All-atom ab initio native structure prediction of a mixed fold (1FME): a comparison of structural and folding characteristics of various beta beta alpha miniproteins.

We performed an all-atom ab initio native structure prediction of 1FME, which is one of the computationally challenging mixed fold beta beta alpha miniproteins, by combining a novel conformational search algorithm (multiplexed Q-replica exchange molecular dynamics scheme) with a well-balanced all-atom force field employing a generalized Born implicit solvation model (param99MOD5/GBSA). The nativelike structure of 1FME was identified from the lowest free energy minimum state and in excellent agreement with the NMR structure. Based on the interpretation of the free energy landscape, the structural properties as well as the folding behaviors of 1FME were compared with other beta beta alpha miniproteins (1FSD, 1PSV, and BBA5) that we have previously studied with the same force field. Our simulation showed that the 28-residue beta beta alpha miniproteins (1FME, 1FSD, and 1PSV) share a common feature of the free energy topography and exhibit the three local minimum states on each computed free energy map, but the 23-residue miniprotein (BBA5) follows a downhill folding with a single minimum state. Also, the structure and stability changes resulting from the two point mutation (Gln1-->Glu1 and Ile7-->Tyr7) of 1FSD were investigated in details for direct comparison with the experiment. The comparison shows that upon mutation, the experimentally observed turn type switch from an irregular turn (1FSD) to type I(') turn (1FME) was well reproduced with the present simulation.