title: pi-Turns: types, systematics and the context of their occurrence in protein structures.
abstract: BACKGROUND
  For a proper understanding of protein structure and folding it is important to know if a polypeptide segment adopts a conformation inherent in the sequence or it depends on the context of its flanking secondary structures. Turns of various lengths have been studied and characterized starting from three-residue gamma-turn to six-residue pi-turn. The Schellman motif occurring at the C-terminal end of alpha-helices is a classical example of hydrogen bonded pi-turn involving residues at (i) and (i+5) positions. Hydrogen bonded and non-hydrogen bonded beta- and alpha-turns have been identified previously; likewise, a systematic characterization of pi-turns would provide valuable insight into turn structures.
  
  
  RESULTS
  An analysis of protein structures indicates that at least 20% of pi-turns occur independent of the Schellman motif. The two categories of pi-turns, designated as pi-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 beta- and alpha-turns, pi-turns have also been identified not only on the basis of the existence of hydrogen bond, but also using the distance between terminal C alpha-atoms, and this resulted in a comparable number of non-hydrogen-bonded pi-turns (pi-NHB). The presence of shorter beta- and alpha-turns within all categories of pi-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.
  
  
  CONCLUSIONS
  pi-turns have been characterized, first using hydrogen bond and the distance between C alpha 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 pi-HB turns, being located on surface cavities, have functional role and there is also sequence conservation.
authors.affiliation: Bioinformatics Centre, Bose Institute, Calcutta, India. bhaskar@bic.boseinst.ernet.in
authors.forename: Bhaskar
authors.initials: B
authors.lastname: Dasgupta
authors.forename: Pinak
authors.initials: P
authors.lastname: Chakrabarti
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D017124:Conserved Sequence; D006860:Hydrogen Bonding; D008958:Models, Molecular; D017433:Protein Structure, Secondary; D011506:Proteins / Q000737:chemistry* / Q000378:metabolism
chemical_list: D011506:Proteins