Delineation of a new structural motif involving NHN γ-turn

Macromolecules are characterized by distinctive arrangement of hydrogen bonds. Different patterns of hydrogen bonds give rise to distinct and stable structural motifs. An analysis of 4114 non-redundant protein chains reveals the existence of a three-residue, (i - 1) to (i + 1), structural motif, having two hydrogen-bonded five-membered pseudo rings (the first, an NH···OC involving the first residue, and the second being NH∙∙∙N involving the last two residues), separated by a peptide bond. There could be an additional hydrogen bond between the side-chain at (i-1) and the main-chain NH of (i + 1). The average backbone torsion angles of -76(±21)° and - 12(±17)° at i creates a tight turn in the polypeptide chain, akin to a γ-turn. Indeed, a search of three-residue fragments with restriction on the terminal C^α ···C^α distance and the existence of the two pseudo rings on either side revealed the presence 14 846 cases of a variant, termed NHN γ-turn, distinct from the NHO γ-turn (2032 cases) that has traditionally been characterized by the presence of NHO hydrogen bond linking the terminal main-chain atoms. As in the latter, the newly identified γ-turns are also of two types-classical and inverse, occurring in the ratio of 1:6. The propensities of residues to occur in these turns and their secondary structural features have been enumerated. An understanding of these turns would be useful for structure prediction and loop modeling, and may serve as models to represent some of the unfolded state or disordered region in proteins.