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Dr. Lai:  The "flap" region of HIV protease undergoes significant
rigidification upon binding of a substrate or ligand.  In this
99-mer protein, I refer to region 45-55, also called the "flap"
region.  There is a crystal structure of an unliganded form, where
the flap is held in place by a crystal contact.  But it is obvious
that the flap has multiple conformations in solution.  In the
liganded form, there are probably 30 crystal structures in the
PDB which show the flap in nearly identical "locked-down" 
conformation.

Kent Stewart
Department of Structural Biology
Abbott Laboratories
Chicago, IL
United States of America
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