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Collagen Types and Structure
- Over 90% of the collagen in the human body is type I collagen.
- 28 types of human collagen have been identified and divided into several groups based on their structure.
- All types of collagen contain at least one triple helix.
- Collagen forms a stable structure through hydrogen bonds and associates into a right-handed super-super-coil called the collagen microfibril.
- Three polypeptides coil to form tropocollagen, which then bind together to form a fibril and further form a fiber.
- Collagen has a regular arrangement of amino acids, with a sequence often following the pattern Gly-Pro-X or Gly-X-Hyp.
- Proline and hydroxyproline constitute about 1/6 of the total sequence, while glycine accounts for 1/3 of the sequence.
- Collagen fibrils have a distinctive D-period repeat of approximately 67nm, resulting in visible bands under electron microscopy.
- Covalent crosslinking occurs within the triple helices and between tropocollagen helices, forming well-organised aggregates.
- Collagen derived from cold-water fish has lower proline and hydroxyproline contents, leading to lower thermal stability compared to mammalian collagen.

Collagen Synthesis and Formation
- Collagen synthesis involves the modification of procollagen, which is formed by the addition of hydroxyl groups to proline and lysine amino acids.
- Hydroxylation reactions are catalyzed by prolyl-4-hydroxylase and lysyl-hydroxylase enzymes.
- Vitamin C is required as a cofactor for these hydroxylation reactions.
- The synthesis of collagen occurs both inside and outside the cell.
- Transcription of mRNA is initiated for the formation of a specific alpha peptide associated with collagen.
- Pre-pro-peptide formation occurs as the mRNA is translated and the signal sequence directs the peptide into the endoplasmic reticulum.
- Pre-pro-peptide is modified to propeptide by removing the signal peptide and undergoing hydroxylation of lysines and prolines.
- Glycosylation takes place by adding glucose or galactose monomers onto the hydroxyl groups of lysines.
- Procollagen is formed when three hydroxylated and glycosylated propeptides twist into a triple helix structure.
- In the Golgi apparatus, procollagen undergoes post-translational modification.
- Oligosaccharides are added to the procollagen.
- The modified procollagen is packaged into secretory vesicles for extracellular secretion.
- Defects in this step can lead to collagenopathies like Ehlers-Danlos syndrome.
- The procollagen is further processed and trimmed before being secreted.
- Tropocollagen is formed outside the cell by removing the loose ends of procollagen.
- Collagen peptidases are responsible for this removal process.
- The remaining tropocollagen undergoes covalent bonding between molecules through lysyl oxidase.
- Lysyl oxidase produces aldehyde groups on lysines and hydroxylysines.
- Multiple tropocollagen molecules form collagen fibrils, which eventually form collagen fibers.

Collagen Functions and Applications
- Collagen plays a key role in determining cell phenotype, cell adhesion, tissue regulation, and infrastructure.
- Non-proline-rich regions of collagen have roles in cell or matrix association/regulation.
- Collagen's unique properties make it important in various biological processes and tissue construction.
- Collagen scaffolds, such as sponges, thin sheets, gels, and fibers, are used in tissue regeneration.
- Collagen scaffolds have favorable properties for tissue regeneration, including pore structure, permeability, hydrophilicity, and in vivo stability.
- Collagen scaffolds support the deposition and growth of cells like osteoblasts and fibroblasts.
- Collagens are widely used in the construction of artificial skin substitutes for severe burns and wounds.
- Artificial skin substitutes containing collagen aid in wound healing and management.
- Collagen-based dressings have unique properties, including resistance against bacteria and promotion of rapid healing.
- Collagen is a key component of skin tissue and supports all stages of wound healing.
- Availability of collagen in the wound bed promotes wound closure and prevents deterioration.
- Collagen serves as a natural wound dressing, providing properties that artificial dressings lack.
- Collagen resists bacterial infection and helps maintain a sterile wound environment.
- Collagen promotes rapid healing of burns by facilitating the formation of healthy granulation tissue.

Collagen Types and Disorders
- Collagen-related diseases often arise from genetic defects or nutritional deficiencies affecting collagen production.
- Genetic defects in collagen genes can lead to disorders such as Osteogenesis imperfecta, Ehlers-Danlos syndrome, and Caffeys disease.
- Different collagen types are associated with specific disorders, such as types II and XI collagenopathies.
- Excessive collagen deposition occurs in scleroderma.
- Mutations in collagen genes can result in various diseases at the tissue level.

Collagen Research and Advances
- Collagen's insolubility was initially a barrier to its study, but extraction from young animals allowed for research breakthroughs.
- Advances in microscopy techniques and X-ray diffraction have provided detailed images of collagen structure.
- Understanding collagen structure is crucial for studying cell-cell and cell-matrix communication, tissue growth and repair, and developmental changes.
- Nanoindentation studies have revealed the heterogeneous nature of collagen fibrils and their different mechanical properties in gap and overlap regions.
- Collagen's arrangement and concentration vary in different tissues to provide specific tissue properties, such as bone's tensile strength.

Merriam-Webster Online Dictionary
collagen (noun)
any of a group of fibrous proteins that occur in vertebrates as the chief constituent of connective tissue fibrils and in bones and yield gelatin and glue upon boiling with water
Collagen (Wikipedia)

Collagen (/ˈkɒləən/) is the main structural protein in the extracellular matrix found in the body's various connective tissues. As the main component of connective tissue, it is the most abundant protein in mammals, making up from 25% to 35% of the whole-body protein content. Collagen consists of amino acids bound together to form a triple helix of elongated fibril known as a collagen helix. It is mostly found in connective tissue such as cartilage, bones, tendons, ligaments, and skin. Collagen makes up 30% of the protein found in the human body. Vitamin C is vital for collagen synthesis. Vitamin E improves the production of collagen.

Tropocollagen molecule: three left-handed procollagens (red, green, blue) join to form a right-handed triple helical tropocollagen.
HCR-FISH visualization of collagen expression in P. waltl

Depending upon the degree of mineralization, collagen tissues may be rigid (bone) or compliant (tendon) or have a gradient from rigid to compliant (cartilage). Collagen is also abundant in corneas, blood vessels, the gut, intervertebral discs, and the dentin in teeth. In muscle tissue, it serves as a major component of the endomysium. Collagen constitutes one to two percent of muscle tissue and accounts for 6% of the weight of the skeletal muscle tissue. The fibroblast is the most common cell that creates collagen. Gelatin, which is used in food and industry, is collagen that has been irreversibly hydrolyzed using heat, basic solutions or weak acids.

Collagen (Wiktionary)

English

Etymology

From French collagène, coined from Ancient Greek κόλλα (kólla, glue) and -γενής (-genḗs, -forming) (see -gen); it is the main substance that animal glues are derived from.

Pronunciation

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