%0 Journal Article
%T pi-Turns: types, systematics and the context of their occurrence in protein structures
%A Bhaskar Dasgupta
%A Pinak Chakrabarti
%J BMC Structural Biology
%D 2008
%I BioMed Central
%R 10.1186/1472-6807-8-39
%X An analysis of protein structures indicates that at least 20% of ¦Ð-turns occur independent of the Schellman motif. The two categories of ¦Ð-turns, designated as ¦Ð-HB and SCH, have been further classified on the basis of backbone conformation and both have AAAa as the major class. They differ in the residue usage at position (i+1), the former having a large preference for Pro that is absent in the latter. As in the case of shorter length ¦Â- and ¦Á-turns, ¦Ð-turns have also been identified not only on the basis of the existence of hydrogen bond, but also using the distance between terminal C¦Á-atoms, and this resulted in a comparable number of non-hydrogen-bonded ¦Ð-turns (¦Ð-NHB). The presence of shorter ¦Â- and ¦Á-turns within all categories of ¦Ð-turns, the subtle variations in backbone torsion angles along the turn residues, the location of the turns in the context of tertiary structures have been studied.¦Ð-turns have been characterized, first using hydrogen bond and the distance between C¦Á atoms of the terminal residues, and then using backbone torsion angles. While the Schellman motif has a structural role in helix termination, many of the ¦Ð-HB turns, being located on surface cavities, have functional role and there is also sequence conservation.Loops and turns are generally mediated by a stretch of residues with varying backbone conformations. They occupy more than 30% of a globular protein [1] and are often found at the active sites of a protein providing molecular recognition, specific interactions between two molecules and helping to maintain globular shape of the protein [2]. In the context of tertiary structure these are used to connect major secondary structures, like ¦Á-helix and ¦Â-sheet [3-7].Turns or shorter loops are more amenable for systematic characterization. Turns of various lengths have been identified, starting from three-residue ¦Ã-turn to six-residue ¦Ð-turn, and the four-residue ¦Â-turn has been subjected to rigorous investigation. ¦Â-turn was first ident
%U http://www.biomedcentral.com/1472-6807/8/39