Replica Exchange Molecular Dynamics Simulations on the Folding of Trpzip4 β-Hairpin

Abstract

β-Hairpin is an essential secondary structure unit of protein. Understanding the formation mechanism of the β hairpin and kinetic stability helps to gain insight into the formation of protein secondary structure as well as the mechanism of protein folding. Replica exchange molecular dynamics (REMD) approach is applied to investigate the folding mechanism of Trpzip4 β-hairpin. The REMD method is more efficient than conventional MD in searching for a representative set of the low energy minima in the existence of a large energy gap between the native state and any of the other possible state. The potential energy, backbone RMSD and solvent-accessible surface area (SASA) present descending trends in the process of REMD at 288 K. Methyl groups (most in threonines) show hydrophobic interactions with SASA decrements. The hydropho- bic interactions and intra hydrogen bond forming drive the folding and maintain the low energy structure. Two low energy structures, β-hairpin and helix-coil conformation, are sampled by REMD. In β-hairpin structure, the hydroxyl group (OH) of Thr49 at β turn is accessible to form hydrogen bonds with carboxyl group (COO - ) or C=O group of Asp46; while the hy- droxyl group (OH) of Thr51 is inclined to interact with water molecule with methyl orienting inwards. As a result, the strong interactions between Asp46 and Thr49 cause a bend at β turn. In helix-coil conformation, hydrophobic interactions play be- tween indole ring and methyl group. One of the zip-in paths of Trpzip4 indicates that, the forming of hydrogen bonds at β turn affects the whole folding process of β hairpin. The forming of hydrogen bonds at β turn takes place first, since it has advantages of distance and strong interactions in both side chains and backbone. Total solvent-accessible surface area de- creases significantly as it folds to the β hairpin structure. REMD method could sample large phase space for the low energy minima, and present scenarios of potential energy distribution on phase space with related variables as hydrophobic SASA, backbone RMSD, and hydrogen bonds for specific conformation. This work sheds some lights on the understanding of β hairpin folding. Keywords β-hairpin; folding; replica exchange molecular dynamics; hydrogen bond; hydrophobic interaction