Estimation of the Hydrophobic Effect in an Antigen-Antibody Protein-Protein Interface

Published

December 19, 2000

Author(s)

E. J. Sundberg, M. Urrutia, B C. Braden, J. Isern, D. Tsuchiya, B. A. Fields, E Malchiodi, J. Tormo, Frederick P. Schwarz, R A. Mariuzza

Abstract

Antigen-antibody complexes provide useful models for analyzing the thermodynamics of protein-protein association reactions. We have employed site-directed mutagenesis, X-ray crystallography, and isothermal titration calorimetry to investigate the role of hydrophobic interactions in stabilizing the complex between the Fv fragment of the anti-hen egg white lysozyme (HEL) antibody D1.3 and HEL. Crystal structures of six FvD1.3-HEL mutant complexes in which an interface tryptophan residue (V_LW92) has been replaced by residues with smaller side chains (alanine, serine, valine, aspartate, histidine and phenylalanine) were determined to resolutions between 1.75 and 2.00 . In the wild-type complex, V_LW92 occupies a large hydrophobic pocket on the surface of HEL and constitutes an energetic hot spot for antigen binding. The looses in apolar buried surface area in the mutant complexes, relative to wild type, range from 25 (V_LF92) to 115 ² (V_LA92), with no significant shifts in the positions of protein atoms at the mutation site for any of the complexes except V_LA92, where there is a peptide flip. The affinities of the mutant Fv fragments for HEL are 10- to 100-fold lower than that of the original antibody. Formation of all six mutant complexes is marked by a decrease in binding enthalpy that exceeds the decrease in binding free energy, such that the loss in enthalpy is partly offset by a compensating gain in entropy. No correlation was observed between decreases in apolar, polar, or aggregate (sum of the apolar and polar) buried surface area in the V_L92 mutant series and changes in the enthalpy of formation. Conversely, there exist linear correlations between losses of apolar buried surface and decreases in binding free energy (R² = 0.937) as well as increases in the solvent portion of the entropy of binding (R²=0.909). The correlation between binding free energy and apolar buried surface area corresponds to 21 cal mol^{-1 -2} (1 cal = 4.185 J) for the effective hydrophobicity at the V_L92 mutation site. Furthermore, the slope of the line defined by the correlation between changes in binding free energy and solvent entropy approaches unity, demonstrating that the exclusion of solvent from the binding interface is the predominant energetic factor in the formation of this protein complex. Our estimate of the hydrophobic contribution to binding at site V₁92 in the D1.3-HEL interface is consistent with values for the hydrophobic effect derived from classical hydrocarbon solubility models. We also show how residue V_LW92 can contribute significantly less to stabilization when buried in a more polar pocket, illustrating the dependence of the hydrophobic effect on local environment at different sites in a protein-protein interface.

Citation

Biochemistry

Volume

39

Issue

50

Pub Type

Journals

Keywords

antibody, antigen, hydrophobic, protein thermodynamics, x-ray crystallography

Citation

Sundberg, E. , Urrutia, M. , Braden, B. , Isern, J. , Tsuchiya, D. , Fields, B. , Malchiodi, E. , Tormo, J. , Schwarz, F. and Mariuzza, R. (2000), Estimation of the Hydrophobic Effect in an Antigen-Antibody Protein-Protein Interface, Biochemistry (Accessed December 12, 2024)