ROLE OF ELECTROSTATIC INTERACTIONS IN THE BINDING OF FLUORESCEIN BY ANTIFLUORESCEIN ANTIBODY 4-4-20

Anti-fluorescein antibodies are excellent model systems for studying the biochemical basis of molecular recognition because a prodigious amount of both physico-chemical and structural information is available for these antibodies. Furthermore, recombinant single-chain antibodies have been produced for several anti-fluorescein antibodies, and site-specific mutagenesis studies have defined the energetic contributions of a number of key active-site residues. In previous studies, we determined the three-dimensional structure of an antigen-binding fragment of a high-affinity anti-fluorescein antibody (4-4-20) in complex with fluorescein. These studies showed that fluorescein binds tightly in an aromatic slot and participates in a network of electrostatic interactions. In this report, we examine the role of electrostatic interactions in the 4-4-20 antigen-combining site by observing the effects of pH on the fluorescence of fluorescein and antigen-binding affinity. These studies showed that the salt link between fluorescein and Arg-L34 in 4-4-20 probably accounts for about -1.5 kcal/mol-1 of the observed free energy of interaction. Furthermore, at pH 10 and higher, the affinity decreases by more than 100-fold (DELTADELTAG-degrees is-approximately-equal-to kcal mol-1). We attributed this decrease to the ionization of Tyr-L32, which probably disrupts a hydrogen bond between tyrosine's hydroxyl group and fluorescein's phenylcarboxylate group. The fluorescence life time of the 4-4-20/fluorescein complex was determined at both pH 8 and pH 10.6. Only one lifetime component (0.38 ns) was observed at pH 8, while two components (0.3 and 3.4 ns) were observed at pH 10.6. Titration experiments showed that the longer lifetime component was not due to unbound fluorescein. This led to the hypothesis that at least two conformers exist for the 4-4-20/fluorescein complex at pH 10.6.