Salt Links Dominate Affinity of Antibody HyHEL-5 for Lysozyme through Enthalpic Contributions*

The binding of murine monoclonal antibody HyHEL-5 to lysozyme has been the subject of extensive crystallographic, computational, and experimental investigations. The complex of HyHEL-5 with hen egg lysozyme (HEL) features salt bridges between Fab heavy chain residue Glu50, and Arg45 and Arg68 of HEL. This interaction has been predicted to play a dominant role in the association on the basis of molecular electrostatics calculations. The association of aspartic acid and glutamine mutants at position 50H of the cloned HyHEL-5 Fab with HEL and bobwhite quail lysozyme (BQL), an avian variant bearing an Arg68 ? Lys substitution in the epitope, was characterized by isothermal titration calorimetry and sedimentation equilibrium. Affinities for HEL were reduced by 400-fold (E50HD) and 40,000-fold (E50HQ) (??G� estimated at 4.0 and 6.4 kcal mol?1, respectively). The same mutations reduce affinity for BQL by only 7- and 55-fold, respectively, indicating a reduced specificity for HEL. The loss of affinity upon mutation is in each case primarily due to an unfavorable change in the enthalpy of the interaction; the entropic contribution is virtually unchanged. An enthalpy-entropy compensation exists for each interaction; ?H� decreases, while ?S� increases with temperature. The ?Cp for each mutant interaction is less negative than the wild-type. Mutant-cycle analysis suggests the mutations present in the HyHEL-5 Fab mutants are linked to those present in the BQL with coupling energies between 3 and 4 kcal mol?1.