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Tropocollagen triple helix.

Collagen is a group of naturally occurring proteins. In nature, it is found exclusively in animals.[1] It is the main protein of connective tissue. It is the most abundant protein in mammals,[2] making up about 25% to 35% of the whole-body protein content.

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.[3] Gelatin, which is used in food and industry, is derived from collagen.


History and background

The molecular and packing structures of collagen have eluded scientists for decades; the first evidence that it possesses a regular structure at the molecular level was presented in the mid-1930s [4][5]. Since that time many prominent scholars, including (but not limited to) Nobel laureate Crick, Pauling, Rich, Yonath, 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 quaternary structure [6][7][8] although this model still required some refinement [9][10][11][12]. 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 [13][14][15]. As with its monomeric structure, several conflicting models alleged that either the packing arrangement of collagen molecules is ‘sheet-like’ or microfibrillar.[16][17] Recently it was confirmed that the microfibrillar structure 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 and hence did not predict the correct conformation of the discontinuous D-periodic pentameric arrangement termed simply: the microfibril[18]

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 possessing the 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 if not 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 regular arrangement of amino acids in each of the three chains of these collagen subunits. The sequence often follows the pattern Gly-Pro-Y or Gly-X-Hyp, where X and Y 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 glycine, proline or hydroxyproline, a fact often missed due to the distraction of the unusual GXY character of collagen alpha-peptides. This kind of regular repetition and high glycine content is found in only a few other fibrous proteins, such as silk fibroin. 75-80% of silk is (approximately) -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. Such high glycine and regular repetitions are never found in globular proteins save for very short sections of their sequence. Chemically-reactive side groups are not needed in structural proteins as they are in enzymes and transport proteins, however collagen is not quite just a structural protein. Due to its key role in the determination of cell phenotype, cell adhesion, tissue regulation and infrastructure, many sections of its non-proline rich regions have cell or matrix association / regulation roles. The relatively high content of Proline and Hydroxyproline rings, with their geometrically constrained carboxyl and (secondary) amino groups, along with the rich abundance of glycine, accounts for the tendency of the individual polypeptide strands to form left-handed helices spontaneously, without any intrachain hydrogen bonding.

Because glycine is the smallest amino acid with no side-chain, it plays a unique role in fibrous structural proteins. In collagen, Gly is required at every third position because the assembly of the triple helix puts this residue at the interior (axis) of the helix, where there is no space for a larger side group than glycine’s single hydrogen atom. For the same reason, the rings of the Pro and Hyp must point outward. These two amino acids help stabilize the triple helix—Hyp even more so than Pro—a lower concentration of them is required in animals such as fish, whose body temperatures are lower than most warm-blooded animals.

Fibrillar structure

1 The tropocollagen subunits spontaneously self-assemble, with regularly staggered ends, into even larger arrays in the extracellular spaces of tissues[19][20]. In the fibrillar collagens, the molecules are staggered from each other by about 67 nm (a unit that is referred to as ‘D’ and changes depending upon the hydration state of the aggregate). Each D-period contains 4 and a fraction collagen molecules. This is because 300 nm divided by 67 nm does not give an integer (the length of the collagen molecule divided by the stagger distance D). Therefore in each D-period repeat of the microfibril, there is a part containing five molecules in cross-section—called the “overlap” and a part containing only 4 molecules, called the "gap".[18] The triple-helices are also arranged in a hexagonal or quasi-hexagonal array in cross-section, in both the gap and overlap regions.[18][21].

There is some covalent crosslinking within the triple helices, and a variable amount of covalent crosslinking between tropocollagen helices forming well organized aggregates (such as fibrils)[22]. Larger fibrillar bundles are formed with the aid of several different classes of proteins (including different collagen types), glycoproteins and proteoglycans to form the different types of mature tissues from alternate combinations of the same key players[20]. Collagen's insolubility was a barrier to the study of monomeric collagen until it was found that tropocollagen from young animals can be extracted because it is not yet fully crosslinked. However, advances in microscopy techniques (Electron Microscopy (EM) and Atomic Force Microscopy (AFM)) and X-ray diffraction have enabled researchers to obtain increasingly detailed images of collagen structure in situ. These later advances are particularly important to better understanding the way in which collagen structure affects cell-cell and cell-matrix communication and how tissues are constructed in growth and repair, and changed in development and disease[23][24].

Collagen fibrils are semi-crystalline aggregates of collagen molecules. Collagen fibers are bundles of fibrils.

Collagen fibrils/aggregates are arranged in different combinations and concentrations in various tissues to provide varying tissue properties. In bone, entire collagen triple helices lie in a parallel, staggered array. 40 nm gaps between the ends of the tropocollagen subunits (approximately equal to the gap region) probably serve as nucleation sites for the deposition of long, hard, fine crystals of the mineral component, which is (approximately) hydroxyapatite, Ca10(PO4)6(OH)2 with some phosphate. It is in this way that certain kinds of cartilage turn into bone. Type I collagen gives bone its tensile strength.

Types and associated disorders

Collagen occurs in many places throughout the body. So far, 29 types of collagen have been identified and described. Over 90% of the collagen in the body, however, are of type I, II, III, and IV.

  • Collagen One: skin, tendon, vascular, ligature, organs, bone (main component of bone)
  • Collagen Two: cartilage (main component of cartilage)
  • Collagen Three: reticulate (main component of reticular fibers), commonly found alongside type I.
  • Collagen Four: forms bases of cell basement membrane
  • Collagen Five: Cells surfaces, hair and placenta

Collagen-related diseases most commonly arise from genetic defects or nutritional deficiencies that affect the biosynthesis, assembly, postranslational modification, secretion, or other processes involved in normal collagen production.

Type Notes Gene(s) Disorders
I This is the most abundant collagen of the human body. It is present in scar tissue, the end product when tissue heals by repair. It is found in tendons, skin, artery walls, the endomysium of myofibrils, fibrocartilage, and the organic part of bones and teeth. COL1A1, COL1A2 osteogenesis imperfecta, Ehlers-Danlos Syndrome, Infantile cortical hyperostosis aka Caffey's disease
II Hyaline cartilage, makes up 50% of all cartilage protein. Vitreous humour of the eye. COL2A1 Collagenopathy, types II and XI
III This is the collagen of granulation tissue, and is produced quickly by young fibroblasts before the tougher type I collagen is synthesized. Reticular fiber. Also found in artery walls, skin, intestines and the uterus COL3A1 Ehlers-Danlos Syndrome
IV basal lamina; eye lens. Also serves as part of the filtration system in capillaries and the glomeruli of nephron in the kidney. COL4A1, COL4A2, COL4A3, COL4A4, COL4A5, COL4A6 Alport syndrome, Goodpasture's syndrome
V most interstitial tissue, assoc. with type I, associated with placenta COL5A1, COL5A2, COL5A3 Ehlers-Danlos syndrome (Classical)
VI most interstitial tissue, assoc. with type I COL6A1, COL6A2, COL6A3 Ulrich myopathy and Bethlem myopathy
VII forms anchoring fibrils in dermal epidermal junctions COL7A1 epidermolysis bullosa dystrophica
VIII some endothelial cells COL8A1, COL8A2 Posterior polymorphous corneal dystrophy 2
IX FACIT collagen, cartilage, assoc. with type II and XI fibrils COL9A1, COL9A2, COL9A3 - EDM2 and EDM3
X hypertrophic and mineralizing cartilage COL10A1 Schmid metaphyseal dysplasia
XI cartilage COL11A1, COL11A2 Collagenopathy, types II and XI
XII FACIT collagen, interacts with type I containing fibrils, decorin and glycosaminoglycans COL12A1 -
XIII transmembrane collagen, interacts with integrin a1b1, fibronectin and components of basement membranes like nidogen and perlecan. COL13A1 -
XIV FACIT collagen COL14A1 -
XV - COL15A1 -
XVI - COL16A1 -
XVII transmembrane collagen, also known as BP180, a 180 kDa protein COL17A1 Bullous pemphigoid and certain forms of junctional epidermolysis bullosa
XVIII source of endostatin COL18A1 -
XIX FACIT collagen COL19A1 -
XX - COL20A1 -
XXI FACIT collagen COL21A1 -
XXII - COL22A1 -
XXIII MACIT collagen - COL23A1 -
XXIV - COL24A1 -
XXV - COL25A1 -
XXIX epidermal collagen COL29A1 Atopic dermatitis[25]

In addition to the above mentioned disorders, excessive deposition of collagen occurs in scleroderma.


In histology, collagen is brightly eosinophilic (pink) in standard H&E slides. The dye methyl violet may be used to stain the collagen in tissue samples.

The dye methyl blue can also be used to stain collagen and immunohistochemical stains are available if required.

The best stain for use in differentiating collagen from other fibers is Masson's trichrome stain.


Action of lysyl oxidase (in French)

Amino acids

Collagen has an unusual amino acid composition and sequence:

  • Glycine (Gly) is found at almost every third residue
  • Proline (Pro) makes up about 9% of collagen
  • Collagen contains two uncommon derivative amino acids not directly inserted during translation. These amino acids are found at specific locations relative to glycine and are modified post-translationally by different enzymes, both of which require vitamin C as a cofactor.

Cortisol stimulates degradation of amino acid from skin collagen.[26]

Collagen I formation

Most collagen forms in a similar manner, but the following process is typical for type I:

  1. Inside the cell
    1. Three peptide chains are formed (2 alpha-1 and 1 alpha-2 chain) in ribosomes along the rough endoplasmic reticulum (RER). These peptide chains (known as preprocollagen) have registration peptides on each end; and a signal peptide is also attached to each
    2. Peptide chains are sent into the lumen of the RER
    3. Signal Peptides are cleaved inside the RER and the chains are now known as procollagen
    4. Hydroxylation of lysine and proline amino acids occurs inside the lumen. This process is dependent on ascorbic acid (Vitamin C) as a cofactor
    5. Glycosylation of specific hydroxylated amino acid occurs
    6. Triple helical structure is formed inside the RER
    7. Procollagen is shipped to the golgi apparatus, where it is packaged and secreted by exocytosis
  2. Outside the cell
    1. Registration peptides are cleaved and tropocollagen is formed by procollagen peptidase.
    2. Multiple tropocollagen molecules form collagen fibrils, via covalent cross-linking by lysyl oxidase which links hydroxylysine and lysine residues. Multiple collagen fibrils form into collagen fibers
    3. Collagen is attached to cell membranes via several types of protein, including fibronectin and integrin.

Synthetic pathogenesis

Vitamin C deficiency causes scurvy, a serious and painful disease in which defective collagen prevents the formation of strong connective tissue. Gums deteriorate and bleed, with loss of teeth; skin discolors, and wounds do not heal. Prior to the eighteenth century, this condition was notorious among long duration military, particularly naval, expeditions during which participants were deprived of foods containing Vitamin C.

An autoimmune disease such as lupus erythematosus or rheumatoid arthritis[27] may attack healthy collagen fibers.

Many bacteria and viruses have virulence factors which destroy collagen or interfere with its production.


Collagen is one of the long, fibrous structural proteins whose functions are quite different from those of globular proteins such as enzymes. Tough bundles of collagen called collagen fibers are a major component of the extracellular matrix that supports most tissues and gives cells structure from the outside, but collagen is also found inside certain cells. Collagen has great tensile strength, and is the main component of fascia, cartilage, ligaments, tendons, bone and skin.[28][29] Along with soft keratin, it is responsible for skin strength and elasticity, and its degradation leads to wrinkles that accompany ageing. It strengthens blood vessels and plays a role in tissue development. It is present in the cornea and lens of the eye in crystalline form. It is also used in cosmetic surgery and burns surgery. Hydrolyzed collagen can play an important role in weight management, as a protein, it can be advantageously used for its satiating power.[citation needed]

Industrial uses

If collagen is sufficiently denatured, e.g. by heating, the three tropocollagen strands separate partially or completely into globular domains, containing a different secondary structure to the normal collagen polyproline II (PPII), e.g. random coils. This process describes the formation of gelatin, which is used in many foods, including flavored gelatin desserts. Besides food, gelatin has been used in pharmaceutical, cosmetic, and photography industries.[30] From a nutritional point of view, collagen and gelatin are a poor-quality sole source of protein since they do not contain all the essential amino acids in the proportions that the human body requires—they are not 'complete proteins' (as defined by food science, not that they are partially structured). Manufacturers of collagen-based dietary supplements claim that their products can improve skin and fingernail quality as well as joint health. However, mainstream scientific research has not shown strong evidence to support these claims.[citation needed] Individuals with problems in these areas are more likely to be suffering from some other underlying condition (such as normal aging, dry skin, arthritis etc.) rather than just a protein deficiency.

From the Greek for glue, kolla, the word collagen means "glue producer" and refers to the early process of boiling the skin and sinews of horses and other animals to obtain glue. Collagen adhesive was used by Egyptians about 4,000 years ago, and Native Americans used it in bows about 1,500 years ago. The oldest glue in the world, carbon-dated as more than 8,000 years old, was found to be collagen—used as a protective lining on rope baskets and embroidered fabrics, and to hold utensils together; also in crisscross decorations on human skulls.[31] Collagen normally converts to gelatin, but survived due to the dry conditions. Animal glues are thermoplastic, softening again upon reheating, and so they are still used in making musical instruments such as fine violins and guitars, which may have to be reopened for repairs—an application incompatible with tough, synthetic plastic adhesives, which are permanent. Animal sinews and skins, including leather, have been used to make useful articles for millennia.

Gelatin-resorcinol-formaldehyde glue (and with formaldehyde replaced by less-toxic pentanedial and ethanedial) has been used to repair experimental incisions in rabbit lungs.[32]

Medical uses

The cardiac valve rings, the central body and the cardiac skeleton of the heart summarily represent a unique and moving collagen anchor to the fluid mechanics of the heart. Individual valvular leaflets are arguably held in shape by collagen under great extremes of pressure. Calcium deposition within collagen occurs as a natural consequence of aging. These fixed points in an otherwise static display of blood and muscle enable current cardiac imaging technology to arrive at ratios essentially stating blood in cardiac input and blood out cardiac output. Specified imaging such as calcium scoring illustrates the utility of this methodology, especially in an aging patient subject to pathology of the collagen underpinning.

Collagen has been widely used in cosmetic surgery, as a healing aid for burn patients for reconstruction of bone and a wide variety of dental, orthopedic and surgical purposes. Some points of interest are:

  1. when used cosmetically, there is a chance of allergic reactions causing prolonged redness; however, this can be virtually eliminated by simple and inconspicuous patch testing prior to cosmetic use, and
  2. most medical collagen is derived from young beef cattle (bovine) from certified BSE (Bovine spongiform encephalopathy) free animals. Most manufacturers use donor animals from either "closed herds", or from countries which have never had a reported case of BSE such as Australia, Brazil and New Zealand.
  3. porcine (pig) tissue is also widely used for producing collagen sheet for a variety of surgical purposes.
  4. alternatives using the patient's own fat, hyaluronic acid or polyacrylamide gel are readily available.

Collagens are widely employed in the construction of artificial skin substitutes used in the management of severe burns. These collagens may be derived from bovine, equine or porcine, and even human, sources and are sometimes used in combination with silicones, glycosaminoglycans, fibroblasts, growth factors and other substances.

Collagen is also sold commercially as a joint mobility supplement[33]. Because proteins are broken down into amino acids before absorption, there is no reason for orally ingested collagen to affect connective tissue in the body, except through the effect of individual amino acid supplementation.

Recently an alternative to animal-derived collagen has become available. Although expensive, this human collagen, derived from donor cadavers, placentas and aborted fetuses, may minimize the possibility of immune reactions.

Although it cannot be absorbed through the skin, collagen is now being used as a main ingredient for some cosmetic makeup.[34]

Fossil record

Because the synthesis of collagen requires a high level of atmospheric oxygen, complex animals may not have been able to evolve until the atmosphere was oxygenic enough for collagen synthesis.[35] The origin of collagen may have allowed cuticle, shell and muscle formation. However, the preservation of collagen in the fossil record is very scarce.[36] There is mounting evidence—which remains controversial—that collagen has been preserved in dinosaur specimens dated as long ago as 80 million years ago.[37]

Also worth of noting are the actinofibrils, collagen fibers present on the wings of pterosaurs.


Julian Voss-Andreae's sculpture Unravelling Collagen (2005), stainless steel, height 11'3" (3.40 m).

Julian Voss-Andreae has created sculptures out of bamboo and stainless steel based on the collagen structure. His piece Unravelling Collagen is, according to him, a "metaphor for aging and growth".[38][39]

See also


  1. ^ Muller, Werner E.G. The Origin of Metazoan Complexity: Porifera as Integrated Animals. Integrated Computational Biology, 43:3–10, 2003.
  2. ^ Gloria A. Di LulloDagger , Shawn M. Sweeney, Jarmo Körkkö, Leena Ala-Kokko, and James D. San Antonio; Mapping the Ligand-binding Sites and Disease-associated Mutations on the Most Abundant Protein in the Human, Type I Collagen; J. Biol. Chem., Vol. 277, Issue 6, 4223-4231, February 8, 2002
  3. ^ Sikorski, Zdzisław E. (2001) Chemical and Functional Properties of Food Proteins. CRC Press. p. 242
  4. ^ Wyckoff, R., R. Corey, and J. Biscoe, X-ray reflections of long spacing from tendon. Science, 1935. 82: p. 175–176.
  5. ^ Clark, G., Parker, E., Schaad, J. and Warren, W.J, New measurements of previously unknown large interplanar spacings in natural materials. J. Amer. Chem. Soc, 1935. 57: p. 1509–1509.
  6. ^ GNR — A Tribute - Resonance - October 2001
  7. ^
  8. ^ G.N. Ramachandran - Nature Structural & Molecular Biology
  9. ^ Fraser, R.D., T.P. MacRae, and E. Suzuki, Chain conformation in the collagen molecule. J Mol Biol, 1979. 129(3): p. 463–81
  10. ^ Okuyama, K., et al., Crystal and molecular structure of a collagen-like polypeptide (Pro-Pro-Gly)10. J Mol Biol, 1981. 152(2): p. 427–43.
  11. ^ Traub, W., A. Yonath, and D.M. Segal, On the molecular structure of collagen. Nature, 1969. 221(5184): p. 914–7.
  12. ^ Bella, J., M. Eaton, B. Brodsky, and H.M. Berman, Crystal and molecular structure of a collagen-like peptide at 1.9 A resolution. Science, 1994. 266(5182): p. 75–81.
  13. ^ Hulmes, D.J. and A. Miller, Quasi-hexagonal molecular packing in collagen fibrils. Nature, 1979. 282(5741): p. 878–80.
  14. ^ Jesior, J.C., A. Miller, and C. Berthet-Colominas, Crystalline three-dimensional packing is general characteristic of type I collagen fibrils. FEBS Lett, 1980. 113(2): p. 238–40.
  15. ^ Fraser, R.D.B. and T.P. MacRae, Unit cell and molecular connectivity in tendon collagen. Int. J. Biol. Macromol., 1981. 3: p. 193–200.
  16. ^ Fraser, R.D., T.P. MacRae, and A. Miller, Molecular packing in type I collagen fibrils. J Mol Biol, 1987. 193(1): p. 115–25.
  17. ^ Wess, T.J., et al., Molecular packing of type I collagen in tendon. J Mol Biol, 1998. 275(2): p. 255–67.
  18. ^ a b c Orgel, J.P., et al., "Microfibrillar structure of type I collagen in situ". Proc Natl Acad Sci U S A, 2006. 103(24): p. 9001–5.
  19. ^ Hulmes, D.J., Building collagen molecules, fibrils, and suprafibrillar structures. J Struct Biol, 2002. 137(1-2): p. 2–10
  20. ^ a b Hulmes, D.J., The collagen superfamily—diverse structures and assemblies. Essays Biochem, 1992. 27: p. 49–67.
  21. ^ Hulmes, D.J. and A. Miller, Quasi-hexagonal molecular packing in collagen fibrils. Nature, 1979. 282(5741): p. 878-80.
  22. ^ Perumal, S., O. Antipova, and J.P. Orgel, Collagen fibril architecture, domain organization, and triple-helical conformation govern its proteolysis. Proc Natl Acad Sci U S A, 2008. 105(8): p. 2824–9.
  23. ^ Sweeney, S.M., et al., Candidate cell and matrix interaction domains on the collagen fibril, the predominant protein of vertebrates. J Biol Chem, 2008. 283(30): p. 21187–97.
  24. ^ Twardowski, T., et al., Type I collagen and collagen mimetics as angiogenesis promoting superpolymers. Curr Pharm Des, 2007. 13(35): p. 3608–21.
  25. ^ Söderhäll C, Marenholz I, Kerscher T, Rüschendorf F, Esparza-Gordillo J, et al., Variants in a Novel Epidermal Collagen Gene (COL29A1) Are Associated with Atopic Dermatitis. PLoS Biology Vol. 5, No. 9, e242 doi:10.1371/journal.pbio.0050242
  26. ^ Houck, J.C.; Sharma, V.K.; Patel, Y.M.; Gladner, J.A. (1968) “Induction of Collagenolytic and Proteolytic Activities by AntiInflammatory Drugs in the Skin and Fibroblasts”. Biochemical Pharmacology 17: 2081,
  27. ^ AJR article about lupus and other collagen disorders
  28. ^ Fratzl, P. Collagen: Structure and Mechanics. Springer, New York, 2008
  29. ^ Buehler, M.J., "Nature designs tough collagen: Explaining the nanostructure of collagen fibrils". Proc Natl Acad Sci U S A, 2006. 103(33): p. 12285–90.
  30. ^ Gelatin's Advantages: Health, Nutrition and Safety
  31. ^ Oldest Glue Discovered
  32. ^ Ann Thorac Surg. 1994 Jun; 57(6): 1622–7
  33. ^ Hydrolyzed Collagen pills usages
  34. ^
  35. ^
  36. ^ We, K. M. T. (1996). "Fossil preservation in the Burgess Shale". Lethaia 29: 107–108. doi:10.1111/j.1502-3931.1996.tb01844.x.  edit
  37. ^ Schweitzer, H.; Zheng, W.; Organ, L.; Avci, R.; Suo, Z.; Freimark, M.; Lebleu, S.; Duncan, B. et al. (May 2009). "Biomolecular Characterization and Protein Sequences of the Campanian Hadrosaur B. Canadensis". Science 324 (5927): 626. doi:10.1126/science.1165069. ISSN 0036-8075. PMID 19407199.  edit
  38. ^ Ward, Barbara (April 2006). "'Unraveling Collagen' structure to be installed in Orange Memorial Park Sculpture Garden". Expert Rev. Proteomics 3 (2): 174. doi:10.1586/14789450.3.2.169. 
  39. ^ Interview with J. Voss-Andreae "Seeing Below the Surface" in Seed Magazine

External links


Up to date as of January 15, 2010

Definition from Wiktionary, a free dictionary

See also collagen




  1. Alternative spelling of Kollagen.
  2. Plural form of Collage.

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