Evidence for the role of the α-helix in the xylosylation reactions involving the glycosaminoglycan-bearing serine of decorin/DS-PGII as shown by1H NMR, CD, and molecular modeling studies

Abstract

The conformation of four peptides (N-terminal acetylated and unacetylated 14-mers DEASGIGPEEHFPENH2and 24-mers AcQKGLFDFMLEDEASGIGPEEHFPENH2with a normal and an oxidized methionine residue) containing the sequence Asp-Glu-Ala-Ser-Gly-Ile-Gly (DEASGIG), which is known to play an important role in the xylosylation reactions involving the glycosaminoglycan-bearing serine of decorin/DS-PGII, were studied by two-dimensional proton NMR techniques, circular dichroism spectroscopy, and molecular dynamics in a methanol–water mixture. The 14-residue peptide comprises the first (i.e., N-terminal) 14 amino acids of the mature decorin protein and the 24-residue peptide incorporates an additional (N-terminal) sequence of 10 amino acids derived from the procore of decorin. The resonance heterogeneity induced by the isomerization of the two prolines (Pro8, Pro13in the 14-mer, and Pro18, Pro23in the 24-mer) in the peptides studied was evaluated from TOCSY and NOESY NMR spectra. The trans-trans, trans-cis, and cis-trans isomers exist in approximate 68:25:7 proportions in the methanol–water mixture. The NOE distance constraints were used as input parameters for molecular dynamics and restrained energy minimization calculations. It was demonstrated that the conformation of the DEASGIG fragment was affected by the presence of the 10 amino acids at the N-terminal end of the 24-mer, and that the serine is part of an α-helix. The results indicate that an α-helix is present in the 24-mer beginning at the N-terminal end with Lys2and ending at Gly15, and suggest that this could be the signal for the xylosylation of serine Ser14. A type VIb β turn was observed, involving the C-terminal cis-proline in the sequence His-Phe-Pro-Glu. Key words: xylosylation, nascent helix, α-helix, csi-proline, type VIb β-turn, molecular dynamics.