A new algorithm for finding the peptide chain turns in a globular protein

An algorithm is presented to identify peptide chain turns from X-ray-elucidated co-ordinate data. Chain turns are those regions in a globular protein where the backbone is folded back upon itself. The concept of a turn is important both because turns constitute recognizable structural units in proteins and because turns are situated at the solvent-accessible surface of the molecule. Current algorithms for turn identification are highly operational in character, often finding false turns and omitting actual ones. The algorithm presented here uses only the C-alpha co-ordinates for every residue in the protein. No other information of any kind is required, and notions about hydrogen bonding at these loci are irrelevant to the geometric nature of the argument. In this sense, the algorithm provides an objective criterion for the recognition of turns as strictly structural components in proteins. The algorithm is used to find the turns in a test set of proteins. Results of this application are in excellent agreement with visual turn identification from physical models.