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

## 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 ($i \rightarrow i\pm 3$ H-bonding).
• A γ-turn is characterized by hydrogen bond(s) in which the donor and acceptor residues are separated by two residues ($i \rightarrow i\pm 2$ H-bonding).
• An α-turn is characterized by hydrogen bond(s) in which the donor and acceptor residues are separated by four residues ($i \rightarrow i\pm 4$ H-bonding).
• A π-turn is characterized by hydrogen bond(s) in which the donor and acceptor residues are separated by five residues ($i \rightarrow i\pm 5$ 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 $\left( 75^{\circ}, -65^{\circ} \right)$ and an inverse form with dihedral angles $\left( -75^{\circ}, 65^{\circ} \right)$. 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 $\mathrm{I}^{\prime}$ 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

## 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 structureTertiary structure→
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 A 310 helix is a type of Secondary structure found (rarely in Proteins Structure The amino acids in a 310-helix are A pi helix (or π-helix) is a type of Secondary structure found in Proteins These structure are particularly common in membrane proteins A beta helix is a Protein structure formed by the association of parallel beta strands in a helical pattern with either two or three faces In Proteins a left-handed polyproline II helix ( PPII, poly-Pro II) is formed when sequential residues all adopt (φψ backbone Dihedral angles In Collagen, the collagen helix, or type 2 helix is a major shape in Quaternary structure. The β sheet (also β-pleated sheet) is the second form of regular Secondary structure in Proteins consisting of beta strands connected laterally The beta hairpin (or beta-beta unit) Structural motif is the simplest Protein motif involving two Beta strands that look like a Hairpin A beta bulge is a localized disruption of the regular Hydrogen bonding of a Beta sheet, usually by inserting a residue with helical Dihedral angles Experimental evidence When Pauling and Corey first proposed the alpha sheet they suggested that it agreed well with Fiber diffraction results from Beta-keratin For the coiled coil shape in general see Coil. A coiled coil is a Structural motif in Proteins in which 2-7 In Proteins the helix-turn-helix ( HTH) is a major Structural motif capable of binding DNA. The EF hand is a Helix-turn-helix Structural domain found in a large family of calcium-binding Proteins It consists of two alpha helices In Biochemistry and Structural biology, secondary structure is the general three-dimensional form of local segments of Biopolymers such as Methionine ( abbreviated as Met or M) is an α- Amino acid with the Chemical formula HO2CCH(NH2CH2CH2SCH3 Alanine (abbreviated as Ala or A) is an α- Amino acid with the Chemical formula HO2CCH(NH2CH3 Leucine (abbreviated as Leu or L) is an α- Amino acid with the Chemical formula HO2CCH(NH2CH2CH(CH32 Glutamic acid (abbreviated as Glu or E) is one of the 20 Alpha Amino acids It is not among the human Essential amino acids Its Glutamine (abbreviated as Gln or Q; the abbreviation Glx or Z represents either glutamate or Glutamic acid) is one of the 20 Lysine (abbreviated as Lys or K) is an α- Amino acid with the Chemical formula HO2CCH(NH2(CH24NH2 Threonine (abbreviated as Thr or T) is an α- Amino acid with the Chemical formula HO2CCH(NH2CH(OHCH3 Isoleucine (abbreviated as Ile or I) is an α- Amino acid with the Chemical formula HO2CCH(NH2CH(CH3CH2CH3 Valine (abbreviated as Val or V) is an α- Amino acid with the Chemical formula HO2CCH(NH2CH(CH32 Phe redirects here For the BitTorrent feature see PHE. For the constellation see Phoenix (constellation. Tyrosine (abbreviated as Tyr or Y) or 4-hydroxyphenylalanine, is one of the 20 Amino acids that are used by cells to synthesize Tryptophan (abbreviated as Trp or W) is one of the 20 standard amino acids, as well as an Essential amino acid in the Human diet Glycine (abbreviated as Gly or G) is the Organic compound with the formula NH2CH2COOH Serine (abbreviated as Ser or S) is an Organic compound with the formula H[[oxygen O]]2 CCH NH sub>2CH2OH Proline (abbreviated as Pro or P) is an α- Amino acid, one of the twenty DNA -encoded amino acids Asparagine (abbreviated as Asn or N; Asx or B represent either asparagine or Aspartic acid) is one of the 20 most common natural Aspartic acid (abbreviated as Asp or D; Asx or B represent either aspartic acid or Asparagine) is an α- Amino acid Not to be confused with Cystine, its oxidized dimer Cysteine (abbreviated as Cys or C) is an α- Amino acid with Histidine (abbreviated as His or H) is one of the 20 standard Amino acids present in Proteins In the Nutritional sense in Arginine (abbreviated as Arg or R) is an α- Amino acid. The L-form is one of the 20 most common natural amino acids In Biochemistry, the primary structure of a biological molecule is the exact specification of its atomic composition and the chemical bonds connecting those atoms (including In Biochemistry and Chemistry, the tertiary structure of a Protein or any other Macromolecule is its three-dimensional structure as defined
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