3 10 helix

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Side view of an 3₁₀-helix of alanine residues in atomic detail. Two hydrogen bonds to the same peptide group are highlighted in magenta; the oxygen-hydrogen distance is 1.83 Å (183 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.

A 3₁₀-helix is a type of secondary structure found (rarely) in proteins.

[edit] Structure

The amino acids in a 3₁₀-helix are arranged in a right-handed helical structure. Each amino acid corresponds to a 120° turn in the helix (i.e., the helix has 3 residues per turn), and a translation of 2.0 Å (=0.2 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 three residues earlier; this repeated $i+3 \rightarrow i$ hydrogen bonding defines a $310$ -helix. Similar structures include the α-helix ( $i+4 \rightarrow i$ hydrogen bonding) and the $π$ -helix ( $i+5 \rightarrow i$ hydrogen bonding).

Top view of the same helix shown above. Three carbonyl groups are pointing upwards towards the viewer, spaced roughly 120° apart on the circle, corresponding to 3.0 amino-acid residues per turn of the helix.

Residues in 3₁₀-helices typically adopt (φ, ψ) dihedral angles near (-49°, -26°). 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 -75°. For comparison, the sum of the dihedral angles for an α-helix is roughly -105°, whereas that for a π-helix is roughly -125°.

The general formula for the rotation angle Ω per residue of any polypeptide helix with trans isomers is given by the equation

$3 \cos \Omega = 1 - 4 \cos^{2} \left[ \left(\phi + \psi \right)/2 \right]$

[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 \| 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→