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This enzyme has 4 chains, each divided into 2 domains. I have given each domain a letter from A to H as if they were distinct chains. Domains A, B, C, and D are similar to the viral neuraminidases. Domains E, F, G and H are described as auxiliary domains. They are part of the same chains as A, B, C, and D respectively. As with other enzymes, the catalytic site is enclosed and physically constrained. I think that the slight unravelling of compact packing of the auxiliary domains around the propeller domains is an artefact of crystallization. (E wraps around D, H around A, F around C and G around B.) 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 catalytic domains. Constraint is produced by the focussed compressive stress of four barrels as well as the auxiliary domains. 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 post of 14 April 2007 on nativeproteins.blogspot.com
Or contact the author, Dr Don Vanselow at dvanselow@hotmail.com
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