casein Micelle Amorphous Calcium Phosphate Mineralization Front Bovine Dentin
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