Epitope mapping of monoclonal antibody HPT-101: a study combining dynamic force spectroscopy, ELISA and molecular dynamics simulations

By combining enzyme-linked immunosorbent assay (ELISA) and optical tweezers-assisted dynamic force spectroscopy (DFS), we identify for the first time the binding epitope of the phosphorylation-specific monoclonal antibody (mAb) HPT-101 to the Alzheimer's disease relevant peptide tau [pThr231/pSer235] on the level of single amino acids. In particular, seven tau isoforms are synthesized by replacing binding relevant amino acids by a neutral alanine (alanine scanning). From the binding between mAb HPT-101 and the alanine-scan derivatives, we extract specific binding parameters such as bond lifetime tau(0), binding length x(ts), free energy of activation Delta G (DFS) and affinity constant K-a (ELISA, DFS). Based on these quantities, we propose criteria to identify essential, secondary and nonessential amino acids, being representative of the antibody binding epitope. The obtained results are found to be in full accord for both experimental techniques. In order to elucidate the microscopic origin of the change in binding parameters, we perform molecular dynamics (MD) simulations of the free epitope in solution for both its parent and modified form. By taking the end-to-end distance d(E-E) and the distance between the alpha-carbons d(C-C) of the phosphorylated residues as gauging parameters, we measure how the structure of the epitope depends on the type of substitution. In particular, whereas d(C-C) is sometimes conserved between the parent and modified form, d(E-E) strongly changes depending on the type of substitution, correlating well with the experimental data. These results are highly significant, offering a detailed microscopic picture of molecular recognition.