Loops and Turns

In addition to α helices and β strands, a folded polypeptide chain contains two other types of secondary structure called loops and turns. (There are also regions of unordered structure, often called coils.)

Loops and turns connect α helices and β strands. The most common types cause a change in direction of the polypeptide chain allowing it to fold back on itself to create a more compact structure.

Loops are not well defined. They generally have hydrophilic residues and they are found on the surface of the protein. Loops that have only 4 or 5 amino acid residues are called turns when they have internal hydrogen bonds. Reverse turns are a form of tight turn where the polypeptide chain makes a 180° change in direction. Reverse turns are also called β turns because they usually connect adjacent β strands in a β sheet.

Reverse turns. (left) Type I β turn. The structure is stabilized by a hydrogen bond between the carbonyl oxygen of the first N-terminal residue (Phe) and the amide hydrogen of the fourth residue (Gly). Note the proline residue at position n + 1 (right) Type II β turn. This turn is also stabilized by a hydrogen bond between the carbonyl oxygen of the first N-terminal residue (Val) and the amide hydrogen of the fourth residue (Asn). Note the glycine residue at position n + 2 [PDB 1AHL (giant sea anemone neurotoxin)]. (Horton et al. 2006)

The two most common common types of β turn are the type I and type II turns shown above. The key point about turns is that they are highly ordered structures stabilized by internal hydrogen bonds. This is why they are counted as the third form of secondary structure (along with the α helix and β strand).

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Horton, H.R., Moran, L.A., Scrimgeour, K.G., perry, M.D. and Rawn, J.D. (2006) Principles of Biochemisty. Pearson/Prentice Hall, Upper Saddle River N.J. (USA)