Mutation-induced change in chignolin stability from π-turn to α-turn.

Chignolin, which consists of 10 amino acids, adopts two stable states in simulations at room temperature at 1 atm: the native and misfolded states. The sequence of chignolin is optimized to form a stable π-turn and thus the native state has a π-turn from Asp3 to Thr8. On the other hand, the misfolded state adopts an α-turn from Asp3 to Gly7. We previously investigated the differences in the stability mechanism of the two states using computational techniques. Our previous detailed energy analysis implied that the native state was stabilized by hydrogen bonding between the side chain atoms of Thr6 and Thr8, and Thr8 was not involved in stabilization of the misfolded state. Thus, we predicted that mutation of Thr8 to a neutral amino acid could stabilize the misfolded structure over the native structure. In the present work, we performed 4 μs molecular dynamics simulations for 19 mutants of the 8th residue. Among them, the T8I, T8F, T8P, T8N, and T8Y mutants, in which the 8th residue was changed to a neutral residue, formed only the misfolded structure at room temperature. Even at high temperature, for the T8P mutant, the native structure was not observed, as the T8P mutant cannot form the native structure because of steric hindrance caused by the distinctive cyclic structure of proline. Interestingly, the T8P mutant at high temperature has and conformations in the Gly7-Pro8 sequence, with the conformation corresponding to the misfolded state. NMR analysis of the T8P mutant supported our results.