Conformation of dentin phosphophoryn adsorbed on hydroxyapatite crystals

Phosphophoryn, the major noncollagenous protein of dentin, was adsorbed on synthetic hydroxyapatite crystals and analyzed by high-resolution solid-state nuclear magnetic resonance (NMR) spectroscopy. Binding of the protein was inhibited by acidic polypeptides, especially by a phosphorylated peptide. After phosphophoryn was incubated with the crystals, the crystals were collected and analyzed by 13C-cross-polarization magic-angle-spinning NMR. Several signals could be assigned to carbons of aspartic acids, taking advantage of the unique amino acid composition of this protein. Chemical shifts of signals of aspartic acids are known to reflect secondary structure of the polypeptide. The chemical shifts obtained from the phosphophoryn indicate that the secondary structure of this protein on the crystal was near to a beta-sheet structure. This result is consistent with the result for poly(Asp) adsorbed on the crystals. The beta-sheet-like structure enables phosphophoryn to extend on the crystal surface and to cover the surface with only a small number of the molecules, resulting in the high inhibitory effect of this protein on crystal growth.