One type of gamma-turn, rather than the other gives rise to chain-reversal in proteins

Abstract

Gamma-turns may be defined by a hydrogen bond between the carbonyl group of one amino acid residue and the amino group of the acid two residues ahead in the sequence. They occur as two types, inverse γ-turns and classic γ-turns (classic γ-turns are usually called just γ-turns but we prefer to add the adjective classic to distinguish them from the word γ-turn, referring collectively to both). Of the two, classic γ-turns are less common and are considered by all authors to be extreme rarities in proteins. However, we find that a number do occur in a sample of proteins of known three-dimensional structure. One occurs at the edge of the second hypervariable region of the light chain in some immunoglobulins. All classic γ-turns except one are associated with a reversal in the main chain direction. In most cases, the turn lies at the loop end of a β-hairpin. By contrast, inverse γ-turns, although giving rise to a kink in the chain, rarely occur within β-hairpins and are seldom situated at a position of reversal, by 180 °, in chain direction.