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collagen

(kŏl'ə-jən) pronunciation

The fibrous protein constituent of bone, cartilage, tendon, and other connective tissue. It is converted into gelatin by boiling.

[Greek kolla, glue + –GEN.]

collagenic col'la·gen'ic (-jĕn'ĭk) or col·lag'e·nous (kə-lăj'ə-nəs) adj.

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Sci-Tech Encyclopedia: Collagen

The major fibrous protein in animals, present in all types of multicellular animals and probably the most abundant animal protein in nature. It is estimated that collagen accounts for about 30% of the total human body protein. Collagen is located in the extracellular matrix of connective tissues. It is part of the interacting network of proteoglycans and proteins that provides a structural framework for both soft and calcified connective tissues. By self-associating into fibrils and by binding to proteoglycans and other matrix components, collagen contributes to tissue integrity and mechanical properties. Collagen interacts with cells through the integrin cell receptors and mediates cellular adhesion and migration. Important roles for collagen have been identified in development, wound healing, platelet aggregation, and aging. Its commercial importance in leather and the production of gelatin and glue have long been recognized. More recently, it is being used as a basis for biomaterials. Examples of its biomedical applications include injectable collagen to lessen facial wrinkles and defects; surgical collagen sponges to increase blood clotting; and artificial skin for the treatment of burns. See also Gelatin.

The classification of an extracellular matrix protein as a collagen is based on the presence of a domain with a distinctive triple-helical conformation. The collagen triple helix consists of three polypeptide chains supercoiled about a common axis and linked by hydrogen bonds. At least 19 distinct molecules have been classified as collagens, and specific types are associated with particular tissues. The most prevalent and well-studied collagens belong to the fibril-forming or interstitial collagen family. The molecules in a fibril are covalently cross-linked by an enzymatic mechanism to strengthen and stabilize them. Inhibition of the enzyme involved in cross-linking results in a dramatic decrease in the tensile strength of tissues, a condition known as lathyrism.

Type I is the most common fibril-forming collagen. Its fibrils make up the mineralized matrix in bone, the strong parallel bundles of fibers in tendon, and the plywoodlike alternating layers in the transparent cornea. Type II is the major fibril-forming collagen in cartilage, while type III is found in blood vessels and skin, together with type I. Basement membranes, which serve to separate cell layers and act as filtration barriers, contain a distinctive group of collagens, denoted as type IV collagens, which are organized into a network or meshlike sheet structure. In the kidney glomerulus, the network based on type IV collagen acts as a filter to determine which molecules will pass from the blood into the urine. See also Bone; Connective tissue; Fibrous protein.

An orderly breakdown of collagen is necessary during development and tissue remodeling. For instance, following childbirth, the uterus reduces in size, which involves a massive degradation of collagen. An abnormal increase in the degradation of cartilage collagen is seen in osteoarthritis. Collagen breakdown also appears to be essential for tumor metastases. A number of hereditary diseases have been shown to be due to mutations in specific collagen genes. Osteogenesis imperfecta (brittle bone) disease is characterized by fragile bones and is due to mutations in type I collagen. Some cartilage disorders are caused by mutations in type II collagen. Ruptured arteries are found in Ehlers-Danlos syndrome type IV, which arises from mutations in type III collagen.

World of the Body: collagen

The word collagen means ‘glue-producing’. Collagen in the body does indeed help to hold it all together, but the notion of glue is not very apt. It strengthens and connects things with a network of tough fibres, rather than sticking them to each other — more like a cat's cradle than an adhesive. Collagen is the protein which forms the ubiquitous white fibres in all the connective tissues of the body, including bone, teeth, cartilage, and tendons; the skin; and all the sheaths, partitions, and supporting frameworks which abound in all organs and tissues. The exception is the central nervous system, which has its own different variety of internal supporting tissue — the glia — though there is collagen in the membranes which cover the brain and spinal cord.

Collagen is one of the ‘structural proteins’ (the other widespread one is elastin), which provide support to the tissues. By crude analogy with string, the principal mechanical property of collagen is its ability to resist distending force (tensile strength), which is vastly greater than its ability to resist compression or twisting (compression and torsion strengths). The tensile strength of collagen is so high as to be comparable, weight for weight, with that of steel. Elastin, by contrast, has a low tensile strength but the important mechanical properties of distensibility and resilience: the capability for relatively long range stretching under load and for returning to the original dimensions when the distending force is removed. Collagen can stretch only by about 2% without damage.

Collagen and elastin fibres often co-exist, notably in tissues which regularly undergo considerable changes in shape, such as skin, lungs, and blood vessels. The essentially inextensible, high tensile strength collagen is able to exist and function alongside the elastic fibres simply by having considerable slack. This can easily be illustrated if you pinch up the skin on the back of the hand: it returns to its original shape on release by virtue of the elastic fibres (a property progressively impaired in old age due to degeneration of the elastic fibres, with consequent increase in skin wrinkling). Now with the fingertips push the same skin on the back of the hand sideways and note that it slides quite freely until displacement comes to a distinct halt (when the collagen has used its slack and the tough fibres are pulled into alignment, resisting the distending force).

Collagen is synthesized by fibroblasts, the living cells present in all connective tissue, so named because they generate fibres — of collagen. There are in fact several types, with minor variations of molecular structure. Like all proteins, collagens are constructed from amino acid units; they are all glycoproteins, meaning that glucose and other simple sugars are attached to the amino acid chains. Each long, thin molecule consists of three chains of over 1000 units; each chain is helical, and the three in turn form a triple helix. A molecule is about 300 nm long — over 3000 end-to-end would measure 1 mm — but in fully-formed collagen they overlap lengthwise, and are also linked side to side, providing longer, wider, and very tough fibres. Again like all proteins in the body, collagen has a finite life span after which it is degraded to the constituent amino acids and replaced by new fibres. The synthesis within the fibroblasts is a complex process; the three chains are separately assembled, and then wound into the triple helix, which is extruded. Once outside the cell, the molecules aggregate and forge links as described.

The complexity of collagen synthesis involves multiple enzymes, so that a congenital deficiency of any of these can lead to some disorder of its formation. This accounts for there being a wide variety of clinical syndromes associated with such disorders: there can be fragile bones, with fractures from minimal trauma; fragile blood vessels with widespread bruising; dental defects; readily dislocating joints; a bent or twisted spine; thin, hyperelastic skin; and poor wound healing. Apart from these inborn defects, deprivation of ascorbic acid (vitamin C) at any time of life interferes with a step in collagen synthesis; the resulting bleeding, bruising, and poor healing are part of the picture of scurvy.

With ageing, habitually exposed areas of skin in white-skinned people show broken and disordered collagen fibres, related to the effects of UV light. Deficient replacement of collagen also contributes to thinning and wrinkling of the skin, and, together with mineral loss, to osteoporosis — decreasing bone mass.

These changes suggest that the continuous production of new fibroblasts, and by them of new collagen, progressively declines. Fibroblasts in culture outside the body divide again and again, but do not continue to replicate indefinitely. When such cultures from different animal species are compared, it is found that the number of cell divisions is related to the lifespan of each species, and is also related inversely to the age of the donor from any one species: a finding of considerable interest in the study of the ageing process.

— Hugh Elder, Sheila Jennett

Uses

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 teeth. Along with soft keratin, it is responsible for skin strength and elasticity, and its degradation leads to wrinkles that accompany aging. 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.

Industrial uses

If collagen is partially hydrolyzed, the three tropocollagen strands separate into globular, random coils, producing gelatin, which is used in many foods, including flavored gelatin desserts. Besides food, gelatin has been used in pharmaceutical, cosmetic, and photography industries.^[2] Nutritionally, collagen and gelatin are poor quality protein since they do not contain all the essential amino acids that the human body requires - they are not complete proteins. 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 any evidence to support these claims. Individuals with problems in these areas are more likely to be suffering from some other underlying condition rather than 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.^[3] 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.^[4]

Medical uses

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:

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
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 and New Zealand.
porcine (pig) tissue is also widely used for producing collagen sheet for a variety of surgical purposes.
due to the care in donor animal breeding and selection, as well as the technology used in the preparation of collagen from animal sources, the chance of immune reactions or disease transmission has been virtually eliminated.^{[citation needed]}
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, as well as for a wide range of dental, orthopedic, and surgical purposes. 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. This lacks supportive research as the proteins would just be broken down into its base amino acids during digestion, and could go to a variety of places besides the joints depending upon need and DNA orders.

Recently an alternative to animal-derived collagen has become available. Although expensive, this human collagen, derived from donor cadavers, placentas and aborted fetuses,^[5] may minimize the possibility of immune reactions.

Composition and structure

The tropocollagen or "collagen molecule" subunit is a rod about 300 nm long and 1.5 nm in diameter, made up of three polypeptide strands, each of which is a left-handed helix, not to be confused with the commonly occurring alpha helix, which is right-handed. These three left-handed helices are twisted together into a right-handed coiled coil, a triple helix, a cooperative quaternary structure stabilized by numerous hydrogen bonds. Tropocollagen subunits spontaneously self-assemble, with regularly staggered ends, into even larger arrays in the extracellular spaces of tissues. There is some covalent crosslinking within the triple helices, and a variable amount of covalent crosslinking between tropocollagen helices, to form the different types of collagen found in different mature tissues — similar to the situation found with the α-keratins in hair. Collagen's insolubility was a barrier to study until it was found that tropocollagen from young animals can be extracted because it is not yet fully crosslinked.

Collagen fibrils are collagen molecules packed into an organized overlapping bundle. Collagen fibers are bundles of fibrils.

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-X-Pro or Gly-X-Hyp, where X may be any of various other amino acid residues. Gly-Pro-Hyp occurs frequently. 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. Such high glycine and regular repetitions are never found in globular proteins. Chemically-reactive side groups are not needed in structural proteins as they are in enzymes and transport proteins. The high content of Proline and Hydroxyproline rings, with their geometrically constrained carboxyl and (secondary) amino groups, 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, 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 thermally stabilize the triple helix — Hyp even more so than Pro — and less of them is required in animals such as fish, whose body temperatures are low.

In bone, entire collagen triple helices lie in a parallel, staggered array. 40 nm gaps between the ends of the tropocollagen subunits probably serve as nucleation sites for the deposition of long, hard, fine crystals of the mineral component, which is (approximately) hydroxyapatite, Ca₅(PO₄)₃(OH), with some phosphate. It is in this way that certain kinds of cartilage turn into bone. Collagen gives bone its elasticity and contributes to fracture resistance.

Types of collagen and associated disorders

Collagen occurs in many places throughout the body. There are 28 types of collagen described in literature.

Collagen diseases commonly arise from genetic defects that affect the biosynthesis, assembly, postranslational modification, secretion, or other processes in the normal production of collagen.

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, the endomysium of myofibrils, fibrocartilage, and the organic part of bone.	COL1A1, COL1A2	osteogenesis imperfecta, Ehlers-Danlos Syndrome
II	Hyaline cartilage, makes up 50% of all cartilage protein	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, 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
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
VIII	some endothelial cells	COL8A1, COL8A2	-
IX	FACIT collagen, cartilage, assoc. with type II and XI fibrils	COL9A1, COL9A2, COL9A3	-
X	hypertrophic and mineralizing cartilage	COL10A1	-
XI	cartilage	COL11A1, COL11A2	Collagenopathy, types II and XI
XII	FACIT collagen, interacts with type I containing fibrils, decorin and glucosaminoglycans	COL12A1	-
XIII	transmembrane collagen, interacts with integrin a1b1, fibronectin and components of basment 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	-	COL23A1	-
XXIV	-	COL24A1	-
XXV	-	COL25A1	-
XXVI	-	EMID2	-
XXVII	-	COL27A1	-
XXVIII	-	COL28A1	-

Staining

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.

Collagen is birefringent when stained with Sirius red F3B (C.I. 35782). ^[6]

Synthesis

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.
- Hydroxyproline (Hyp), derived from proline.
- Hydroxylysine, derived from lysine. Depending on the type of collagen, varying numbers of hydroxylysines have disaccharides attached to them.

Collagen I formation

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

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
Outside the cell
1. Registration peptides are cleaved and tropocollagen is formed by procollagen peptidase.
2. Multiple tropocollagen molecules form collagen fibrils, and 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. In the human body, a malfunction of the immune system, called an autoimmune disease, results in an immune response in which healthy collagen fibers are systematically destroyed with inflammation of surrounding tissues. The resulting disease processes are called Lupus erythematosus, and rheumatoid arthritis, or collagen tissue disorders.^[7]

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

Collagen in art

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

Julian Voss-Andreae has created sculptures based on the collagen structure out of bamboo and stainless steel. His piece "Unraveling Collagen" is, according to the artist, a "metaphor for aging and growth"^[8]^[9].

References

^ 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
^ http://www.gmap-gelatin.com/gelatin_adv.html
^ http://www.archaeology.org/online/news/glue.html
^ Ann Thorac Surg. 1994 Jun; 57(6): 1622-7
^ http://www.lipaugmentation.com/bioimplants.htm
^ Junqueira LCU, Bignolas G, Brentani RR. Picrosirius staining plus polarization microscopy, a specific method for collagen detection in tissue sections. Histochem J 1979).
^ AJR article about lupus and other collagen disorders
^ Ward, Barbara (April 2006). "'Unraveling Collagen' structure to be installed in Orange Memorial Park Sculpture Garden". Expert Rev. Proteomics 3 (2): 174.
^ Interview with J. Voss-Andreae "Seeing Below the Surface" in Seed Magazine

Additional images

Collagen

Action of lysyl oxydase (in French)

External links

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Collagen

Histology: connective tissue
Classification	proper (loose/areolar, dense, adipose brown and white, reticular) embryonic (mucous, mesenchymal) specialized (cartilage, bone, blood)
Extracellular matrix	ground substance (tissue fluid) fibers (collagen, reticular fiber, elastic fibers)
Cells	resident (fibroblast, adipocyte, chondroblast, osteoblast), wandering cell

Protein: fibrous proteins
Collagen	Type-I (COL1A1) - Type-II (COL2A1) - Type-III - Type-IV - Type-V - Type XI (COL11A2) - Type-XVII - Type-XVIII
Keratin/Cytokeratin	type I (10, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21), type II (1, 2A, 3, 4, 5, 6A, 7, 8, 9), Hair (Type I, Type II), Beta
other	Elastin - Cytoskeletal

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Translations: Translations for: Collagen

Dansk (Danish)
n. - kollagen

Nederlands (Dutch)
collageen

Français (French)
n. - collagène

Deutsch (German)
n. - Kollagen, Knorpelleim

Ελληνική (Greek)
n. - (χημ.) κολλαγόνο

Italiano (Italian)
collagene

Português (Portuguese)
n. - colágeno (m) (Histol.)

Русский (Russian)
коллаген

Español (Spanish)
n. - colágeno

Svenska (Swedish)
n. - collagen

中文（简体） (Chinese (Simplified))
胶原质

中文（繁體） (Chinese (Traditional))
n. - 膠原質

한국어 (Korean)
n. - 교원질, 콜라겐

日本語 (Japanese)
n. - 膠原

العربيه (Arabic)
‏(الاسم) مولد الغراء في الجسم, كولاجين, مهلمن‏

עברית (Hebrew)
n. - ‮קולגן (פרוטאין)‬

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