Mineral deposition in the extracellular matrices of vertebrate tissues: identification of possible apatite nucleation sites on type I collagen.

The possible means by which type I collagen may mediate mineralization in normal vertebrate bone, tendon, dentin and cementum as well as in pathological mineral formation are not fully understood. One consideration in this regard is that the structure of the protein is somehow important in binding calcium and phosphate ions in a stereochemical configuration conducive to nucleation of apatite crystals. In the present study, type I collagen, packed in a quarter-staggered arrangement in two dimensions and a quasi-hexagonal model of microfibrillar assembly in three dimensions, has been examined in terms of several of its charged amino acid residues. These included glutamic and aspartic acid, lysine, arginine, hydroxylysine and histidine, whose positions along the three alpha-chain axes of the collagen molecule were determined with respect to each other. It was found that the locations of these residues specified sites uniquely suited as potential apatite nucleation centers following binding of calcium and phosphate ions. From this analysis, it would appear that type I collagen provides a template of charged amino acid residues that dictates ion binding critical to subsequent nucleation events for mineral formation in vertebrate tissues.