Investigation of cis/trans proline isomerism in a multiply occurring peptide fragment from human salivary proline-rich glycoprotein

The solution-state conformations of eight proline-containing peptide fragments found in human salivary proline-rich glycoprotein (PRG) were investigated in 2 × distilled water (treated with metal ion chelating resin) using ¹³C-nuclear magnetic resonance (NMR) and circular dichroism (CD) spectroscopy. The peptide sequences and acronyms were as follows: PRG9-2 = NH₂-G(I)-P(2)-CONH₂, PRG9-3 = NH₂-G(1)-P(2)-P(3)-CONH₂,PRG9-4 = NH₂-G(1)-P(2)-P(3)-P(4)-CONH₂, PRG9-5 = NH₂-G(1)-P(2)-P(3)-P(4)-H(5)-CONH₂,PRG9-6 = NH₂-G(1)-P(2)-P(3)-P(4)-H(5)-P(6)-CONH₂, PRG9-7 = NH₂-G(1)-P(2)-P(3)-P(4)-H(5)-P(6)-G(7)-CONH₂, PRG9-8 = NH₂-G(1)-P(2)-P(3)-P(4)-H(5)-P(6)-G(7)-K(8)-CONH₂ and PRG9-9 = NH₂-G(1)-P(2)-P(3)-P(4)-H(5)-P(6)-G(7)-K(8)-P(9)-CONH₂. Sequence-specific resonance assignments from the ¹³C-NMR spectra indicated that the trans proline isomer dominated the conformations of the peptides. CD results clearly showed the presence of the poly-l -proline II helix as the major conformation in PRG9-3 → PRG9-5, supplemented by β- and/or γ-turns in PRG9-6 → PRG9-9. These data suggest that in “metal free” water, native PRG could contain several small poly-l -proline II helices along with β- and/or γ-turns. Since proline is the major amino acid present in native PRG, these localized conformations may contribute to PRG's global conformation and act as a primary force in determining its biological activities.