United-Residue (UNRES) Langevin Dynamics Simulations of trpzip2 Folding

The folding mechanism of beta-hairpins might provide fundamental knowledge that helps us to understand how large proteins fold. However, all-atom molecular dynamics is difficult to do large-scale and long-time simulations, even for small peptides. In this article, we report a large-scale simulation of the beta-hairpin trpzip2 by Langevin dynamics with united-residue (UNRES) force field. We obtained 374 successfully folding trajectories of trpzip2. The results show that trpzip2 can fold into the native structure by adopting two different mechanisms: (1) zipper, and (2) simultaneous zipper and collapse. The former occurs with a larger probability. Furthermore, the folding of trpzip2 is heterogeneous. Our results clarify the inconsistencies in the current picture of the folding mechanisms of trpzip2 and other similar beta-hairpins.