title: Expanded turn conformations: characterization and sequence-structure correspondence in alpha-turns with implications in helix folding.
abstract: Like the beta-turns, which are characterized by a limiting distance between residues two positions apart (i, i+3), a distance criterion (involving residues at positions i and i+4) is used here to identify alpha-turns from a database of known protein structures. At least 15 classes of alpha-turns have been enumerated based on the location in the phi,psi space of the three central residues (i+1 to i+3)-one of the major being the class AAA, where the residues occupy the conventional helical backbone torsion angles. However, moving towards the C-terminal end of the turn, there is a shift in the phi,psi angles towards more negative phi, such that the electrostatic repulsion between two consecutive carbonyl oxygen atoms is reduced. Except for the last position (i+4), there is not much similarity in residue composition at different positions of hydrogen and non-hydrogen bonded AAA turns. The presence or absence of Pro at i+1 position of alpha- and beta-turns has a bearing on whether the turn is hydrogen-bonded or without a hydrogen bond. In the tertiary structure, alpha-turns are more likely to be found in beta-hairpin loops. The residue composition at the beginning of the hydrogen bonded AAA alpha-turn has similarity with type I beta-turn and N-terminal positions of helices, but the last position matches with the C-terminal capping position of helices, suggesting that the existence of a "helix cap signal" at i+4 position prevents alpha-turns from growing into helices. Our results also provide new insights into alpha-helix nucleation and folding.
authors.affiliation: Department of Biochemistry, Bose Institute, Calcutta, India.
authors.forename: Bhaskar
authors.initials: B
authors.lastname: Dasgupta
authors.forename: Lipika
authors.initials: L
authors.lastname: Pal
authors.forename: Gautam
authors.initials: G
authors.lastname: Basu
authors.forename: Pinak
authors.initials: P
authors.lastname: Chakrabarti
mesh_terms: D000595:Amino Acid Sequence; D030562:Databases, Protein; D006860:Hydrogen Bonding; D008969:Molecular Sequence Data; D017510:Protein Folding; D017433:Protein Structure, Secondary; D011506:Proteins / Q000737:chemistry*; D055672:Static Electricity
chemical_list: D011506:Proteins