Solid-phase synthesis and characterization of human salivary statherin: a tyrosine-rich phosphoprotein inhibitor of calcium phosphate precipitation.

Human statherin, at low molecular weight (M 5380 Da. 43 amino acid residues) acidic tyrosine-rich phosphoprotein secreted mainly by salivary glands, has been synthesized successfully for the first time following standard solid-phase Fmoc chemistry. Synthesis of this phosphoprotein was accomplished using preformed phosphoserin building blocks. The phosphorylated protein thus synthesized was analyzed and compared with the native molecule and was found to have identical characteristics in its entirety, is evidenced by various analytical methods including mass spectral analysis. Analysis of both the synthetic and native statherin by circular dichroism spectroscopy showed an increase in helicity upon the addition of an organic cosolvent, trifluoroethanol (50%, vol/vol), indicating the presence of potentially amphipathic helical regions. Circular dichroism studies and hydrophobic moment calculations on this synthetic phosphoprotein revealed that the molecule adopts an amphipathic helical conformation at the N-terminus connected to a long poly-L-proline type II segment, which, in turn, is linked to an extended beta-strand. In correlation with previous studies. It appears that the strong binding affinity of statherin for hydroxyapatite can be attributed primarily to the N-terminal sequence, which prefers to adopted helical conformation and provides both electrostatic and hydrogen bonding interactions, thereby inhibiting its mineralization. Production of this highly homogenous synthetic statherin by chemical means may circumvent the prevailing obstacles encountered in conducting its tertiary structural investigations under various physiological conditions.