  Protein data bank entries obtain distinct, reproducible flexibility
characteristics determined by normal mode analyses of their three dimensional
coordinate files. We study the effectiveness and sensitivity of this technique
by analyzing the results on one class of glycosidases: family 10 xylanases. A
conserved tryptophan that appears to affect access to the active site can be in
one of two conformations according to X-ray crystallographic electron density
data. The two alternate orientations of this active site tryptophan lead to
distinct flexibility spectra, with one orientation thwarting the oscillations
seen in the other. The particular orientation of this sidechain furthermore
affects the appearance of the motility of a distant, C terminal region we term
the mallet. The mallet region is known to separate members of this family of
enzymes into two classes.
