Alpha helix
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Image:AlphaHelixProtein.jpg 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.
The experimental work to provide the basic dimensions of the helix had been done by William Astbury on keratin using X-ray diffraction in 1937. However the correct interpretation of the data was difficult and Pauling puzzled over it for years. It was not until January 1948, while in Oxford, that Pauling caught a cold and went to bed. After a while he became bored and drew the chain on a strip of paper and folded it into a helix. After a few attempts at folding, he eventually produced the model which had the hydrogen bonds in the correct places. Pauling then worked with Corey and Branson to confirm his idea before publication.
The amino acids in an α helix are arranged in a helical structure, 5.4 Angstroms or .54 nanometres wide. Each amino acid results in a 100° turn in the helix, and corresponds to a translation of 0.15 nanometres 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+4) can establish a hydrogen bond with the C=O group of amino acid (n).
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.
Ordinarily, a helix has an overall dipole moment caused by the aggregate effect of all the individual dipoles from the carbonyl groups of the peptide bond pointing along the helix axis. This can lead to destabilization of the helix through entropic effects. 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 neutralize this helix dipole. Less common (and less effective) is C-terminal capping with a positively charged amino acid like lysine.
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 1.2 nanometres, the same as the width of the major groove in B-form DNA.
α helices are one of the basic structural elements in proteins, together with beta sheets.
The peptide backbone of an α helix has 3.6 amino acids per turn.
See also
References
- David Eisenberg, "The discovery of the α-helix and β-sheet, the principal structural features of proteins". Proceedings of the National Academy of Sciences USA. (2003). 100:11207-11210. http://www.pnas.org/cgi/content/full/100/20/11207
External links
eo:Alfa-helico es:Hélice alfa fr:Hélice alpha nl:Alfa-helix ja:Αへリックス pl:Helisa alfa sv:Alfahelix