Quantitative evaluation of congo red binding to amyloid-like proteins with a beta-pleated sheet conformation

The binding of Congo red to several purified amyloid-like peptides having a beta-pleated sheet conformation was quantitatively examined. Congo red binds preferentially to the beta-pleated sheet conformation of both insulin fibrils and poly-L-lysine. Congo red does not bind nearly so well to poly-L-serine or polyglycine, despite the fact that these peptides also have a beta-pleated sheet conformation. Binding to insulin fibrils was saturable with an apparent Bmax of 2 moles of Congo red per mole of insulin fibrils and an apparent KD of 1.75 x 10(-7) M. Binding to beta-poly-L-lysine was similar but had a much higher apparent Bmax of 43. Binding of Congo red to beta-poly-L-lysine was pH dependent and appeared to be determined by the number of protonated lysine residues in the 250 amino acid peptide. We present a new hypothesis in which Congo red binds to amyloid-like proteins via bonds between the two negatively charged sulfonic acid groups of Congo red and two positively charged amino acid residues of two separate protein molecules which are properly oriented by virtue of the beta-pleated sheet conformation of the peptide backbone.