Pi helix
From Wikipedia, the free encyclopedia
A pi helix (or <math>\boldsymbol\pi</math>-helix) is a type of secondary structure found (rarely) in proteins.
[edit] Structure
The amino acids in an <math>\pi</math>-helix are arranged in a right-handed helical structure. Each amino acid corresponds to a 87° turn in the helix (i.e., the helix has 4.1 residues per turn), and a translation of 1.15 Å (=0.115 nm) along the helical axis. Most importantly, the N-H group of an amino acid forms a hydrogen bond with the C=O group of the amino acid five residues earlier; this repeated <math>i+5 \rightarrow i</math> hydrogen bonding defines a <math>\pi</math>-helix. Similar structures include the <math>3_{10}</math> helix (<math>i+3 \rightarrow i</math> hydrogen bonding) and the α-helix (<math>i+4 \rightarrow i</math> hydrogen bonding).
Residues in <math>\pi</math>-helices typically adopt (φ, ψ) dihedral angles near <math>(-55^{\circ}, -70^{\circ})</math>. More generally, they adopt dihedral angles such that the ψ dihedral angle of one residue and the φ dihedral angle of the next residue sum to roughly <math>-125^{\circ}</math>. For comparison, the sum of the diheral angles for a <math>3_{10}</math> helix is roughly <math>-75^{\circ}</math>, whereas that for the α-helix is roughly <math>-105^{\circ}</math>. The general formula for the rotation angle <math>\Omega</math> per residue of any polypeptide helix with trans isomers is given by the equation
- <math>
3 \cos \Omega = 1 - 4 \cos^{2} \left[ \left(\phi + \psi \right)/2 \right] </math>
[edit] See also
[edit] References
- Pauling L, Corey RB and Branson HR. (1951) "The Structure of Proteins: Two Hydrogen-Bonded Helical Configurations of the Polypeptide Chain", Proc. Nat. Acad. Sci. Wash., 37, 205.
| Protein secondary structure | ||
|---|---|---|
| Helices: | α-helix | 310 helix | π-helix | β-helix | Polyproline helix | Collagen helix | |
| Extended: | β-strand | Turn | Beta hairpin | Beta bulge | |
| 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→ | |
 | This protein-related article is a stub. You can help Wikipedia by expanding it. |
