Collagen is a very variable protein, forming the basis of many connective and support tissues. It is a fibrous structural protein, with a distinctive structure.
In its most common form (tropocollagen) it is actually made up of 3 polypeptide chains which wind together to form into a triple helix - the quaternary level of protein structure. These are not alpha helices - in fact they are said to be left handed, opposite to the alpha helix. This model shows a section of 30 amino acid residues in each chain.
There is a high proportion (about 30%) of the simplest amino acid glycine
which having virtually no side chain nestles well into the helix, and also an unusual imino
- together with a modified version of it - hydroxyproline
Interestingly, this modification requires ascorbic acid (vitamin C) as an enzyme co-factor, which explains the symptoms of scurvy.
In other (globular) proteins, proline interferes with helical structures, especially the alpha helix, but it gives a tight structure to the strands in collagen.
Collagen has great tensile strength because under tension the 3 strands interlock rather than unwind. In real life variable amounts of cross-linking occurs, producing different arrangements for different functions in the body. Its structure of fibres, themselves composed of fibrils made of bundles of these triple helices, lends itself well to fibrous tissues such as tendons and ligaments, and also to skeletal tissues such as cartilage, bone and teeth which are further strengthened by mineral deposition. It is also partly responsible for the structure of skin and blood vessels.
The name collagen is based on the fact that skin and bones can be boiled to produce glue, as a result of the triple helices unwinding in all directions!