Extension of the classical classification of β-turns

The functional properties of a protein primarily depend on its three-dimensional (3D) structure. These properties have classically been assigned, visualized and analysed on the basis of protein secondary structures. The beta-turn is the third most important secondary structure after helices and beta-strands. beta-turns have been classified according to the values of the dihedral angles. and. of the central residue. Conventionally, eight different types of beta-turns have been defined, whereas those that cannot be defined are classified as type IV beta-turns. This classification remains the most widely used. Nonetheless, the miscellaneous type IV beta-turns represent 1/3rd of beta-turn residues. An unsupervised specific clustering approach was designed to search for recurrent new turns in the type IV category. The classical rules of beta-turn type assignment were central to the approach. The four most frequently occurring clusters defined the new beta-turn types. Unexpectedly, these types, designated IV1, IV2, IV3 and IV4, represent half of the type IV beta-turns and occur more frequently than many of the previously established types. These types show convincing particularities, in terms of both structures and sequences that allow for the classical beta-turn classification to be extended for the first time in 25 years.