NB

Collagen contains specific amino acids – Glycine, Proline, Hydroxyproline and Arginine. These amino acids have a regular arrangement in each of the three chains of these collagen subunits. The sequence often follows the pattern Gly-Pro-X or Gly-X-Hyp, where X may be any of various other amino acid residues. Proline or hydroxyproline constitute about 1/6 of the total sequence. 
Glycine accounts for 1/3 of the sequence meaning that approximately half of the collagen sequence is not glycine, proline or hydroxyproline. In addition, the regular repetition and high glycine content is found in only a few other fibrous proteins, such as silk fibroin.
In silk 75-80% is -Gly-Ala-Gly-Ala- with 10% serine, and elastin is rich in glycine, proline, and alanine (Ala), whose side group is a small, inert methyl group. High glycine contents are not found in globular proteins except in very short sections of their sequence. Because glycine is the smallest amino acid with no side chain, it plays a unique role in fibrous structural proteins. 
Collagens do not contain chemically reactive side groups unlike in enzymes and transport proteins. Collagen determines cell phenotype, cell adhesion, tissue regulation and infrastructure and its non-proline rich regions have cell or matrix association/regulation roles. 
Left handed helices are formed because of the high content of proline and hydroxyproline rings, with their geometrically constrained carboxyl and (secondary) amino groups along with abundance of glycine. The left handed helices are formed without any intrachain hydrogen bonding.