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Title
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Solvent rearrangement in an antigen-antibody interface introduced by site-directed mutagenesis of the antibody combining site.
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Authors
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X.Ysern,
B.A.Fields,
T.N.Bhat,
F.A.Goldbaum,
W.Dall'Acqua,
F.P.Schwarz,
R.J.Poljak,
R.A.Mariuzza.
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Ref.
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J Mol Biol, 1994,
238,
496-500.
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PubMed id
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Abstract
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The three-dimensional structure of a site-directed mutant of the bacterially
expressed Fv fragment from monoclonal antibody D1.3, complexed to the specific
antigen lysozyme has been determined to a nominal resolution of 1.8 A using
X-ray diffraction data. The replacement of VL Trp92 by Asp allows two water
molecules to occupy space taken by Trp92 in the wild-type complex, in agreement
with a previous observation that water molecules play an important role in
stabilizing this antigen-antibody complex. The equilibrium constant for the
binding of the mutant Fv to the antigen decreases by three orders of magnitude
(from 2.3 x 10(8) M-1 to 2.6 x 10(5) M-1). Titration calorimetry shows that this
results from a smaller negative binding enthalpy (delta delta H = -16 kJ mol-1
at 24 degrees C), whereas the value of the binding entropy is not affected.
Since in the complex between the mutated Fv and antigen the buried area has
decreased relative to that of the wild-type Fv by about 150 A2, the contribution
of the buried unit area to the decrease in free energy (delta Gzero) is
approximately 117 J mol-1 (28 cal mol-1) per A2. The loss of interatomic
contacts in replacing Trp by Asp permits an approximate calculation for the
contribution of van der Waals interactions made by Trp92 in this complex, which
gives an average of 2.1 kJ mol-1 (0.5 kcal mol-1) for contacts between carbon
atoms.
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