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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.
The sulfur atoms of pairs of cys142 can be seen coloured yellow on the y axis. These form intersubunit disulfide links in the native form and in the crystal. In the reassembled pdb file the first entry is the N-terminal amino acid numbered 501 as it is in the original 1B9B.pdb so that it is clear which data have been transformed. Consequently cys142 is numbered cys642.
For further information about this protein, see the post of 7 January 2012 on nativeproteins.blogspot.com Or contact the author, Dr Don Vanselow at dvanselow@hotmail.com
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