Situations of gamma-turns in proteins. Their relation to alpha-helices, beta-sheets and ligand binding sites

Gamma-turns occur as one of two possible enantiomers with regard to their main-chain structure, called classic and inverse. Of these, inverse ones are more common. Unlike other hydrogen bonds, those in inverse gamma-turns include a large proportion that are weak. If such hydrogen bonds are included, these turns may be said to be abundant in proteins. A significant number of inverse gamma-turns, usually weak ones, exist as consecutive turns in a structural feature, now called the 2.2(7)-helix, proposed for polypeptides as long ago as 1943 by Huggins. Most of these features occur within strands of beta-sheet. The less-weak inverse gamma-turns fall into several structural subgroups. They are frequently situated directly at either end of alpha-helices or of strands of beta-sheet, or adjacent to certain loop motifs. In general, they are well conserved during evolution and some are found at key positions in proteins. One occurs in the first hypervariable loop in the heavy chain of immunoglobulins.