Effect of amino-acid substitutions on Alzheimer's amyloid-beta peptide-glycosaminoglycan interactions

One of the major clinical features of Alzheimer's disease is the presence o f extracellular amyloid plaques that are associated with glycosaminoglycan- containing proteoglycans. It has been proposed that proteoglycans and glyco saminoglycans facilitate amyloid fibril formation and/or stabilize these ag gregates. Characterization of proteoglycan-protein interactions has suggest ed that basic amino acids in a specific conformation are necessary for glyc osaminoglycan binding. Amyloid-beta peptide (A beta) has a cluster of basic amino acids at the N-terminus (residues 13-16, His-His-Gln-Lys), which are considered critical for glycosaminoglycan interactions. To understand the molecular recognition of glycosaminoglycans by A beta, we have examined a s eries of synthetic peptides with systematic alanine substitutions. These in clude: His13 --> Ala, His14 --> Ala, Lys16 --> Ala, His13His14Lys16 --> Ala and Arg5His6 --> Ala. Alanine substitutions result in differences in both the secondary and fibrous structure of A beta 1-28 as determined by circula r dichroism spectroscopy and electron microscopy. The results demonstrate t hat the His-His-Gln-Lys region of A beta, and in particular His13, is an im portant structural domain, as Ala substitution produces a dysfunctional fol ding mutant. Interaction of the substituted peptides with heparin and chond roitin sulfate glycosaminoglycans demonstrate that although electrostatic i nteractions contribute to binding, nonionic interactions such as hydrogen b onding and van der Waals packing play a role in glycosaminoglycan-induced A beta folding and aggregation.