Turn (biochemistry)
Encyclopedia
A turn is an element of secondary structure
in proteins where the polypeptide chain reverses its overall direction.
s are in close approach (< 7 Å), while the corresponding residues do not form a regular secondary structure
element such as an alpha helix
or beta sheet
. Contrary to helices, the backbone dihedral angles are not (roughly) constant for all the residues in the turn.
Although the close approach of the two terminal Cα atoms is usually correlated with the forming of one or two hydrogen bond
s between the corresponding residues, such hydrogen bond is not strictly required in the definition of the turn. That said, in most cases the H-bonding and Cα-distance definitions are equivalent.
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 (also called its mirror-image turn) by changing the sign on all of its dihedral angles. (The inverse turn is not a true mirror image since the chirality of the Cα atoms 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 β-turn have been identified, varying mainly 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 usually distinguished with a prime, e.g., type I and type I' β-turns.
double turn. Multiple turns (up to 7-fold) occur in proteins, and they are found to be more common than single turns.
, antiparallel β-strands.
(a rather confusing name, since a β-hairpin may contain many types of turns - α,β,γ, etc.)
β-hairpins may be classified according to the number of residues that make up the turn - that is, that are not part of the flanking β-strands . If this number is X or Y (according to two different definitions of β sheets) the β hairpin is defined as X:Y
. In one view, turns play a critical role in folding by bringing together and enabling or allowing interactions between regular secondary structure elements. This view is supported by mutagenesis studies indicating a critical role for particular residues in the turns of some proteins. Also, nonnative isomers of X-Pro peptide bond
s in turns can completely block the conformational folding of some proteins. In the opposing view, turns play a passive role in folding. This view is supported by the poor amino-acid conservation observed in most turns. Also, non-native isomers of many X-Pro peptide bond
s in turns have little or no effect on folding.
Secondary structure
In biochemistry and structural biology, secondary structure is the general three-dimensional form of local segments of biopolymers such as proteins and nucleic acids...
in proteins where the polypeptide chain reverses its overall direction.
Definition
According to the most common definition, a turn is a structural motif where the Cα atoms of two residues separated by few (usually 1 to 5) peptide bondPeptide bond
This article is about the peptide link found within biological molecules, such as proteins. A similar article for synthetic molecules is being created...
s are in close approach (< 7 Å), while the corresponding residues do not form a regular secondary structure
Secondary structure
In biochemistry and structural biology, secondary structure is the general three-dimensional form of local segments of biopolymers such as proteins and nucleic acids...
element such as an alpha helix
Alpha helix
A common motif in the secondary structure of proteins, the alpha helix is a right-handed coiled or spiral conformation, in which every backbone N-H group donates a hydrogen bond to the backbone C=O group of the amino acid four residues earlier...
or beta sheet
Beta sheet
The β sheet is the second form of regular secondary structure in proteins, only somewhat less common than the alpha helix. Beta sheets consist of beta strands connected laterally by at least two or three backbone hydrogen bonds, forming a generally twisted, pleated sheet...
. Contrary to helices, the backbone dihedral angles are not (roughly) constant for all the residues in the turn.
Although the close approach of the two terminal Cα atoms is usually correlated with the forming of one or two hydrogen bond
Hydrogen bond
A hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, such as nitrogen, oxygen or fluorine, that comes from another molecule or chemical group. The hydrogen must be covalently bonded to another electronegative atom to create the bond...
s between the corresponding residues, such hydrogen bond is not strictly required in the definition of the turn. That said, in most cases the H-bonding and Cα-distance definitions are equivalent.
Tight turns
Turns are classified according to the separation between the two end residues:- In an α-turn the end residues are separated by four peptide bonds (
). - In a β-turn (the most common form), by three bonds (
). - In a γ-turn, by two bonds (
). - In a δ-turn, by one bond (
). - In a π-turn, by five bonds (
).
| Type |  |  |  |  |
|---|---|---|---|---|
| I | -60 | -30 | -90 | 0 |
| II | -60 | 120 | 80 | 0 |
| VIII | -60 | -30 | -120 | 120 |
| I' | 60 | 30 | 90 | 0 |
| II' | 60 | -120 | -80 | 0 |
| VIa1 | -60 | 120 | -90 | 0* |
| VIa2 | -120 | 120 | -60 | 0* |
| VIb | -135 | 135 | -75 | 160* |
| IV | turns excluded from all the above categories |
|||
-turn types. Types VIa1, VIa2 and VIb turns are subject to the additional condition that residue 
(*) must be a cis-Within each type, turns may be further classified by their backbone dihedral angles (see Ramachandran plot
Ramachandran plot
-Introduction and early history:A Ramachandran plot , originally developed in 1963 by G. N. Ramachandran C. Ramakrishnan and V...
). A turn can be converted into its inverse turn (also called its mirror-image turn) by changing the sign on all of its dihedral angles. (The inverse turn is not a true mirror image since the chirality of the Cα atoms 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 β-turn have been identified, varying mainly 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 usually distinguished with a prime, e.g., type I and type I' β-turns.
Loops
An ω-loop is a catch-all term for a longer, extended or disordered loop without fixed internal hydrogen bonding.Multiple turns
In many cases, one or more residues are involved in two partially overlapping turns. For example, in a sequence of 5 residues, both residues 1-4 and residues 2-5 form a turn; in such a case, one speaks of a double turn. Multiple turns (up to 7-fold) occur in proteins, and they are found to be more common than single turns.Hairpins
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 β-hairpin connects two hydrogen-bondedHydrogen bond
A hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, such as nitrogen, oxygen or fluorine, that comes from another molecule or chemical group. The hydrogen must be covalently bonded to another electronegative atom to create the bond...
, antiparallel β-strands.
(a rather confusing name, since a β-hairpin may contain many types of turns - α,β,γ, etc.)
β-hairpins may be classified according to the number of residues that make up the turn - that is, that are not part of the flanking β-strands . If this number is X or Y (according to two different definitions of β sheets) the β hairpin is defined as X:Y
Role in protein folding
Two hypotheses have been proposed for the role of turns in protein foldingProtein folding
Protein folding is the process by which a protein structure assumes its functional shape or conformation. It is the physical process by which a polypeptide folds into its characteristic and functional three-dimensional structure from random coil....
. In one view, turns play a critical role in folding by bringing together and enabling or allowing interactions between regular secondary structure elements. This view is supported by mutagenesis studies indicating a critical role for particular residues in the turns of some proteins. Also, nonnative isomers of X-Pro peptide bond
Peptide bond
This article is about the peptide link found within biological molecules, such as proteins. A similar article for synthetic molecules is being created...
s in turns can completely block the conformational folding of some proteins. In the opposing view, turns play a passive role in folding. This view is supported by the poor amino-acid conservation observed in most turns. Also, non-native isomers of many X-Pro peptide bond
Peptide bond
This article is about the peptide link found within biological molecules, such as proteins. A similar article for synthetic molecules is being created...
s in turns have little or no effect on folding.