Alpha helix

In proteins, the α helix is a major structural motif in secondary structure. It was first postulated by Linus Pauling, Robert Corey, and Herman Branson in 1951 based on the known crystal structures of amino acids and peptides and Pauling's prediction of planar peptide bonds.

Related Topics:
Protein - Secondary structure - Linus Pauling - Robert Corey - Herman Branson - 1951 - Crystal structures - Amino acid - Peptide - Peptide bond

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The amino acids in an α helix are arranged in a helical structure, about 5 Å (0.5 nm) wide. Each amino acid results in a 100° turn in the helix, and corresponds to a translation of 1.5 Å (0.15 nm) along the helical axis. The helix is tightly packed; there is almost no free space within the helix. All amino acid side-chains are arranged at the outside of the helix. The N-H group of amino acid (n) can establish a hydrogen bond with the C=O group of amino acid (n+4).

Related Topics:
Helical - Å - Nm - N-H - Hydrogen bond - C=O

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Short polypeptides usually are not able to adopt the alpha helical structure, since the entropic cost associated with the folding of the polypeptide chain is too high. Some amino acids (called helix breakers) like proline and glycine will disrupt the helical structure.

Related Topics:
Polypeptide - Entropic - Proline - Glycine

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Ordinarily, a helix has a buildup of positive charge at the N-terminal end and negative charge at the C-terminal end which is a destabilizing influence. As a result, α helices are often capped at the N-terminal end by a negatively charged amino acid (like glutamic acid) in order to stabilise the helix dipole. Less common (and less effective) is C-terminal capping with a positively charged amino acid like lysine.

Related Topics:
N-terminal end - C-terminal end - Amino acid - Glutamic acid - Lysine

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α helices have particular significance in DNA binding motifs, including helix-turn-helix motifs, leucine zipper motifs and zinc finger motifs. This is because of a structural coincidence: The diameter of the α helix is 120 nm, the same as the width of the major groove in B-form DNA.

Related Topics:
DNA - Helix-turn-helix - Leucine zipper - Zinc finger

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α helices are one of the basic structural elements in proteins, together with beta sheets.

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The peptide backbone of an α helix has 3.6 amino acids per turn.

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