Homo sapiens (human)?
Helices are possibly the most prominent motif of secondary structure in biochemistry. Made famous by the double helix of DNA, we also find helices in proteins, mainly as alpha-helices. The beta-strands that we also find in proteins are actually degenerate forms of helices. Helices were known to structural chemists long before the structures of DNA and proteins were known. They show very distinct patterns on fiber diffraction photographs. Linus Pauling used this information to predict alpha-helices before they were first observed. Alpha-helices in proteins are single-stranded. Individual amino acids turn in such a way that consecutive amino acids form the cork-screw like pattern of a helix. They are right-handed: if you take the four finger of the right hand to form the shape of a barrel then your thumb points into the direction of the advancing helix. These helices were prominent features in the first protein structures determined. However another form of protein helix was known for a long time, collagen. Collagen is part of the connective tissue. Based on fiber diffraction it was predicted that it would form a left-handed triple helix, but it took until the 1990s before these models could be experimentally verified.
The result you see here. Collagen has a particular sequence: the basic building block is glycine- proline-hydroxyproline. This is a peculiar arrangement because both glycine and proline are special amino acids. The basic form of an amino acid residue in a protein is (-NH-RCH-CO-) where R stands for the side chain atoms of the residue. In glycine R is H, making it the most flexible residue which can occur in almost any position and in proline R consists of three consecutive methyl groups -CH2-CH2-CH2. But the side chain bends such that the last methyl of proline bends makes a bond with the nitrogen. This makes proline a very rigid residue compared to other amino acids. Hydroxyproline has a hydroxyl (OH) attached to the second methyl. Individual strands of protein here form rather stretched-out helices, almost like beta-strands, but winding left-handed. Then three such helices wind around each other to from a superhelix, a left-handed triple helix. Defects in the structure of the collagen helix are behind a variety of diseases. A very simple example is caused by lack of vitamin C which causes a disease caused scurvy. In this case the body cannot attach the hydroxyl group to from hydroxyproline.
Protein Data Bank (PDB)
Bella, J. Eaton, M. Brodsky, B. Berman, H.M.; "Crystal and molecular structure of a collagen-like peptide at 1.9 A resolution."; Science; (1994) 266:75-81 PubMed:7695699.
author: Arno Paehler