Impact of Phosphorylation on the Conformational Landscape and Aggregation Behavior of Aβ40: A Comprehensive Computational Study.

Phosphorylation of amyloid-β (Aβ40) at Ser8 and Ser26 exerts opposing effects on fibril formation: Ser8 phosphorylation promotes aggregation, whereas Ser26 phosphorylation strongly inhibits it. Using replica exchange with solute tempering (REST2) simulations and coarse-grained modeling, we reveal the atomic-level mechanisms underlying these effects. Ser26 phosphorylation forms a highly stable pS26-K28 salt bridge that competes with and disrupts the native E22/D23-K28 interaction required for hairpin formation and fibrillization, yielding a compact, globular conformation that is aggregation-incompetent. In contrast, Ser8 phosphorylation stabilizes the hairpin structure with a preformed N-terminal attachment (the S* structure) via pS8-K16/K28 salt bridges, lowering the entropic barrier for N-terminal alignment. This mechanism aligns with experimental observations that pS8 fibrils gain stability due to N-terminal incorporation into the fibril core, thereby promoting fibrillization. Together, these results demonstrate that monomeric conformational landscapes directly encode aggregation propensity, providing a mechanistic framework for understanding how post-translational modifications modulate amyloid assembly pathways.