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As with other enzymes, the catalytic site is enclosed and physically constrained. 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.
Note that, as in all enzyme barrels containing helices, these helices are orientated as if the barrel has a right-handed twist. Analysis of the structural elements within an alpha helix suggests that this twist maximises stiffness through the central axis of the barrel. The calculation suggests that maximum stiffness is achieved when the helices are inclined 25° away from the direction of the barrel. Conversely a left-handed twist would maximise longitudinal stiffness at the periphery. A left-handed twist is seen in almost all structural proteins composed of bundles of alpha helices, such as keratin, presumably because stiffness at the periphery provides resistance to bending.
For further information about this protein, see post of 5 May 2011 on nativeproteins.blogspot.com
Or contact the author, Dr Don Vanselow at dvanselow@hotmail.com
This protein also features in the poster "Symmetry, pseudo-symmetry and evolution in protein structures" and the talk "Effect of Crystallization on Protein Quaternary Structure" linked in the blog posts of 9 December 2011.
Further details of JSmol available here