Alpha helix

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Alpha helix （α-helix） yes Protein secondary structure One of the main forms of. Anaphora Peptide chain Main chain around Central axis Regular spiral rise, every 3.6 Amino acid residue The spiral rises one circle and moves upward 0.54nm, so pitch 0.54nm, two amino acid The distance between residues is 0.15 nm. The direction of the spiral is Right-hand spiral 。 amino acid Side chain The R group extends to the outside of the helix Peptide bond Of carbonyl Oxygen forms a hydrogen bond with the fourth N-H, and the direction of the hydrogen bond is basically parallel to the long axis of the helix. Since all peptide bonds in the peptide chain can form hydrogen bonds, the α - helix is very stable.

Chinese name: Alpha helix
Type: Secondary structure of protein

Pitch: 0.54nm
Radius of spiral: 0.23nm
English: α -helix

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Main features

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Alpha helix

① Peptide chain Advance by spiral coiling, each spiral consists of 3.6 amino Acid composition, coil spacing（ pitch ）5.44 A; ② Helical structure It is stabilized by regularly arranged hydrogen bonds. The arrangement of hydrogen bonds is as follows: Amino acid residue N - H and its amino side amino acid C=O of residue forms hydrogen bond. The ring thus formed is closed by a hydrogen bond and contains 13 atoms. Therefore, the alpha helix is often accurately expressed as 3.6/13 helix. Spiral winding methods generally include right hand rotation and left hand rotation protein What actually exists in the molecule is Right-hand spiral 。

Structure of alpha helix^[1]

Formative factors

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① Side chain The charge property and size of the group have influence, glycine Because the side chain is too small, conformation Unstable, yes Alpha helix The destroyer of; ② Continuous existence of amino acids with the same charge Residue ；③ non-existent proline Residues, proline due to its Imino group One less hydrogen atom The hydrogen bond cannot be formed, and the C α - N bond cannot rotate, so it is the breaker of the alpha helix. When proline appears in the peptide chain, the alpha helix will be interrupted, forming a "nodule".

Dipole distance

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All hydrogen bonds in the α - helix point in the same direction along the helix axis. Every Peptide bond Both have dipole distances due to the polarity of N-H and C=O.

Capping

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The so-called capping is to provide only hydrogen bond for N-H and C=O exposed at the end Consonant body (partner), and fold other parts of the protein to promote non-polar with the end Residue Of Hydrophobic action 。

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