Amyloid precursor protein and amyloid beta, molecular models. Amyloid precursor protein (APP, left) is cleaved by enzymes, with one of the residues being the amyloid beta peptide (right), which has about 40 amino acid residues. Amyloid beta changes shape and aggregates into long fibrils. These fibrils form dense plaques on nerve cells, a characteristic feature of Alzheimer's disease.