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Recently, a cyclic peptide ligand, cyclo-Ac-[CHPQG-PPC]-NH2, that binds to
streptavidin with high affinity was discovered by screening phage libraries.
From the streptavidin-bound crystal structures of cyclo-Ac-[CHPQGPPC]-NH2 and of
a related but more weakly binding linear ligand, FSHPQNT, we designed linear
thiol-containing streptavidin binding ligands, FCH-PQNT-NH2 and Ac-CHPQNT-NH2,
which are dimerized catalytically by the streptavidin crystal lattice of space
group I222, as demonstrated by high performance liquid chromatography and mass
spectrometry. The catalytic dimerization relies on presentation of the ligand
thiols toward one another in the lattice. The streptavidin crystal
lattice-mediated catalysis achieved by structure-based design is the first
example of catalysis of a chemical reaction by a protein crystal lattice. The
spontaneous and crystal catalyzed rates of disulfide formation were determined
by high performance liquid chromatography at pH 3.1, 4.0, 5.0, and 6.0. The
ratio of the catalyzed to uncatalyzed rate was maximal at pH 3.1 (kcat/kuncat =
3.8), diminishing to 1.2 at pH 6.0. The crystal structures of the
streptavidin-bound dimerized peptide ligands, FCHPQNT-NH2 dimer at 1.95 A and
Ac-CHPQNT-NH2 dimer at 1.80 A, are described and compared with the structures of
streptavidin-bound FSHPQNT monomer and cyclo-Ac-[CHPQGPPC]-NH2 dimer.
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