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Collagen - What is Collagen?
Collagen is a group of naturally occurring proteins. In nature, it is found exclusively in animals, especially in the flesh and connective tissues of mammals. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content. Collagen, in the form of elongated fibrils, is mostlyfound in fibrous tissues such as tendon, ligament and skin, and is also abundant in cornea, cartilage, bone, blood vessels, the gut, and intervertebral disc. In muscle tissue it serves as a major component of endomysium. Collagen constitutes 1% to 2% of muscle tissue, and accounts for 6% of the weight of strong, tendinous muscles. Gelatin, which is used in food and industry, is collagen that hasbeen irreversibly hydrolyzed.

An atomic-force-microscopy image of collagen fibrils in a collagen fiber. Image Credit: UChicago Argonne LLC The molecular and packing structures of collagen have eluded scientists over decades of research. The first evidence that it possesses a regular structure at the molecular level was presented in the mid-1930s.

Since that time many prominent scholars,including Nobel laureates Crick, Pauling, Rich and Yonath and others including Brodsky, Berman, and Ramachandran, concentrated on the conformation of the collagen monomer. Several competing models, although correctly dealing with the conformation of each individual peptide chain, gave way to the triple-helical "Madras" model which provided an essentially correct model of the molecule's quaternarystructure although this model still required some refinement. The packing structure of collagen has not been defined to the same degree outside of the fibrillar collagen types, although it has been long known to be hexagonal ...or quasi-hexagonal. As with its monomeric structure, several conflicting models alleged that either the packing arrangement of collagen molecules is 'sheet-like' ormicrofibrillar. The microfibrillar structure of collagen fibrils in tendon, cornea and cartilage has been directly imaged by electron microscopy. In 2006, it was confirmed that the microfibrillar structure of adult tendon as described by Fraser, Miller, Wess (amongst others) was closest to the observed structure, although it over-simplified the topological progression of neighboring collagen molecules andhence did not predict the correct conformation of the discontinuous D-periodic pentameric arrangement termed simply: the microfibril.

Collagen Molecular Structure
The ''tropocollagen'' or "collagen molecule" is a subunit of larger collagen aggregates such as fibrils. It is approximately 300 nm long and 1.5 nm in diameter, made up of three polypeptide strands (called alpha chains), each possessingthe conformation of a left-handed helix (its name is not to be confused with the commonly occurring alpha helix, a right-handed structure). These three left-handed helices are twisted together into a right-handed coiled coil, a triple helix or "super helix", a cooperative quaternary structure stabilized by numerous hydrogen bonds.

With type I collagen and possibly all fibrillar collagens ifnot all collagens, each triple-helix associates into a right-handed super-super-coil that is referred to as the collagen microfibril. Each microfibril is interdigitated with its neighboring microfibrils to a degree that might suggest that they are individually unstable although within collagen fibrils they are so well ordered as to be crystalline. A distinctive feature of collagen is the regulararrangement of amino acids 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. With glycine accounting for the 1/3 of the sequence, this means that approximately half of the collagen sequence is not...
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