However, the collagen triple helix is not an alpha helix.
In article <Z1BU6.119623$ff.894474 at news-server.bigpond.net.au>, "Mitchell
Isaacs" <misaacs at spam_me_not.student.usyd.edu.au> wrote:
> "Eckart Bindewald" <bindewald at ti.uni-mannheim.de> wrote in message
> news:3B224173.7C6F9DFA at ti.uni-mannheim.de...> > Hello!
> > Often it is cited, that left-handed alpha-helices are very rare in
> proteins, but DO exist. However,
> > I could not find a reference to an example structure. Who can inform me
> about a protein structure
> > which contains a left-handed alpha-helix (PDB codes preferred)?
> I don't think you can get an extensive left-handed helix with L-amino acids,
> but if you did it would be with glycines (no side chain, no steric
> hindrance - glycine is not optically active). Glycine residues adopting the
> left-handed helical position are quite common - check out
>http://www.expasy.org/swissmod/course/text/chapter2.htm>> Actually, one that may interest you: I took this from
>http://academic.mu.edu/bisc/siebenlistk/413proteinstructure.pdf>> "Protropocollagen is the monomer unit and this molecule folds into a Left
> Handed Helix containing 3.3 amino acids per turn. Three protropocollagen
> molecules, three left handed helixes combine to form a triple helix - The
> Collagen Triple Helix or Tropocollagen. This triple helix has a Right
> Handed Twist, and contains ten X-GLY-Y repeat units per turn. The glycine
> residues of the collagen molecule line up on the interior of the triple
> helix. Tropocollagen triple helixes associate with each other to form long
> strong fibers. The triple helixes associate with each other in a staggered
> overlapping fashion involving 75 to 80% of the molecule."