Estimating the twist of beta-strands embedded within a regular parallel beta-barrel structure.

The parallel beta-barrel is a recurrent structural motif found in a large variety of different enzymes belonging to the family of alpha/beta-proteins. It has been shown previously that the hyperboloid can be considered as a scaffold describing the parallel beta-barrel structure. To assess restraints on beta-strand twist imposed by a given scaffold geometry, the notion of scaffold twist, Ts, is introduced. From Ts, the beta-strand twist (Tw beta) expected for a given scaffold geometry can be derived and it is verified that this computed twist can be used to identify beta-barrels characterized by good hydrogen bonding. It is noted that Tw beta is only slightly affected for beta-barrels differing in the number (N) of beta-strands, suggesting that restraints on main-chain conformation of beta-strands are not likely to account for the N = 8 invariability observed in natural parallel beta-barrels thereby strengthening previous work rationalizing this constancy.