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As with other enzymes, the catalytic site is enclosed and physically constrained. This enzyme acts on the terminal residue of an oligosaccharide and therefore must provide a passageway through which the oligosaccharide can extend from the outside to the active site. The passageway is most likely at each point of meeting of 3 subunits. Constraint is produced by the focussed compressive stress of four barrels. The orientation of the barrels deviates from a regular tetrahedron to provide the optimum spatial distribution of stress for catalysis at the active site.
For further information about this protein, see posts of 15 April 2015 and 14 April 2007 on nativeproteins.blogspot.com
Or contact the author, Dr Don Vanselow at dvanselow@hotmail.com
Further details of JSmol available here