Site-specific stabilization and destabilization of ? helical peptides upon phosphorylation and O-GlcNAcylation

Abstract

Helices (?-helix) are the most common type of secondary structure motif present in proteins. In this study, we have investigated the structural influence of phosphorylation and O-GlcNAcylation, common intracellular post-translational modifications (PTMs), on the ?-helical conformation. The simulation studies were performed on the Baldwin model ?-helical peptide sequence (Ac-AKAAAAKAAAAKAA-NH2). The Baldwin sequences were chosen due to the availability of site-specific experimental post-translational data for cross-validation with the simulations. The influence of PTMs was examined across the span of the ?-helix, namely, at the N-terminus, position 10 (interior region), and the C-terminus for both serine and threonine residues placed at these positions. Molecular dynamics (MD) simulations revealed that phosphorylation and O-GlcNAcylation at the N-terminus lead to the stabilization of the helical conformation. PTMs in the interior or the C-terminus were found to disrupt helicity, with the disruption being more pronounced for PTMs in the interior region, in accordance with experimental studies. It was found that phosphorylation-derived destabilization was mainly due to the formation of an intraresidue HN�PO32� electrostatic interaction and interactions between the phosphate group and the side chain of adjacent lysine residues (NH3���PO32�). Hydrophobic and steric clashes were the main causes of destabilization in the case of O-GlcNAcylation. The structural disruptions were found to be more pronounced for PTM at the threonine site when compared to the serine site. The salt-bridge-dependent stability of the ?-helix was found to be highly position specific, an i ? i + 4 interaction stabilizing the helix, with other placements leading to the destabilization of the helix.