Turns revisited: A uniform and comprehensive classification of normal, open, and reverse turn families minimizing unassigned random chain portions

Turns are irregular secondary structure elements with a hydrogen bond or a specific C alpha-C alpha distance between the first and the last residue. They are up to six residues in length. Here, we present a uniform classification for all normal (COi - NHi+n hydrogen bond), open (a C alpha(i)-C alpha(i+n) distance up to 10 A), and reverse (NHi - COi+i hydrogen bond) turn families based on current structural data. Considering the large amount of data evaluated, this classification likely covers quite comprehensive]), most of the possible conformations of turns. All turn structures of a nonredundant dataset of 1903 protein chains were retrieved using Relibase and clustered using emergent self-organizing maps. This leads to three normal, four open, and five reverse turn families with several new turn-types. Based on the amino acid propensities, the achieved separation into normal, open, and reverse turn families seems convincing. In combination with P-sheet and helix classification on average 96% of the given protein chain can now be successfully classified.