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3.2 Sheets
3.2.3 Twists
The classical beta sheets originally proposed are planar but most sheets observed in globular proteins are twisted (0 to 30 degrees per residue). Antiparallel beta sheets are more often twisted than parallel sheets. This twist is always of the same handedness, but unfortunately, it has been described using two conflicting conventions in the literature. If defined in terms of the angle of strand crossings, the twist is left-handed and if defined in terms of the progressive twist of the hydrogen-bonding direction, the twist is right-handed. Two-stranded beta strands show the largest twists. At least two separate explanations have been offered as reasons for the observed twist. One, based on statistical reasoning suggests that the greater amount of allowed phi, psi space to the "right" of the classical beta strand region would produce a right-handed twist (Chothia, 1973). Another suggests a systematic distortion of the tetrahedral nitrogen, seen in X-ray crystal structures would favor a right-handed twist. A classical example is the antiparallel beta strand in the bovine pancreatic trypsin inhibitor.
Figure 12. A severely twisted antiparallel beta hairpin from the pancreatic trypsin inhibitor (5PTI).
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No Title - 31 MAY 96
written by Kurt D. Berndt
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