Cooperativity in Calcium Ion Binding to Repetitive, Carboxylate‐serylphosphate Polypeptides and the Relationship of This Property to Dentin Mineralization

Analogs of phosphophoryn, a calcium-binding phosphorylated protein found in mineralized dentin, were synthesized by solid phase peptide synthesis. The dentin phosphophoryn appears to contain some sequence blocks of (Asp-PhosphoSer)n. As this sequence is difficult to synthesize, polymers of (alpha-L-Glu-L-Ser) were prepared. The 30-peptide, (alpha-L-Glu-L-Ser)15, was phosphorylated by reaction with orthophosphoric acid in the presence of trichloroacetonitrile in anhydrous dimethylsulfoxide. Calcium ion binding studies were carried out with both the 30-peptide and phosphorylated 30-peptide. The conformation of the original 30-peptide, (Glu-Ser)15, was examined, in the presence and absence of calcium ion, by circular dichroism measurements. Purified bovine phosphophoryn, previously studied by the same techniques, was partially dephosphorylated by alkaline phosphatase, and its calcium ion binding properties were determined. Dephosphorylation to 31% of the initial phosphorus content reduced the number of high affinity sites to approximately 30% of the initial value. However, the stoichiometry of binding indicated that both phosphate and carboxylate groups participate in the high affinity binding and that the binding constant was decreased only slightly. Partial phosphorylation of the 30-mer raised the calcium binding constant, Ka, from 2.1 x 10(2) to 3.3 x 10(3) M-1 and increased the amount of binding from an electrostatic equivalent number of sites to a stoichiometric equivalent number. Concomitant with binding, there was a transition from random coil to beta-like structure. These data suggest that the repetitive (Asp-PhosphoSer)n regions in phosphophoryn and the (Glu-PhosphoSer)n sequence of the synthetic polymer have special conformations which favor the unidentate binding of calcium to the carboxyl groups and phosphate groups. and which enhance the binding affinities of the carboxyl groups in such sequences in a cooperative fashion.