Loop (biochemistry)
A turn is an element of secondary structure in proteins where the polypeptide chain reverses its overall direction.
According to one definition, a turn is a structural motif where the Cα atoms of two residues separated by few (usually 1 to 5) peptide bonds are close (less than 7 Å [0.70 nm]), while the residues do not form a secondary structure element such as an alpha helix or beta sheet with regularly repeating backbone dihedral angles. Although the proximity of the terminal Cα atoms usually correlates with formation of a hydrogen bond between the corresponding residues, a hydrogen bond is not a requirement in this turn definition. That said, in many cases the hydrogen-bonding and Cα-distance definitions are equivalent.
Turns are classified according to the separation between the two end residues:
turns excluded from all the above categories
Within each type, turns may be further classified by their backbone dihedral angles (see Ramachandran plot). A turn can be converted into its inverse turn (in which the main chain atoms have opposite chirality) by changing the sign on its dihedral angles. (The inverse turn is not a true enantiomer since the Cα atom chirality is maintained.) Thus, the γ-turn has two forms, a classical form with (φ, ψ) dihedral angles of roughly (75°, −65°) and an inverse form with dihedral angles (−75°, 65°). 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. The classical and inverse β-turns are distinguished with a prime, e.g., type I and type I′ beta turns. If an i → i + 3 hydrogen bond is taken as the criterion for turns, the four categories of Venkatachalam (I, II, II′, I′) suffice to describe all possible beta turns. All four occur frequently in proteins but I is most common, followed by II, I′ and II′ in that order.
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