Abstract
Oligomeric intermediates are possible cytotoxic species in diseases associated with amyloid deposits. Understanding the early steps of fibril formation at atomic details may provide useful information for the rational therapeutic design. In this study, using the heptapeptide GNNQQNY from the yeast prion-like protein Sup35 as a model system, for which a detailed atomic structure of the fibril formed has been determined by x-ray microcrystallography, we investigated its oligomer-formation process from monomer to tetramer at the atomistic level by means of a molecular dynamics simulation with explicit water. Although the number of simulations was limited, the qualitative statistical data gave some interesting results, which indicated that the oligomer formation might start from antiparallel beta-sheet-like dimers. When a new single peptide strand was added to the preformed dimers to form trimers and then tetramers, the transition time from disorder aggregates to regular ones for the parallel alignment was found to be obviously much less than for the antiparallel one. Moreover, the parallel pattern also statistically stayed longer, providing more chances for oligomer extending, although the number of parallel stack events was almost equal to antiparallel ones. Therefore, our simulations showed that new strands might prefer to extend in a parallel arrangement to form oligomers, which agrees with the microcrystal structure of the amyloid fibril formed by this peptide. In addition, analysis of the pi-pi stacking of aromatic residues showed that this type of interaction did not play an important role in giving directionality for beta-strand alignment but played a great influence on stabilizing the structures formed in the oligomer-formation process.