Collagen is a structural protein. The beam of collagen is able to withstand greater load than steel wire of the same diameter. It is a major component of connective tissue and the most abundant protein in mammals. In humans, it accounts for about 30% of the total protein mass. Collagen provides strength and elasticity to the ligaments, cartilage, organs envelopes, epithelial basement membrane, as well as internal skin layer . One collagen molecule consists of three protein strands twisted into a spiral. Such a triple-helical structures are then combined into larger bands. Collagen synthesis occurs mainly in fibroblasts of the connective tissue, but some variation of this protein can be synthesized by epithelial cells .
Abnormalities in collagen synthesis lead to the connective tissue degradation. One of the classic examples is scurvy. This disease may occur due to lack of the vitamin C, since this compound is a cofactor of the proteins that synthesize collagen [3, 4].
Some pathogenic microorganisms such as Clostridium perfringens can cleave and destroy collagen using the enzyme collagenase . This bacteria live in soil and may become a reason of a gas gangrene in case of a contact with a wound or damage in a human body .
Collagen in a native and in modified forms is widely used in regenerative medicine and cosmetic surgery since it is important in the process of the wound healing and affects the skin elasticity [7, 8]. This protein is also used in the food industry: gelatin is a partially degraded collagen, which is extracted from the connective tissues of the livestock .
Each of the collagen subunits is 1050 amino acids in length. Collagen sequence consists of Gly-Pro-X repeats (triplets of glycine, proline (or hydroxyproline) and random amino acid going one after another. Three subunits together form a stable right-handed helix which is 300 nm long and 1.5 nm thick . Collagen molecules are connected with each other through hydroxyproline residues at the ends of chains. Triple helix strands are displaced from one another by about a quarter of their length. The resulting collagen fibrils eventually may reach several microns in length and become from 50 to 200 nm thick. Collagen synthesis starts in the fibroblasts cytoplasm, and the assembly of the fibrils occurs in the extracellular space. Different types of collagen vary in their ability to form crosslinks with auxiliary proteoglycan molecules. Type I collagen, mainly found in ligaments, forms long and strong bundles while the type II collagen, present in cartilage, forms finer and randomly oriented fibrils that are interconnected to form a dense network .
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