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Figure 1.
Structural superposition of bound wild-type (white), S45A
(blue), D128A (red), and S45A/D128A double-mutant (green)
streptavidin structures. (A) Stereoview of the overall
structures. (B). Details of the binding pocket described in the
text. The bound structures were superimposed by least-squares
fit of the 65 C[alpha] atoms in the [beta]-barrel core of each
monomer. The nearly identical structure of these cores can be
seen in the overlay of the [beta]-sheet backbone in the
[beta]-barrel core. The protein adjusts to the two mutations by
a combination of the shifts found in the individual mutants,
with the main-chain near S45A shifting toward biotin, the
main-chain near D128A shifting away from biotin, a water
molecule replacing the missing D128 carboxylate, and the biotin
shifting away from the bottom of the pocket (the new water in
the D128A structure is hidden by that in the double mutant due
to nearly complete overlap). Hydrogen bonds involving Q24, S45,
D128, and biotin in wild-type streptavidin are shown by dotted
lines.
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