title: Variations in antigen-antibody association kinetics as a function of pH and salt concentration: a QSAR and molecular modeling study.
abstract: The relationship between three environmental factors (ionic strength, pH, and temperature) and antigen-antibody binding kinetics was investigated using QSAR (quantitative structure-activity relationship) and molecular modeling approaches. The interaction used for this analysis is that between the camel antibody fragment cAbLys3 and lysozyme. Binding kinetics were measured using a Biacore 2000 instrument, at NaCl concentrations between 50 and 500 mM, at pH's between 5 and 10, and at temperatures between 15 and 30 degrees C, according to multivariate experimental designs. Variations in kinetic on- and off-rate parameters were up to 400- and 16-fold, respectively. Mathematical models that relate log k(on) to experimental conditions were developed. They indicated an influence of all three factors, with a clear dependency between pH and NaCl concentration for their effect on k(on). These models were able to predict on-rate parameters under new experimental conditions. Titration calculations using continuum electrostatics were performed on the crystallographic structures of the isolated and bound proteins to gain structural insight for the on-rate enhancement observed at pH <6.5 and low salt concentrations. These calculations rule out electrostatic steering linked to global and/or local charge variations in the molecules as the factor responsible for the on-rate enhancement at low pH. His 111 of cAbLys3, located at the binding interface, can adopt two side chain orientations with different intramolecular contacts. The results of the calculations suggest an alternative mechanism whereby the conformation of the interfacial His 111 depends on the charge, and these differences in conformation may influence the solvation energy and the subsequent binding kinetics. Our results stress the complex relationship between environmental conditions and molecular binding properties.
authors.affiliation: UMR 7104, Structural Biology and Genomics Department, Institut de Génétique et de Biologie Moléculaire et Cellulaire, CNRS/INSERM/ULP, BP 10142 F-67404 Illkirch Cedex, France.
authors.forename: Annick
authors.initials: A
authors.lastname: Dejaegere
authors.forename: Laurence
authors.initials: L
authors.lastname: Choulier
authors.forename: Virginie
authors.initials: V
authors.lastname: Lafont
authors.forename: Erwin
authors.initials: E
authors.lastname: De Genst
authors.forename: Danièle
authors.initials: D
authors.lastname: Altschuh
mesh_terms: D000818:Animals; D000906:Antibodies / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D000936:Antigen-Antibody Complex / Q000737:chemistry*; D000941:Antigens / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D008958:Models, Molecular; D015999:Multivariate Analysis; D009113:Muramidase / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D011485:Protein Binding; D021281:Quantitative Structure-Activity Relationship; D012492:Salts / Q000737:chemistry*; D013696:Temperature
chemical_list: D000906:Antibodies; D000936:Antigen-Antibody Complex; D000941:Antigens; D012492:Salts; D009113:Muramidase