STRONGLY TWISTED AND COILED β-HAIRPINS AND THEIR ROLE IN PROTEIN FOLDING

A strongly twisted and coiled β-hairpin can be represented as a double-helical structure in which the strands are twisted and coiled in a right-handed sense. The double superhelix has both a concave and a convex surface. A distinctive feature of this structure is that it is always formed by the right-turned β-hairpin when viewed from the concave side of the superhelix. Another feature of this structure is that the cp, φ, ψ values alternate along the polypeptide chain and cp, φ, ψ values of the “inside” residues located on the concave surface have a strong tendency to fall in the β_E-region (extended conformations) and those of the “outside” residues in the β_p-region (polyproline region) of the Ramachandran map. A stereochemical analysis shows that glycines in “inside” and prolines in “outside” positions have to facilitate formation of coiled β-hairpins. Statistical data confirm these conclusions. Thus, the strongly twisted and coiled β-hairpin has the unique fold itself and can be taken as the starting structure in modeling of protein folds and folding pathways. The larger protein folds are obtained by stepwise addition of secondary structural elements to the root β-hairpin in accordance with a set of simple rules. A structural tree constructed using this approach includes 112 non-homologous proteins classified as a subclass of (α + β)-proteins and referred to as wrap-proteins.