A turn is an element of secondary structure in proteins. In Biochemistry and Structural biology, secondary structure is the general three-dimensional form of local segments of Biopolymers such as
According to the most common definition, a turn is defined by the close approach of two Cα atoms (< 7 Å), when the corresponding residues are not involved in a regular secondary structure element such as an alpha helix or beta sheet. In Biochemistry and Structural biology, secondary structure is the general three-dimensional form of local segments of Biopolymers such as A common motif in the Secondary structure of Proteins the alpha helix (α-helix is a right-handed coiled conformation resembling a spring, in which The β sheet (also β-pleated sheet) is the second form of regular Secondary structure in Proteins consisting of beta strands connected laterally
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Types of turns
Turns are grouped by their hydrogen bonding and by their backbone dihedral angles. A hydrogen bond results from a Dipole-dipole force between an Electronegative atom and a Hydrogen atom bonded to Nitrogen, Oxygen In Aerospace engineering, the Dihedral is the Angle between the two wings see Dihedral.
At the level of hydrogen bonds, the nomenclature is similar to that of helices. A common motif in the Secondary structure of Proteins the alpha helix (α-helix is a right-handed coiled conformation resembling a spring, in which
- A β-turn (the most common form) is characterized by hydrogen bond(s) in which the donor and acceptor residues are separated by three residues (
H-bonding). - A γ-turn is characterized by hydrogen bond(s) in which the donor and acceptor residues are separated by two residues (
H-bonding). - An α-turn is characterized by hydrogen bond(s) in which the donor and acceptor residues are separated by four residues (
H-bonding). - A π-turn is characterized by hydrogen bond(s) in which the donor and acceptor residues are separated by five residues (
H-bonding).
Finally, an ω-loop is a catch-all term for a longer loop with no internal hydrogen bonding.
Strictly speaking, a turn is defined by the close approach (< 7 Å) of Cα atoms and need not have a well-formed hydrogen bond. A hydrogen bond results from a Dipole-dipole force between an Electronegative atom and a Hydrogen atom bonded to Nitrogen, Oxygen Thus, it is more correct to define a β-turn by the close approach of Cα atoms of residues separated by three peptide bonds, a γ-turn by the close approach of Cα atoms of residues separated by two peptide bonds, etc. A peptide bond is a Chemical bond formed between two Molecules when the Carboxyl group of one molecule reacts with the A peptide bond is a Chemical bond formed between two Molecules when the Carboxyl group of one molecule reacts with the In most cases, the H-bonding and Cα-distance definitions are equivalent.
Within each hydrogen-bonding type, turns may be classified by their backbone dihedral angles. In Aerospace engineering, the Dihedral is the Angle between the two wings see Dihedral. 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 
and an inverse form with dihedral angles 
. 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 
β-turns.
Hairpins vs. diverging turns
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-bonded, antiparallel β-strands. A hydrogen bond results from a Dipole-dipole force between an Electronegative atom and a Hydrogen atom bonded to Nitrogen, Oxygen
However, turns can cause less drastic changes in direction and may connect regular secondary structure elements that do not interact with each other. Such turns are called diverging turns.
Role in protein folding
Two hypotheses have been proposed for the role of turns in protein folding. Protein folding is the physical process by which a Polypeptide folds into its characteristic and functional three-dimensional structure. In one view, turns play a critical role in folding by bringing together and fostering 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 bonds in turns can completely block the conformational folding of some proteins. A peptide bond is a Chemical bond formed between two Molecules when the Carboxyl group of one molecule reacts with the 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 bonds in turns have little or no effect on folding. A peptide bond is a Chemical bond formed between two Molecules when the Carboxyl group of one molecule reacts with the
See also
References
- Venkatachalam CM (1968) "Stereochemical Criteria for Polypeptides and Proteins. In Biochemistry and Structural biology, secondary structure is the general three-dimensional form of local segments of Biopolymers such as V. Conformation of a System of 3 Linked Peptide Units", Biopolymers, 6, 1425-1436.
- Némethy G and Printz MP. (1972) "The γ-Turn, a Possible Folded Conformation of the Polypeptide Chain. Comparison with the β-Turn", Macromolecules, 5, 755-758.
- Lewis PN, Momany FA and Scheraga HA. (1973) "Chain Reversals in Proteins. ", Biochim. Biophys. Acta, 303, 211-229.
- Toniolo C. (1980) "Intramolecularly Hydrogen-Bonded Peptide Conformations", CRC Crit. Rev. Biochem. , 9, 1-44.
- Richardson JS. (1981) "The anatomy and taxonomy of protein structure", Adv. Protein Chem. , 34, 167-339.
- Rose GD, Gierasch LM and Smith JA. (1985) "Turns in peptides and proteins", Adv. Protein Chem. , 37, 1-109.
- Milner-White EJ and Poet R. (1987) "Loops, bulges, turns and hairpins in proteins", TIBS, 12, 189-192.
- Wilmot CM and Thornton JM. (1988) "Analysis and Prediction of the Different Types of β-Turn in Proteins", J. Mol. Biol. , 203, 221-232.
- Milner-White EJ. (1990) "Situations of Gamma-turns in Proteins: Their Relation ot Alpha-helices, Beta-sheets and Ligand Binding Sites", J. Mol. Biol. , 216, 385-397.
- Pavone V, Gaeta G, Lombardi A, Nastri F, Maglio O, Isernia C, and Saviano M. (1996) "Discovering Protein Secondary Structures: Classification and Description of Isolated α-Turns", Biopolymers, 38, 705-721.
- Rajashankar KR and Ramakumar S. (1996) "π-Turns in proteins and peptides: Classification, conformation, occurrence, hydration and sequence", Protein Sci. , 5, 932-946.
| Protein secondary structure | ||
|---|---|---|
| Helices: | α-helix | 310 helix | π-helix | β-helix | Polyproline helix | Collagen helix | |
| Extended: | β-strand | Turn | Beta hairpin | Beta bulge | α-strand | |
| Supersecondary: | Coiled coil | Helix-turn-helix | EF hand | |
| Secondary structure propensities of amino acids | ||
| Helix-favoring: | Methionine | Alanine | Leucine | Glutamic acid | Glutamine | Lysine | |
| Extended-favoring: | Threonine | Isoleucine | Valine | Phenylalanine | Tyrosine | Tryptophan | |
| Disorder-favoring: | Glycine | Serine | Proline | Asparagine | Aspartic acid | |
| No preference: | Cysteine | Histidine | Arginine | |
| ←Primary structure | Tertiary structure→ | |
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