α-Turns in protein structure

Abstract

The occurrence of 5 → 1 type of hydrogen bonds (α-turn) in proteins has been studied using a data set comprising of 107 proteins with resolution ≤2.0 Å. A very large majority of such α-turn segments (96%) form part of regular α-helices. The examples (84) which do not form part of an α-helix are termed 'isolated α-turns' and are grouped into two major families and seven minor groups along with two isolated examples based on the similarity of conformational angles, The family with large number of examples (50) have (φ, ψ) angles close to an α-helix and hence belong to the class of the shortest α-helices, The 'end to end' distances of these α-turns vary between 4.7 and 6.7 Å, the range being nearly the same as that of α-helices. The propensity calculations show that some amino acids such as Glu, Ser and Thr have statistically significant higher preferences to occur in α-turns than in α-helices, In addition to the 5 → 1 type, the residues in the α-turn are involved in hydrogen bonds with other parts of the chain, The residues are in general more hydrophilic compared to those in α-helices, In many cases (70%) the α-turn occurs at the ends of extended strands, and whenever it occurs at the loop regions connecting two extended strands, it brings about a hairpin bend.