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Figure 3.
Figure 3. Stereoview comparisons of substrate recognition
by fIXa and other homologs. (a) The divergent structures of the
fIXa 99-loop (yellow) compared with those of factor Xa (white)
and tPA (red) place Tyr99 either in a relaxed position that
blocks substrate binding in the S2 site (thick lines) or in a
strained position that blocks the S4 site (thin lines). The
inhibitor (purple, Image -Phe-Pro-Arg with proline atoms omitted
for clarity) position is taken from the superposition of porcine
fIXa. (b) The conformation of the fIXa 99-loop (yellow) is
stabilized by a hydrogen bond between Tyr94 (phenylalanine in
fXa) and Lys98, whereas the fXa 99-loop (gray) conformation is
prevented by the position of Tyr177 (threonine in fXa).
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