Three-dimensional structures of idiotypically related Fabs with intermediate and high affinity for fluorescein.

Multi-disciplinary studies of fluorescein-protein conjugates have led to the generation of a family of antibodies with common idiotypes and affinities for fluorescein ranging over five orders of magnitude. The high affinity 4-4-20 prototype traps the ligand in a highly complementary binding slot, which is lined by multiple aromatic side-chains. An antibody (9-40) of intermediate affinity belongs to the same idiotypic family as 4-4-20 and shares substantial amino acid identities within the VL and VH domains. To establish the structural basis for the affinity differences, we solved the crystal structure of the 9-40 Fab-fluorescein complex at a resolution of 2.3A. Similar to 4-4-20, 9-40 binds fluorescein in a tight aromatic slot with its xanthenonyl ring system accommodated by end-on insertion. However, the combined effects of the amino acid substitutions have resulted in reorganization of the binding site, with the HCDR3 loops showing the greatest differences in conformations. Access to the binding site of 9-40 is substantially more open, leaving the fluorescein's phenylcarboxylate moiety partially exposed to solvent. In addition to the usage of a different D (diversity) mini-gene encoding the HCDR3 loop, the decrease in fluorescein affinity in the 9-40 antibody family appears to be correlated with the substitution of histidine (9-40) for arginine (4-4-20) in position 34 of the antibody light chains.

Discrete bathochromic shifts exhibited by fluorescein ligand bound to rabbit polyclonal anti-fluorescein Fab fragments.

Eleven individual hyperimmune rabbit polyclonal anti-fluorescein Fab fragment preparations were resolved into heterogeneous subfractions based on differential dissociation times from a specific adsorbent. Four Fab subfractions (i.e., 0.1-, 1.0-, 10-, and 100-day elutions) that differed in affinity were characterized and classified according to the extent of the bathochromic shift in the absorption properties of antibody-bound fluorescein ligand. Absorption maxima of bound fluorescein were shifted in all cases to two distinct narrow ranges, namely, 505 to 507 nm or 518 to 520 nm relative to 491 nm for free fluorescein. There was no direct correlation between the two spectral shift populations and antibody affinity, fluorescence polarization, fluorescence quenching, or fluorescence lifetimes of bound ligand. Fluorescence emission maxima varied with the bathochromic shift range. Bound fluorescein ligand, with absorption maxima of 505 to 507 nm and 518 to 520 nm showed fluorescence emission maxima of 519 to 520 nm and 535 nm, respectively. The two spectral shift ranges differed by approximately 14 to 15 nm and/or energies of approximately 1.5 kcal mol(-1) relative to each other and to the absorption maximum for free fluorescein. Spectral effects on the antibody-bound ligand were discussed relative to solvent-water studies and the atomic structure of a high-affinity liganded anti-fluorescein active site.