collagen, any of a group of proteins found in skin, ligaments, tendons, bone and cartilage, and other connective tissue. Cells called fibroblasts form the various fibers in connective tissue in the body. The fibroblasts produce three types of fibers to form the ground substance: collagen, elatin, and the reticulum. Collagen consists of groups of white inelastic fibers with great tensile strength. These fibers include fine fibrils, which are composed of even finer filaments, visible only through the electron microscope. Collagen protein contains an unusually high percentage of the amino acids proline and hydroxyproline. X-ray diffraction studies provide evidence that the protein forms a wavy band, a coiled chain with periodic, i.e., repeating, arrangement of its amino acids. Cartilage is composed of fibrous collagen in an amorphous gel. The organic (nonmineral) content of bone is made up largely of collagen fibers with calcium salt crystals lying adjacent to each segment of the fiber; the fibers and salt crystals combined form a structure with compressional and tensile strength comparable to that of reinforced concrete. A group of diseases, often termed collagen, or connective tissue, diseases, involve a variety of alterations in the connective tissue fibers; rheumatoid arthritis, rheumatic fever, lupus, and scleroderma are included in this group. Some of these diseases may involve an autoimmune response, in which the immune mechanism injures or destroys the individual's own tissues (see immunity). Collagen dissolved in boiling water becomes denatured to form gelatin.

Any of a class of organic compounds, the most abundant proteins in the animal kingdom, occurring widely in tendons, ligaments, dentin (see tooth), cartilage, and other connective tissues. Their molecules share a triple-helix configuration. Collagens occur as whitish, inelastic fibres of great tensile strength and low solubility in water. Soluble when first synthesized (the form used in personal-care preparations), collagen changes to a more stable, insoluble form. Glue made from collagen in animal hides and skins is a widely used adhesive. Specially treated forms of collagen are used in medicine and surgery (including lip implants and other cosmetic surgery), in prostheses, and as sausage casings. Collagen is converted to gelatin by boiling it in water.

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Collagen is the main protein of connective tissue in animals and the most abundant protein in mammals, making up about 50% of the whole-body protein content.

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 skin. 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. Hydrolyzed collagen can play an important role in weight management. As a protein, it can be advantageously used for its satiating power.

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 flavoured gelatin desserts. Besides food, gelatin has been used in pharmaceutical, cosmetic, and photography industries. 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. 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.

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:

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. 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.
5. 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. 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, may minimize the possibility of immune reactions.

Collagen is now being used as a main ingredient for some cosmetic makeup.

Conformation and structure

Collagen structure is complex. Its conformation can be considered at the monomeric level (individual) collagen molecules and/or at its aggregate level, how the monomers are arranged i.e. their packing structure (fibrils, networks, etc. - see table below).

History and Background

The molecular and packing structures of collagen have eluded scientists for decades; the first evidence that it possess a regular structure at the molecular level was presented in the mid-1930s . Since that time many prominent scholars, including (but not limited to) Nobel laureate Crick, and Pauling, Rich, Yonath, Brodsky, Berman and Ramachandran concentrated on the conformation of the collagen monomoer. 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 molecules quaternary structure 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’ or microfibrillar . Recently it was confirmed that the microfibrillar structure as described by Fraser, Miller, Wess (amongst others) was closet to the observed structure, although it over-simplified the topological progression of neighboring collagen molecules and hence did not predict the correct confirmation of the discontinuous D-periodic pentameric arrangement termed simply: the microfibril .

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 peptides), 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 or proline, 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 then most warm-blooded animals.

Fibrillar Structure

The tropocollagen subunits spontaneously self-assemble, with regularly staggered ends, into even larger arrays in the extracellular spaces of tissues. In the fibrillar collagens, the molecules are staggered from each other by about 67nm (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 5 molecules in cross-section – called the “overlap” and a part containing only 4 molecules. The triple-helices are also arranged in a hexagonal or quasi-hexagonal array in cross-section, in both the gap and overlap regions.

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). 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. 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.

Collagen fibrils are collagen molecules packed into an organized overlapping bundle. 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 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)₂with 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. There are more then 28 types of collagen described in literature. 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 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, skin, artery walls, the endomysium of myofibrils, fibrocartilage, and the organic part of bones and teeth.	COL1A1, COL1A2	osteogenesis imperfecta, Ehlers-Danlos Syndrome
II	Hyaline cartilage, makes up 50% of all cartilage protein. Vitreous humour of the eye. Fibrocartilage.	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
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	- EDM2 and EDM3
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 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	-
XXVI	-	EMID2	-
XXVII	-	COL27A1	-
XXVIII	-	COL28A1	-
XXIX	epidermal collagen	COL29A1	Atopic Dermatitis

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

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.

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:

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, 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.

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

Art

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.

See also

• Collagenase, the enzyme involved in collagen breakdown and remodelling.
• Osteoid
• Fibrous protein
• Ehlers-Danlos Syndrome
• Marfan Syndrome

References

Additional images

External links

• The Collagen Protein
• Hydrolyzed Collagen (Gelatin)
• 12 types of collagen
• Database of type I and type III collagen mutations
• Science.dirbix Collagen
• Computer-generated animations of the assembly of Type I and Type IV Collagens