Interaction of epitope-related and -unrelated peptides with anti-p24 (HIV-1) monoclonal antibody CB4-1 and its Fab fragment

✍ Scribed by Karin Welfle; Rolf Misselwitz; Wolfgang Höhne; Heinz Welfle

• Publisher: John Wiley and Sons
• Year: 2003
• Tongue: English
• Weight: 147 KB
• Volume: 16
• Category: Article
• ISSN: 0952-3499
• DOI: 10.1002/jmr.607

✦ Synopsis

Abstract

The binding of four epitope‐related peptides and three library‐derived, epitope‐unrelated peptides of different lengths (10–14 amino acids) and sequence by anti‐p24 (HIV‐1) monoclonal antibody CB4‐1 and its Fab fragment was studied by isothermal titration calorimetry. The binding constants K~A~ at 25 °C vary between 5.1 × 10^7^ M
^−1^ for the strongest and 1.4 × 10^5^ M
^−1^ for the weakest binder. For each of the peptides complex formation is enthalpically driven and connected with unfavorable entropic contributions; however, the ratio of enthalpy and entropy contributions to Δ__G__^0^ differs markedly for the individual peptides. A plot of −Δ__H__^0^ vs −T__Δ__S^0^ shows a linear correlation of the data for a wide variety of experimental conditions as expected for a process with Δ__C__~p~ much larger than Δ__S__^0^. The dissimilarity of Δ__C__~p~ and Δ__S__^0^ also explains why Δ__H__^0^ and T__Δ__S^0^ show similar temperature dependences resulting in relatively small changes of Δ__G__^0^ with temperature. The heat capacity changes Δ__C__~p~ upon antibody–peptide complex formation determined for three selected peptides vary only in a small range, indicating basic thermodynamic similarity despite different key residues interacting in the complexes. Furthermore, the comparison of van't Hoff and calorimetric enthalpies point to a non‐two‐state binding mechanism. Protonation effects were excluded by measurements in buffers of different ionization enthalpies. Differences in the solution conformation of the peptides as demonstrated by circular dichroic measurements do not explain different binding affinities of the peptides; specifically a high helix content in solution is not essential for high binding affinity despite the helical epitope conformation in the crystal structure of p24. Copyright © 2003 John Wiley & Sons, Ltd.