Turn (biochemistry)

According to one definition, a turn is a structural motif where the Cα atoms of two residues separated by a few (usually 1 to 5) peptide bonds are close (less than 7 Å [0.70 nm]).

[1] The proximity of the terminal Cα atoms often correlates with formation of an inter main chain hydrogen bond between the corresponding residues.

At least eight forms of the beta turn occur, varying in whether a cis isomer of a peptide bond is involved and on the dihedral angles of the central two residues.

A hairpin is a special case of a turn, in which the direction of the protein backbone reverses and the flanking secondary structure elements interact.

For example, a beta hairpin connects two hydrogen-bonded, antiparallel β-strands (a rather confusing name, since a β-hairpin may contain many types of turns – α, β, γ, etc.).

In one view, turns play a critical role in folding by bringing together and enabling or allowing interactions between regular secondary structure elements.

Apart from prediction, this method can also be used to find the minimum number of mutations required to initiate or break a beta turn in a protein at a desired location.