1x9q
From Proteopedia
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4m5.3 anti-fluorescein single chain antibody fragment (scFv)
Overview
Here, we compare an antibody with the highest known engineered affinity, (K(d)=270 fM) to its high affinity wild-type (K(d)=700 pM) through, thermodynamic, kinetic, structural, and theoretical analyses. The 4M5.3, anti-fluorescein single chain antibody fragment (scFv) contains 14, mutations from the wild-type 4-4-20 scFv and has a 1800-fold increase in, fluorescein-binding affinity. The dissociation rate is approximately, 16,000 times slower in the mutant; however, this substantial improvement, is offset somewhat by the association rate, which is ninefold slower in, the mutant. Enthalpic contributions to binding were found by calorimetry, to predominate in the differential binding free energy. The crystal, structure of the 4M5.3 mutant complexed with antigen was solved to 1.5A, resolution and compared with a previously solved structure of an, antigen-bound 4-4-20 Fab fragment. Strikingly, the structural comparison, shows little difference between the two scFv molecules (backbone RMSD of, 0.6A), despite the large difference in affinity. Shape complementarity, exhibits a small improvement between the variable light chain and variable, heavy chain domains within the antibody, but no significant improvement in, shape complementarity of the antibody with the antigen is observed in the, mutant over the wild-type. Theoretical modeling calculations show, electrostatic contributions to binding account for -1.2 kcal/mol to -3.5, kcal/mol of the binding free energy change, of which -1.1 kcal/mol is, directly associated with the mutated residue side-chains. The, electrostatic analysis reveals several mechanistic explanations for a, portion of the improvement. Collectively, these data provide an example, where very high binding affinity is achieved through the cumulative effect, of many small structural alterations.
About this Structure
1X9Q is a Protein complex structure of sequences from Homo sapiens with ACT and FLU as ligands. Full crystallographic information is available from OCA.
Reference
Substantial energetic improvement with minimal structural perturbation in a high affinity mutant antibody., Midelfort KS, Hernandez HH, Lippow SM, Tidor B, Drennan CL, Wittrup KD, J Mol Biol. 2004 Oct 22;343(3):685-701. PMID:15465055
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