REVERSIBLE RANDOM COIL BETA-SHEET TRANSITION OF THE ALZHEIMER BETA-AMYLOID FRAGMENT (25-35)

The beta-amyloid protein (39-43 amino acid residues) is the major cons tituent of the amyloid deposits found in brain of patients with Alzhei mer's disease. Using circular dichroism spectroscopy, we have studied the secondary structure and the aggregation of fragment 25-35 of the b eta-amyloid protein (beta AP(25-35)OH) under a variety of conditions. beta AP(25-35)OH in solution at pH 4.0 or 5.5 exhibits a concentration -dependent random coil reversible arrow beta-sheet transition. The equ ilibrium is characterized spectroscopically by an isodichroic point an d can be described quantitatively by a simple association model with a ssociation constants between 1.8 x 10(4) M(-1) (non-cooperative model, nucleation parameter sigma = 1) and 2.9 x 10(4) M(-1) (cooperative mo del, sigma = 0.2). The enthalpy of association is Delta H approximate to -3 kcal/mol as determined by titration calorimetry. The equilibrium is shifted completely toward beta-structured fibrils at pH 7.4 where the Met-35 carboxyl group is fully charged. In contrast, removal of th e charged carboxy terminus by amidation locks the equilibrium in the r andom coil conformation. Model calculations suggest an antiparallel be ta-sheet structure involving residues 28-35 which is stabilized at bot h ends of the beta-sheet by ion pairs formed between Lys-28 and Met-35 . Removal of fibrils via millipore filtration leads to solutions with random coil monomers only. Seeding these solutions with a few fibrils establishes a new random coil reversible arrow beta-sheet equilibrium.