Pi helix
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![Side view of a standard π-helix of L-alanine residues in atomic detail. Two hydrogen bonds to the same peptide group are highlighted in magenta; the oxygen-hydrogen distance is 1.65 Å (165 pm). The protein chain runs upwards, i.e., its N-terminus is at the bottom and its C-terminus at the top of the figure. Note that the sidechains point slightly downwards, i.e., towards the N-terminus.](../../../../images/shared/thumb/2/2a/Pi_helix_neg55_neg70_sideview.png/250px-Pi_helix_neg55_neg70_sideview.png)
A pi helix (or π-helix) is a type of secondary structure found in proteins. These structure are particularly common in membrane proteins
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[edit] Standard structure
The amino acids in a standard π-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 i+5→i hydrogen bonding defines a π-helix. Similar structures include the 310 helix (i+3→i hydrogen bonding) and the α-helix (i+4→i hydrogen bonding).
![Top view of the same helix shown above. Four carbonyl groups are pointing upwards towards the viewer, spaced roughly 87° apart on the circle, corresponding to 4.1 amino-acid residues per turn of the helix.](../../../../images/local/thumb/f/ff/Pi_helix_topview.png/150px-Pi_helix_topview.png)
Residues in π-helices typically adopt (φ, ψ) dihedral angles near (-55°, -70°). 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 -125°. For comparison, the sum of the diheral angles for a 310 helix is roughly -75°, whereas that for the α-helix is roughly -105°. The general formula for the rotation angle Ω per residue of any polypeptide helix with trans isomers is given by the equation
[edit] Left-handed structure
In principle, a left-handed version of the π-helix is possible by reversing the sign of the (φ, ψ) dihedral angles to (55°, 70°). This pseudo-"mirror-image" helix has roughly the same number of residues per turn (4.1) and helical pitch (1.5 angstroms or 150 picometers). It is not a true mirror image, because the amino-acid residues still have a left-handed chirality. A long left-handed π-helix is unlikely to be observed in proteins because, among the naturally occurring amino acids, only glycine is likely to adopt positive φ dihedral angles such as 55°.
[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 | ||
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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→ |