The asparagine-stabilized beta-turn of apamin: contribution to structural stability from dynamics simulation and amide hydrogen exchange analysis.

Molecular dynamics simulations of bee venom apamin, and an analogue having an Asn to Ala substitution at residue 2 (apamin-N2A), were analyzed to explore the contribution of hydrogen bonds involving Asn2 to local (beta-turn residues N2, C3, K4, A5) and global stability. The wild-type peptide retained a stable conformation during 2.4 ns of simulation at 67 degrees C, with high beta-turn stability characterized by backbone-side chain hydrogen bonds involving beta-turn residues K4 and A5, with the N2 side chain amide carbonyl. The loss of stabilizing interactions involving the N2 side chain resulted in the loss of the beta-turn conformation in the apamin N2A simulations (27 or 67 degrees C). This loss of beta-turn stability propagates throughout the peptide structure, with destabilization of the C-terminal helix connected to the N-terminal region by two disulfide bonds. Backbone stability in a synthetic peptide analogue (apamin-N2A) was characterized by NMR and amide hydrogen exchange measurements. Consistent with the simulations, loss of hydrogen bonds involving the N2 side chain resulted in destabilization of both the N-terminal beta-turn and the C-terminal helix. Amide exchange protection factors in the C-terminal helix were reduced by 9-11-fold in apamin N2A as compared with apamin, corresponding to free energy (deltaDeltaG(uf)) of around 1.5 kcal M(-1) at 20 degrees C. This is equivalent to the contribution of hydrogen bond interactions involving the N2 side chain to the stability of the beta-turn. Together with additional measures of exchange protection factors, the three main contributions to backbone stability in apamin that account for virtually the full thermodynamic stability of the peptide have been quantitated.