{
  "id": "collagen",
  "title": "Collagen",
  "lang": "en",
  "source_lang": "en",
  "summary": "Collagen is the most abundant structural protein in animals, forming triple helices that give tissues tensile strength.",
  "category": "science",
  "subcategory": "biology",
  "type": "concept",
  "aliases": [
    "Collagens",
    "collagen protein"
  ],
  "generated_at": "2026-10-08",
  "model": "pro/claude-haiku-5-5",
  "url": "https://aiwiki.page/en/collagen/",
  "translations": {
    "zh-Hans": {
      "url": "https://aiwiki.page/zh/collagen/",
      "json": "https://aiwiki.page/zh/collagen.json"
    }
  },
  "sections": [
    {
      "heading": "",
      "text": "Collagen is a family of fibrous proteins and the most abundant protein in mammals. It is the main structural component of the extracellular matrix, where it gives skin, bone, tendon, cartilage, and blood vessel walls their tensile strength and shape. Its defining feature is a triple helix built from three chains in which every third residue is a glycine, so the sequence repeats as Gly-X-Y, with X and Y often being amino acids proline and hydroxyproline. Collagen is one of the few proteins whose modified residues depend on vitamin C for their formation, which links its chemistry directly to nutrition and to disease when that supply fails.\n\nVertebrates have at least 28 distinct collagen types, encoded by dozens of genes and differing in chain composition, supramolecular architecture, and tissue distribution. The fibrillar types form the strong, cross-linked fibers of bone and tendon, while network-forming types such as type IV make up basement membranes. Understanding collagen therefore requires connecting molecular structure, the biosynthetic pathway that modifies and assembles the chains, the hierarchical organization into fibrils, and the way cells remodel the matrix throughout life.\n\nThis entry first describes the triple helix and the post-translational steps that produce it, then explains how molecules assemble into fibrils and how the types are classified. It goes on to cover tissue functions, turnover and degradation, inherited and acquired disorders, the history of structural research, practical applications in medicine and industry, and the questions that remain open."
    },
    {
      "heading": "Definition and distribution",
      "text": "Collagen is a protein family defined by a structural feature rather than by shared sequence. The classification of an extracellular matrix protein as a collagen is based on the presence of a domain with a distinctive triple-helical conformation. In that domain, three polypeptide chains twist around one another and are held together by hydrogen bonds. Because membership depends on this domain, the boundary of the family is not fixed. Counts differ with the definition used. One account describes the vertebrate superfamily as having 28 members in vertebrates, each comprising three of 46 α-chains, while another counts only the types that form continuous triple helices, noting that 33 collagen gene products are expressed in a tissue-specific manner, leading to 19 triple-helical collagens. Proteins with interrupted helices, and short non-helical segments, are therefore included or excluded depending on the author.\n\nCollagen is one of the most abundant proteins in animals. Estimates of its share of total body protein vary: one review gives 25–35% of the total body protein, while a patent-literature summary gives about 30%. It occurs in nearly every organ, including skin and other epithelial tissues, tendons, bone, blood vessels, cartilage, ligaments and teeth. Tissue-specific proportions differ sharply. In humans and most other animals, type I collagen forms more than 90% of the organic mass of bone, and it is also the main collagen of skin, tendon and cornea. In the cornea, type I fibrils are arranged in plywood-like alternating layers within the transparent tissue. In skin, collagen constitutes about three-quarters of the dry weight.\n\nName, origin and early study\n\nThe name derives from the Greek κόλλα (kólla), meaning \"glue\", and suffix -γέν, -gen, denoting \"producing\". English dictionaries record the term first recorded in 1860–65, but the substance itself had been used for millennia. Boiling skin and sinews of animals yielded an adhesive, and collagen adhesive was used by Egyptians about 4,000 years ago. A carbon-dated collagen lining on rope baskets is more than 8,000 years old, according to one reported find.\n\nEarly chemical work focused largely on gelatin rather than on native collagen. Gelatin is collagen that has been irreversibly hydrolyzed using heat, basic solutions or weak acids. Native collagen is far less convenient to work with: its fibers are insoluble, and they form insoluble fibers possessing high tensile strength. Soluble gelatin, by contrast, was the practical material that could be extracted, purified and measured in the laboratory, and it had commercial value. Its applications later extended to food, pharmaceutical, cosmetic, and photography industries. The structural questions about the intact triple helix were therefore addressed only later, once X-ray and chemical methods were applied to the fibrous protein itself.\n\nInvertebrates and other animal groups\n\nCollagen is not restricted to vertebrates. It is found in organisms ranging from sponges to humans, and fibrillar collagens are present in almost all animals, where they form the well-known striated fibrils. Invertebrate collagen superfamilies are generally smaller than vertebrate ones: their collagen gene sets contain fewer members than those of vertebrates. Nevertheless, the core architecture is conserved. A sponge fibrillar collagen gene has been characterized whose product is homologous to vertebrate and sea urchin fibrillar collagens. Likewise, the length of the major triple helix is conserved in both invertebrates and vertebrates, despite the presence of some imperfections.\n\nSome invertebrate lineages have collagen-related proteins that have no vertebrate counterpart. A family of short-chain spongin proteins is present in invertebrates (except Ecdysozoa), including nonsponge organisms but is undetectable in vertebrates. Demosponges, which lack basement membranes, carry a short-chain collagen family related to type IV collagen. Recent work has pushed the comparison further: a 2025 study reported finding mammalian-type collagens I and III within sponge spongin, which would suggest that these collagens have an ancient structural role. This claim rests on a single study and should be treated as provisional."
    },
    {
      "heading": "Molecular structure of the triple helix",
      "text": "Three chains in a superhelix\n\nEach collagen molecule is built from three polypeptide chains. Each chain adopts a left-handed polyproline II (PPII) conformation, the same extended helix seen in proline-rich peptide segments, and the three chains then wind around a common axis to form a right-handed superhelix about 1.5 nm in diameter. Because the chains are supercoiled, the triple helix is more rigid than any single chain would be. Its tight packing depends on the sequence rules described below, and the same triple-helical motif appears in many non-collagen proteins as well.\n\nThe Gly-X-Y repeat\n\nThe primary structure of each chain is a repeat of the tripeptide Gly-X-Y, in which every third residue is glycine. X and Y can be any amino acid, but they are most often proline in the X position and 4-hydroxyproline in the Y position. The reason glycine is required at every third position is geometric. Where the three chains converge near the helix axis, there is no room for any side chain larger than a hydrogen atom. Glycine, which has the smallest side chain, is the only residue that can sit at this central core. The X and Y positions lie on the outer surface and can accommodate a wider range of side chains. Sequence variation therefore affects the chemistry of the surface, while the glycine constraint preserves the architecture. When glycine is replaced by a larger residue, the local helix is destabilized and folding is delayed.\n\nProline, hydroxyproline and interchain hydrogen bonds\n\nThe pyrrolidine ring of proline restricts the backbone torsion angles, which lowers the conformational entropy of the chain and favors the PPII-like geometry needed for the helix. The ring can pucker in two ways, and in collagen the ring pucker of 4-hydroxyproline is stabilized by the 4R hydroxyl group, which acts through stereoelectronic effects and hydrogen bonding. Hydroxylation therefore contributes substantially to stability at body temperature, and the modification must occur while the chains are still single strands, because the folded triple helix is no longer a substrate for the enzyme. Recent solid-state NMR and energy-landscape modelling suggest that hydroxylation at the Y position can also alter the puckering preferences of neighbouring X-position prolines.\n\nThe three chains are held together by a network of hydrogen bonds. The amide hydrogen of each glycine forms a hydrogen bond with a carbonyl oxygen of the X residue on a neighbouring chain, which links the strands laterally. Charge-pair interactions, such as lysine–aspartate pairs, add further stabilization in some collagen types and are used in designed synthetic peptides.\n\nHelix length, composition and interruptions\n\nHelix length varies greatly among collagen types. The fibrillar types I, II, III, V and XI contain a contiguous Gly-X-Y stretch of roughly 1000 residues, which makes them the most sensitive to any break in the repeat. Many non-fibrillar collagens, such as type IV, contain numerous interruptions, which cause localized kinks or unwinding and, in general, reduce thermal stability and slow folding. Interruptions also change the pattern of salt bridges along the molecule. Folding begins at the C-terminal end, where three chains are first registered, and then propagates toward the N-terminus in a zipper-like manner.\n\nChain composition is also variable. Although the three chains of a single collagen type can be identical, heterotrimers are more common than homotrimers. Type I collagen, for example, is composed of two α1(I) chains and one α2(I) chain, while type II is generally a homotrimer of α1(II) chains. The three chains are offset from one another by a single residue, so the register of each chain relative to the others also matters for folding and assembly.\n\nAt the two ends of each triple helix, short non-helical regions called telopeptides remain after processing. Although they are small, the telopeptides are not structurally trivial. They carry the lysine residues that become covalently cross-linked during fibril formation, and experimental work suggests that they contribute catalytically to fibril assembly, whereas the information needed for molecular recognition resides largely in the triple-helical domain. Full molecular details of the telopeptide conformations remain incompletely known.\n\nFinally, the triple helix in tissues is not identical to the ideal structure seen in short synthetic peptides. Fibre X-ray diffraction of native tendon collagen indicates a looser helical symmetry than that of model peptides, which suggests that the native molecule is somewhat more flexible than the idealized structure implies."
    },
    {
      "heading": "Biosynthesis and post-translational modification",
      "text": "Collagen biosynthesis begins on ribosomes attached to the rough endoplasmic reticulum (ER), where each pro-α chain is built as a longer precursor. An N-terminal signal sequence directs the growing chain across the ER membrane, so that translocation happens during translation. Hydroxylation of proline and lysine residues within the Gly-X-Y repeat region begins as the chain passes into the ER lumen. Modifications at this stage matter because the enzymes involved act only on single-stranded chains; the responsible enzymes recognize only single-stranded substrates, so these changes precede triple-helix formation.\n\nProlyl 4-hydroxylase (P4H) is the most consequential of these enzymes. It is a tetramer with two catalytic α subunits and two β subunits that are identical to protein disulfide isomerase (PDI), so the enzyme doubles as a folding chaperone. It converts proline in X-Pro-Gly sequences into 4-hydroxyproline. Lysyl hydroxylases, encoded by the PLOD genes, carry out the parallel reaction on lysine. Both enzyme families belong to the 2-oxoglutarate-dependent dioxygenases: they require 2-oxoglutarate and oxygen as co-substrates, with Fe²⁺ and ascorbic acid (vitamin C) as cofactors. Ascorbate is consumed in the reaction and must be replenished, which is why its deficiency is so damaging. Without adequate hydroxylation, procollagen trimer formation depends on correct hydroxylation of proline residues in the triple helix. Chains that are under-hydroxylated fold poorly and are held back in the ER. Under-hydroxylated procollagen is retained in the ER and, when ascorbate is absent, the chains do not fold correctly and are not secreted. The resulting defect in collagen stability underlies scurvy, which is described in a later section.\n\nLysine hydroxylation is followed by glycosylation. Some hydroxylysine residues receive sugar units through galactosyltransferase and glucosyltransferase, producing galactosyl-hydroxylysine and glucosyl-galactosyl-hydroxylysine. These sugars are not required for triple-helix formation as such, but they affect the cross-linking pathway and the properties of the final fibrils.\n\nTriple-helix assembly is initiated at the C-terminal end of the chains. The globular C-propeptide domains of the three α chains fold separately and then associate, which selects the correct chains and fixes their register. Triple-helix formation then propagates in a zipper-like manner from the C-terminal toward the N-terminal end. Interchain disulfide bonds in the C-propeptide help stabilize this association, but they are not strictly required for chain recognition or helix nucleation; mutant chains lacking them still assemble. Human procollagen folds into its native triple-helical conformation at about 30–34 °C, but not at higher temperatures. Because body temperature is higher than this range, the folding is assisted by chaperones. The collagen-specific chaperone HSP47, a serpin family member, binds the triple helix at Gly-X-Arg motifs. Its role is not the usual one of binding unfolded chains: it preferentially stabilizes the folded helix. More than twenty HSP47 molecules are needed to stabilize a single triple helix at body temperature. HSP47 also helps load procollagen into large transport vesicles on the way to the Golgi apparatus, and deleting the HSP47 gene in mice causes embryonic lethality because collagen biosynthesis is severely disrupted.\n\nAfter secretion, the propeptides are removed. Bone morphogenetic protein 1 (BMP1) acts as the C-proteinase, and ADAMTS2 acts as the N-proteinase. These enzymes trim the propeptides off, and the resulting molecules are called tropocollagen. The tropocollagen molecules then pack into fibrils, and that step leads into the next section. The same proteolytic step also governs the cross-linking machinery. Lysyl oxidase (LOX) is secreted as a proenzyme, and procollagen C-proteinase cleaves it into the mature enzyme and the LOX propeptide. Once active, the enzyme is a copper-dependent amine oxidase. It uses molecular oxygen and copper ions to oxidize the terminal ε-amino group of a lysine residue near either end of the tropocollagen molecule. The resulting aldehyde, allysine, reacts with neighboring lysine side chains, and this reaction starts the covalent cross-linking that stabilizes fibrils. The chemistry of the mature cross-links is covered later in this article."
    },
    {
      "heading": "Hierarchical assembly into fibrils and fibers",
      "text": "Collagen assembly begins with single molecules of tropocollagen. Each is a rod roughly 300 nm long and about 1.5 nm across, and the molecules are arranged in a quarter-stagger pattern in which neighbors are shifted along their length by a fixed step of about 67 nm aligned rodlike tropocollagen molecules, ~300 nm long and ~1.5 nm in diameter, are axially offset from each other by their D-period, a multiple of ~67 nm, and pack laterally to form fibrils. The simplest model of this packing, the Hodge–Petruska scheme, treats five molecules as a repeating unit. Because the molecule length is not a whole multiple of the offset, five tropocollagen molecules staggered according to the Hodge-Petruska scheme create alternating 'overlap' regions of length 0.46D that contain five molecules and 'gap' regions of length 0.54D that contain four molecules. This alternation of gap and overlap zones is what produces the cross-striated appearance seen by electron microscopy. The five-molecule unit is not fully settled: the cross-section of the microfibril is more nearly hexagonal than pentagonal, and the theoretical foundations of these packing schemes remain unclear.\n\nThe 67 nm period is conserved across collagen types, but it is not fixed. Measured values vary with tissue and hydration state; for example, a period of 65 nm has been reported in type III-rich tissues such as dermis and cornea. Individual microfibrils assemble into fibrils that are typically 50-200 nm in diameter. Fibrils then align side by side to form bundles and, ultimately, fibers.\n\nLateral cross-linking and mechanical strength\n\nFibril stability depends on covalent cross-links, which form only after the molecules are in register. The process begins when collagen aldehydes are derived from a few specific lysine or hydroxylysine side-chains by the action of lysyl oxidase. These aldehydes then react with partner residues on neighboring molecules. The first products are divalent aldimine and oxo-imine cross-links, which can later mature into trivalent hydroxylysyl-pyridinoline and lysyl-pyridinoline cross-links. Because each trivalent cross-link ties three chains together, the network becomes much stronger than the divalent intermediates alone. The chemistry of the mature lysine-derived product is less well established, since the chemical structure(s) of the mature cross-linking amino acids on the lysine aldehyde pathway are unknown, but hydroxypyridinium residues have been identified as mature products on the hydroxylysine aldehyde route.\n\nWhich pathway operates depends on the tissue. The operating pathway is governed by whether lysine residues are hydroxylated in the telopeptide sites where aldehyde residues will later be formed by lysyl oxidase. The lysine-aldehyde route dominates in adult skin, cornea, sclera, and rat tail tendon, whereas the hydroxylysine-aldehyde route dominates in bone, cartilage, ligament, most tendons and most internal connective tissues of the body. Bone is especially complex, with a mix of trivalent pyridinolines and pyrroles and divalent ketoimine cross-links. The amount of cross-linking also tracks mechanical demand: in tissues where tensile strength is crucial, collagen is highly cross-linked. Loss of divalent cross-links weakens the material; in one study of osteoporotic vertebral bone, a reduced stability towards acetic acid and pepsin, and a substantial reduction in the concentration of the divalent collagen cross-links was observed compared with matched controls.\n\nNon-fibrillar network architectures\n\nNot all collagens form banded fibrils. Network-forming types assemble into open lattices, and their geometry differs from that of fibrillar types. Type IV collagen α chains form chicken-wire structures by end-to-end associations stabilized by lysine-derived cross-linking and interchain disulfide bonds, rather than through side-by-side staggering. In the basement membrane, type IV networks are irregular, whereas type VIII and X form hexagonal networks. Type VIII forms a hexagonal lattice in Descemet's membrane of the cornea, and this open, porous meshwork is thought to contribute compressive strength while providing an open, porous meshwork. Type VI takes a different form: its tetramers aggregate linearly to form beaded filaments, but also associate laterally through the globular domains so forming a network. These architectures show that the hierarchical, staggered assembly of fibrillar collagen is one of several strategies, not the only one."
    },
    {
      "heading": "Classification and diversity of collagen types",
      "text": "Vertebrates have a large collagen family, but the exact count depends on the source and on which proteins are included. Recent reviews list 28 vertebrate types, while other reviews give 29, and older texts give 19. Collagens are sorted by domain architecture and by how the molecules assemble outside the cell, not by what they do. This means that tissue distribution and function overlap across groups, and a type's family is set by its structure. The main groups are fibrillar, basement membrane (network-forming), fibril-associated (FACIT), beaded-filament, multiplexin, transmembrane, and short-chain collagens.\n\nFibrillar collagens\n\nTypes I, II, III, V and XI form the classical fibrils that give tissues tensile strength. Type I is the most abundant. Its molecule consists of two α1(I) chains and one α2(I) chain, and it forms fibrils in tendon, bone, skin, cornea and vessel walls. Type II is a homotrimer of α1(II) chains and forms the fibrils of cartilage. Type III is also a homotrimer, made of three identical α1(III) chains encoded by COL3A1, and it is found in skin, vessels and other soft tissues. Types V and XI are minor fibrillar collagens that do not usually form pure fibrils. Instead, they co-assemble with the major types to form heterotypic fibrils: type V with type I, and type XI with type II. As a result, skin fibrils contain both type I and type III, and cartilage fibrils contain types II and XI. Type V and XI thus set the diameter and assembly of fibrils formed from the dominant collagen. Their chains are ordinarily heterotrimeric, and they are considered part of the fibrillar group.\n\nBasement membrane and network-forming collagens\n\nType IV is the principal collagen of basement membranes. Instead of forming fibrils, it forms thin sheet-like networks, with a chicken-wire appearance in electron micrographs. Mammals express six distinct α chains, α1(IV) to α6(IV), encoded by six genes, and these chains can assemble into several heterotrimeric protomers. Three are well characterized: α1α1α2, which is the most abundant and is found in all mammalian basement membranes; α3α4α5, which is restricted largely to the kidney glomerulus, lung and brain; and α5α5α6, which is found in smooth muscle, skin and the kidney's Bowman's capsule. The minor α3–α6 protomers are present only in higher organisms. The composition of the network therefore varies between tissues, which explains why earlier studies of corneal basement membrane produced conflicting reports about the presence of type IV. The α3α4α5 assembly is central to the pathology of Alport syndrome, because defects in any one chain prevent the complex from forming. Other network-forming collagens include types VI, VIII and X. Type VI forms fine beaded microfibrils that are widely distributed, while types VIII and X belong to the short-chain group discussed below.\n\nFACIT collagens\n\nFibril-associated collagens with interrupted triple helices (FACIT) are built from short triple-helical segments separated by non-collagenous domains. They do not form fibrils themselves. Instead, they bind at regular intervals to the surface of fibrils made of fibrillar collagens and modify how fibrils interact with the surrounding matrix. Type IX, the prototypical member, is associated with type II fibrils in cartilage and stabilizes them. Type XII is found on the surface of type I fibrils, and type XIV is also associated with type I fibrils. These molecules are thought to act as molecular bridges between fibrils and other matrix components, including components of basement membranes. Other collagens with FACIT-like features, such as types XIX, XX, XXI and XXVI, are sometimes included in this group, and the boundaries of the group are not uniformly defined across sources.\n\nShort-chain, transmembrane and other specialized types\n\nShort-chain collagens, including types VIII and X, have a triple-helical region roughly half the length of that in fibrillar collagens. They form mesh-like structures, and some are found in the endothelial and mineralizing cartilage tissues. Transmembrane collagens, also called MACITs (membrane-associated collagens with interrupted triple helices), include types XIII, XVII, XXIII and XXV. They are type II membrane proteins with a short cytoplasmic N-terminal domain, a transmembrane segment, and an extracellular collagenous domain. Type XVII is a structural component of hemidesmosomes, which anchor epidermal cells to the basement membrane, and it is also the target of the autoantibodies in bullous pemphigoid. Type XIII and XVII are found in other tissues as well, reflecting the fact that their functions are not restricted to one location. The MACIT subgroup is evolutionarily ancient: invertebrates typically encode a single MACIT, which the vertebrate subgroup later expanded through genome duplications. Multiplexin collagens, such as types XV and XVIII, have multiple triple-helical domains and interruptions and are placed in a separate category. Types that have collagen-like sequences but do not fit these families are often excluded from the collagen family altogether.\n\nGrouping by domain structure has practical limits. Some types, such as type IV, cannot be neatly assigned to one function, and the same molecule can serve as a structural element in one tissue and a signaling or adhesion molecule in another. For this reason, classification schemes are revised as new proteins are identified and as assembly data accumulate."
    },
    {
      "heading": "Tissue distribution and biological functions",
      "text": "Tendon, ligament and bone\n\nIn tendons and ligaments, collagen fibrils are packed into bundles aligned with the axis of loading, so that the fibrils, rather than the surrounding matrix, bear most of the tension when the tissue is stretched. Bone is a composite in which collagen supplies the organic framework and tensile toughness, while mineral crystals of calcium phosphate are deposited within and around the collagen fibrils to provide stiffness and resistance to compression. Type I collagen is the dominant form in these tissues. Without the collagen framework, bone would be brittle, and without the mineral it would be too compliant to bear weight.\n\nCartilage\n\nIn hyaline articular cartilage, CII accounts for 95% of the collagen content and constitutes 60% of the dry weight of the cartilage, which makes type II collagen the defining structural protein of the tissue. Its fibrils are organised in zones. In the superficial zone they run parallel to the joint surface and resist shear, whereas in the deep zone they run perpendicular to the surface and anchor the cartilage to the subchondral bone. Collagen fibers in the deep zone extend radially from the tidemark, a division between calcified and non-calcified cartilage. Here, the type II collagen fibers have the largest diameter serving to anchor the soft tissue to the sub-chondral bone.\n\nResistance to compression depends on an interaction between the collagen network and the proteoglycans. Large aggregates of aggrecan bound to hyaluronic acid draw in water, generating swelling pressure, and the collagen fibrils restrain that swelling. The collagen fibrils in cartilage also indirectly affect the compressive strength through limiting the swelling and hydration from proteoglycans. The collagen network therefore limits how far the matrix can swell and gives the tissue a resilient, load-bearing structure. Type II fibrils also contribute directly to tensile and shear strength, and the loss or disorganisation of this network is a central feature of osteoarthritis.\n\nBasement membranes and the kidney\n\nBasement membranes are thin sheets of extracellular matrix built largely from type IV collagen, which forms a network that is distinct from the fibrillar collagens. The glomerular basement membrane (GBM) of the kidney is a key example. It is composed of five (alpha1 to alpha5) of the six chains of type IV collagen, organized into an alpha1.alpha2(IV) and an alpha3.alpha4.alpha5(IV) network. In human GBM, the α3 to α5 chains predominate. The α3α4α5 network is the main tension-resisting component of the healthy membrane, and it forms part of the molecular sieve through which blood is filtered.\n\nBecause the membrane is a permselective barrier, its collagen network must stay intact. The scaffold tethers other macromolecules within the membrane, and when it is lost or assembled incorrectly, the filter leaks. Mutations in the genes encoding α3, α4 or α5 prevent formation of the α3α4α5 network, and this defect underlies Alport syndrome, which causes progressive kidney failure. Pathogenic variants in these genes are numerous, and they illustrate how a single collagen network can be essential to renal filtration.\n\nSignalling through integrins and discoidin domain receptors\n\nCells do not only bear collagen fibres; they also sense them. The most widely expressed mammalian collagen receptors are four members of the β1 integrin family and two homologous receptor tyrosine kinases (RTKs), the discoidin domain receptors DDR1 and DDR2. Integrins recognise short sequence motifs within the triple helix, such as the high-affinity GFOGER motif, whereas DDRs bind collagen through their own discoidin-type extracellular domain. The two receptor classes recognise distinct sequences, and integrins predominate in adhesion to collagen I, although DDRs also contribute to it.\n\nDDRs differ from integrins in their signalling kinetics. The DDRs exist as pre-formed homodimers independent of ligand binding. Collagen binding induces receptor autophosphorylation with distinctly slow and prolonged kinetics. Through these pathways, collagen contact influences many aspects of cell proliferation, adhesion, and migration, as well as remodeling of the extracellular matrix. The two receptors have different tissue distributions: DDR1 is mainly expressed in epithelial cells, particularly of the lung, kidney, mammary gland, and gastrointestinal tract, whereas DDR2 is primarily found in cells of mesenchymal origin, such as fibroblasts and smooth muscle cells.\n\nThe signals can also antagonise one another. In some epithelial cell lines, DDR1 binding to collagen negatively regulates the function of α2β1, involving the phosphatase SHP-2 and suppression of the activation of transcriptional activators Stat1 and Stat3, whereas in pancreatic cancer cells the two receptors cooperate to drive collagen-induced epithelial-to-mesenchymal transition. In vivo, loss of DDR2 disturbs skeletal development, and DDR2-null mice show altered collagen fibre organisation. These findings show that collagen receptors act as sensors that couple the physical state of the matrix to cell behaviour."
    },
    {
      "heading": "Turnover, remodeling and degradation",
      "text": "Collagen in adult tissue is not a fixed scaffold. The extracellular matrix is renewed continuously, though usually slowly, as old collagen is broken down and new molecules are deposited. The pace differs greatly among tissues. Aspartic acid racemization studies put the half-life of human articular cartilage collagen at about 117 years, that of intervertebral disc collagen at about 95 years, and that of skin collagen at about 15 years. Rodent measurements using stable isotopes gave half-lives of roughly 74 days for skin and 45 days for muscle collagen. In the human liver, the fractional synthesis rates of types I and III collagen corresponded to half-lives of about four months to a year, and the turnover rate rose with the severity of fibrosis. Turnover can also shift with disease: in hypertensive rats, the half-life of aortic and mesenteric artery collagen fell to about 17 days, while skin and tail tendon collagen were unaffected.\n\nEnzymatic cleavage of the triple helix\n\nThe intact triple helix resists most proteases. The main exception is a group of vertebrate collagenases in the enzyme family of matrix metalloproteinases (MMPs), including MMP-1, MMP-8 and MMP-13. These enzymes cut the interstitial collagens I, II and III at a single site in each chain, roughly three-quarters of the way from the N-terminus. In the α1(I) chain the scissile bond lies between Gly775 and Ile776. Cleavage depends on local unwinding rather than simple access: MMP-1 binds the helix and loosens a short stretch of it before the peptide bond is hydrolyzed, and in the transition state only a few interchain hydrogen bonds are broken. Once the ¾ and ¼ fragments form, they denature at body temperature and become vulnerable to gelatinases and other less specific proteases. MMP-2 and MMP-9 can also attack native collagen at this locus, whereas MMP-3 binds triple-helical collagen without cutting it. Bacterial collagenases, such as those from Clostridium histolyticum, differ in that they can cut the helix at several positions.\n\nThe activity of MMPs is restrained mainly by tissue inhibitors of metalloproteinases (TIMPs), endogenous proteins that bind the enzymes and block their catalytic sites. Because the balance between MMPs and TIMPs sets the net rate of matrix breakdown, shifts in this balance are a central feature of tissue remodeling and of pathological matrix destruction.\n\nControl by growth factors and mechanical load\n\nThe most influential signal for collagen synthesis is transforming growth factor β (TGF-β), a cytokine that activates fibroblasts and drives extracellular matrix production. In cultured human dermal fibroblasts, TGF-β3 raised collagen protein synthesis to as much as 200% of control and increased procollagen mRNA, and this effect partly operated through TGF-β1. Within tissues, TGF-β is largely stored in an inactive latent complex in the matrix, which means its availability depends on activation as well as on synthesis. TGF-β also promotes the conversion of fibroblasts into myofibroblasts, which contributes to excessive matrix deposition in fibrosis.\n\nMechanical load acts through the same cells. Fibroblasts and tenocytes sense force through integrins and the cytoskeleton. Repeated stretching of tendon cells activates AKT and mTOR signaling, which regulates mRNA translation and collagen expression, and integrin β1 signaling depends on integrin-linked kinase. Stretching of dermal fibroblasts increases COL1A1 and TGF-β1 expression. Conversely, when the actin cytoskeleton is disassembled, TGF-β type II receptor levels fall through a microRNA-21-dependent mechanism, reducing Smad2/3 activation and collagen production. Loss of mechanical tension therefore lowers both the responsiveness to TGF-β and the output of matrix proteins.\n\nAging and reduced turnover\n\nBecause collagen turns over slowly, its chemical modifications accumulate. Non-enzymatic reactions between protein amino groups and reactive carbonyl compounds derived from sugars or dicarbonyls generate advanced glycation end products (AGEs), which can form cross-links. These cross-links are effectively irreversible, so long-lived collagen acts as a record of cumulative exposure. Ageing also changes the nature of the cross-links themselves, shifting the balance of collagen chemistry in tissues such as the lung. In aged dermis, collagen fibrils become fragmented and their production declines. In the lung, collagen appears to consist of two pools: a recently synthesized fraction that is readily degraded, and a heavily cross-linked, stable fraction that resists turnover. As the stable pool grows and synthesis falls, the tissue's capacity to renew its matrix diminishes, and the accumulated covalent cross-links and AGEs stiffen the matrix and impair its repair."
    },
    {
      "heading": "Genetic and acquired disorders",
      "text": "Most cases of osteogenesis imperfecta (OI) arise from dominant changes in the type I collagen genes. About 85%–90% of cases involve dominantly inherited variants in COL1A1 and COL1A2, and most of these change a glycine residue, altering the structure of type I collagen. The reason a single substitution is so damaging lies in the Gly-X-Y repeat. Glycine is the only residue small enough to sit at the crowded central axis of the triple helix, so swapping it for a bulkier side chain prevents the NH group of that glycine from forming its hydrogen bond with a carbonyl on a neighbouring chain. Replacing a glycine at the centre of the helix therefore blocks interchain hydrogen bond formation. Among the variant classes, glycine substitutions are the most frequent, followed by changes at splice sites that lead to exon skipping; duplications or deletions that shift the Gly-X-Y register, and defects in the C-propeptide, are less common. Phenotypes span a wide range, from mild type I OI to progressively deforming type III and lethal type II forms. Glycine-to-serine is the most common missense change, and the position of the substitution along the helix influences the severity of dental involvement. Dental aberrations are strongly associated with qualitatively altered type I collagen. Biallelic (homozygous or compound heterozygous) cases are rare and are generally associated with a more severe or Ehlers–Danlos-like presentation.\n\nEhlers–Danlos syndromes (EDS) are a clinically heterogeneous group. Variants in at least 20 genes have been found to cause the Ehlers-Danlos syndromes. The causes are not all structural collagen genes: some encode enzymes or chaperones that process collagen. Genes such as ADAMTS2, FKBP14, PLOD1 and TNXB encode proteins that process, fold or interact with collagen, and variants in them disrupt collagen production or processing. In the classical type, in about 50% of patients there is a mutation in one of the two type V collagen genes, COL5A1 and COL5A2. Vascular EDS is mostly caused by variants in COL3A1, although rarely this type is caused by certain COL1A1 gene variants. The cardiac-valvular type is linked to COL1A2, and the dermatosparaxis type to ADAMTS2. These changes weaken connective tissues in the skin, bones, and other parts of the body.\n\nAlport syndrome, a kidney disorder, is caused by defects in type IV collagen: pathogenic variants in the COL4A3, COL4A4, and COL4A5 genes, which code for type IV collagen, cause Alport syndrome. Stickler syndrome involves the collagen types of cartilage and vitreous. Pathogenic variants in the COL2A1, COL9A1, COL9A2, COL9A3, COL11A1 and COL11A2 genes, which code for type II, IX, and XII collagen, cause Stickler syndrome.\n\nScurvy is the classic example of an acquired collagen disorder. Prolyl and lysyl hydroxylases, the enzymes that modify collagen, depend on vitamin C as a cofactor. Vitamin C keeps the iron in these enzymes in its active ferrous state. When it is absent or depleted, the iron oxidizes to the ferric form, and the enzyme becomes inactive. Without hydroxyproline, chains cannot fold into the triple-helix structure and are prone to degradation before secretion from the cell. Clinical signs appear when plasma concentrations of ascorbic acid drop below 10 micromolar. The skin and vessel walls suffer first: scurvy classically involves fragility of blood vessels. As scurvy progresses, wound healing is impaired due to the loss of mature collagen, which allows wounds to remain open. Perifollicular bleeding and corkscrew hairs are also described. The link between diet and this disease was made long before the molecular basis was known, when James Lind described the spotted skin of scorbutic patients in 1753.\n\nFibrosis is the excessive accumulation of collagen and other matrix components in response to repeated or chronic tissue injury. Fibroblasts and activated myofibroblasts deposit more collagen than the tissue can remodel, and the resulting scar tissue replaces functional parenchyma. In the liver, activated hepatic stellate cells drive septal fibrosis that can progress to cirrhosis. In the lung, idiopathic pulmonary fibrosis stiffens the alveolar walls and impairs gas exchange. In the kidney, interstitial and glomerular scarring is a common final pathway of chronic kidney disease. In each organ, the disorganised deposits disrupt normal tissue architecture, and the balance between synthesis and degradation of collagen is shifted toward accumulation."
    },
    {
      "heading": "Research history",
      "text": "Work on collagen's architecture began with X-ray fiber diffraction, which revealed regular repeats in tendon and related fibers but could not by itself give the atomic arrangement. Astbury was the first to suggest a structure for collagen in 1938, which combined trans and cis peptide units. In a 1940 paper with Bell, he instead described a single extended chain whose amide bonds were all cis (see the 1940 proposal). The cis/trans question mattered because the bonds in most natural proteins are overwhelmingly trans; cis-peptide bonds are rare, accounting for less than 0.1% of the bonds in natural proteins. Astbury's mixed model did not explain all the features of the collagen X-ray pattern.\n\nPauling and Corey's 1951 proposal came from the same era as their α-helix and β-sheet work. Three polypeptide strands were held together in a helical conformation by hydrogen bonds, but their hydrogen bonding required two of the three peptide bonds in each triplet to be cis. This model was also not in good agreement with the observed X-ray pattern, so neither early model could account for the diffraction data.\n\nThe decisive step came in 1954 from G. N. Ramachandran and Gopinath Kartha at the University of Madras. Working in relative isolation, with limited biochemical and X-ray data, they conceptualized the collagen model. Their structure was a right-handed triple helix of three staggered, left-handed PPII helices with all peptide bonds in the trans conformation and two hydrogen bonds within each triplet. The group had begun recording collagen diffraction patterns in 1952 at the suggestion of J. D. Bernal, and they drew on tendon and elastoidin from shark fin rays for their data (elastoidin diffraction patterns from shark fin ray collagen). Their 1955 follow-up introduced a coiling of the three chains around a common axis, an idea Crick had earlier used for α-helices packing.\n\nThe two-hydrogen-bond version drew criticism. Rich and Crick argued that it contained a few short interatomic contacts, and suggested that only structures with one hydrogen bond for every three residues would be free of bad contacts. In 1955, Alexander Rich and Francis Crick published a refined model in Nature, which they called structure I and structure II. All four groups working on the problem agreed that structure II was more satisfactory stereochemically and fit the X-ray picture better. Cowan, McGavin and North reached a similar supercoiled conformation in the same year (this view was proposed independently in 1955). The accepted refinement, with a single interstrand N–H(Gly)⋯O=C(Xaa) hydrogen bond per triplet and a tenfold helical symmetry with a 28.6 Å axial repeat, is the 10/3 helix. Later, Okuyama and colleagues proposed a 7/2 helix with a 20 Å axial repeat, based on the crystal structure of (Pro–Pro–Gly)10 and fiber data, and argued that it better describes native collagen; this view remains a minority alternative (they introduced it as a new model in 1977).\n\nModel peptides provided the next line of evidence. Short synthetic chains of polyglycine and polyproline played an important role in elucidating the triple-helix structure, and collagen-like peptides were later designed to test folding and stability. High-resolution crystal structures of such peptides, including a 2 Å structure of a model peptide reported in 1999, showed how sequence shapes local conformation within the triple helix (the 1999 structure provided the first visualization of how sequence defines local conformational variation). From the late 1970s and 1980s, the cloning of collagen complementary DNAs and genes allowed the amino acid sequences of individual chains to be read directly, which in turn revealed the Gly-X-Y pattern and the many collagen types that followed."
    },
    {
      "heading": "Applications in medicine, food, and industry",
      "text": "Gelatin\n\nMost collagen used commercially is not sold as intact fibre but converted into gelatin. Gelatin is produced by the partial hydrolysis (breakdown) of collagen extracted from the skin, bones, and/or connective tissues of animals, with bovine bones and hides, porcine skin, and fish skin, scales or offal as common raw materials. Porcine and bovine gelatin is generally obtained from skin or hides and, to a lesser extent, from bones, cartilages, and tendons. Fish by-products are increasingly important: fish skin and bone represent about 30% of the weight of processing waste and contain higher collagen content.\n\nThe process has to break the covalent cross-links that stabilise collagen in mature tissue, yet keep the chains from being cut at random. Heating in an autoclave dissolves the material but causes random hydrolysis of the bonds in the tropocollagen, resulting in gelatin of poor quality. The preferred approach works at lower temperature: the telopeptides and, hence, the cross-links are hydrolysed at a low temperature, leaving the triple helix intact and rendering the tropocollagen extractible. Industrial pretreatment also determines the product's character. Acid-pretreated gelatin (\"type A\") is most used for the less covalently cross-linked collagen found in pig skin, whereas alkaline-pretreated gelatin (\"type B\") can be applied to more complex collagen found in bovine hides.\n\nGelatin's functions explain its breadth of use. In food it serves as an emulsifier, stabiliser, clarifying agent and coating material. It is also used in pharmaceuticals, photographic products, textiles and paper, but also in the pharmaceutical, the photographic, the textile and the paper industry. Hard and soft capsule shells, a major pharmaceutical use, rely on the same gelling behaviour.\n\nScaffolds, sponges and wound dressings\n\nCollagen's excellent biocompatibility and controllable biodegradation make it a common biomaterial. Collagen scaffolds have been used as a dermal equivalent to induce fibroblast infiltrations and dermal regeneration in the treatment of chronic wounds and diabetic foot ulcers. The main obstacle is speed: untreated collagen degrades too quickly for many in vivo applications, so controlling the rate of breakdown is central to design.\n\nChemical cross-linking is the standard remedy. Traditional glutaraldehyde has drawbacks, so methods using a water soluble carbodiimide have been developed. Cross-linking decreased the enzymatic degradation and increased the thermal denaturation temperature in vitro. Other agents show similar effects: one epoxidised natural polysaccharide reduced the degradation rate from 89.45% to 38.66% in a hydrogel study. Composite materials that combine collagen with polysaccharides such as alginate and hyaluronic acid are also under investigation for chronic wound care, where the aim is a dressing that absorbs exudate and stops bleeding.\n\nHemostatic performance is a key target. One crosslinked collagen wound-care patent reports hemostasis in less than sixty seconds; hemostasis in 30 seconds has been demonstrated. The same source describes a dressing that demonstrates a collagenase degradation of less than 10 percent by weight after thirty days. Such targets reflect the widely cited design goal that the degradation rate of a wound dressing ideally matches the regeneration rate of the injured tissue.\n\nRecombinant collagen\n\nAnimal-derived collagen carries risks of pathogen transmission and batch-to-batch variation, which has motivated production in engineered microbes, plants and cell lines. Recombinant collagen I has already been produced in transgenic mice, tobacco plants, silk worms, insect cell lines, and mammalian cell lines. The main technical problem is that the thermal stability of collagen is dependent on the hydroxylation of proline by prolyl-4-hydroxylase, an enzyme present in sufficient amounts only in mammalian cell expression systems. For that reason, in most other hosts it is necessary to also clone both subunits of the P4H enzyme.\n\nYeast is a leading option. A gelatin-mimetic protein based on the human collagen I alpha chain was expressed in Pichia pastoris, the protein was secreted into the cultivation medium and the final yield was 3.4 g·L^−1, and in cell-culture tests its performance was indistinguishable from animal-derived gelatin. Bacteria offer cheaper growth but limited modification capacity. Fermentation-based collagen can be produced in large quantities from E. coli, with average production levels yielding greater quantities than conventionally produced collagen from mammalian yeast expression systems. Plant systems have also been developed, in which human collagen sequences are adapted to tobacco-relative hosts and secreted into the extracellular space.\n\nMany \"non-animal\" products are not full-length triple-helical proteins. Some yeast systems instead secrete collagen fragments with defined length, composition, and physiochemical properties that serve as substitutes for animal-derived gelatins. Buyers and regulators should therefore check what exactly is being sold.\n\nCosmetic and dietary products\n\nHydrolysed collagen is widely marketed in creams and as an ingested supplement for skin, joints and hair. The evidence base for these claims is limited and heterogeneous, and trials are often small. One trade-association update summarises a study of a fermentation-derived product reporting that it significantly improved skin collagen density and elasticity, but findings of this kind are not yet established as consistent across independent studies. Regulation also differs by region: ingested collagen is generally governed as a food or dietary supplement, and the health claims that manufacturers may make are restricted in some jurisdictions and permitted in others."
    },
    {
      "heading": "Current research and open questions",
      "text": "Non-collagenous domains are now seen as active in choosing which chains associate and in folding them correctly, but structure-prediction tools have trouble modeling this region. In a community assessment of AlphaFold2 applications, collagen triple helices were among the complex types that often failed, because their unusually repetitive residue composition defeats the method. AlphaFold folded a von Willebrand factor domain almost perfectly but could not assemble the collagen triple helix, even though it assigned high confidence to the collagen residues. The newer AlphaFold3 is a clear improvement. It predicted a hexamer of triple-helical trimers for the C1q collagenous stem, but with only three copies of each chain it predicted a trimer of trimers instead, showing that the results depend on how many chains the user supplies. In that case the confidence metrics were below 0.5, suggesting very low confidence. Chain selection itself has been mapped experimentally. The NC2 domain of type IX collagen guides three distinct α chains into the leading, middle and trailing positions of the triple helix. For fibrillar types, the selection signal lies in the C-terminal propeptides, which is why these regions are central to the question.\n\nSingle-molecule mechanics is another active area. Atomic force microscopy has stretched individual collagen molecules repeatedly without failure. One study reported that an individual triple-helix molecule can be stretched without failing to a contour length of up to 900 nm, nearly three times its native length, over thousands of stretching cycles. Molecular dynamics models of the whole molecule sliding and unravelling gave a peak force of around 500 pN. A different approach, centrifuge force microscopy, tests stability by measuring how fast trypsin cuts the helix under load. In that work, the cleavage rate increased with force, suggesting a force-enhanced destabilization of the triple helix. Such findings support the broader idea that mechanical actuation of cryptic sites in proteins can control downstream signalling. Whether forces on collagen fibrils expose sites that change cell behavior in living tissue remains to be shown directly.\n\nStability and stagger in heterotrimers are closely linked to fibril formation. In a heterotrimer with unrestricted chain placement, an arbitrary staggering process could generate multiple structurally distinct conformations in the absence of a control mechanism. For type I collagen, there are three different staggers possible, where the α2 chain is placed in the leading, middle or trailing position. Homotrimers behave differently from heterotrimers. Their impaired ability to form fibrils and denaturation of homotrimers is about 100 times slower than for heterotrimers both show that chain composition matters. Modeling work suggests that the interactions favour the formation of right-handed, pentameric microfibrils with a staggered axial periodicity of approximately 67 nm. Whether each collagen type has a specific stagger that binds its ligands or resists degradation still remains somewhat unresolved.\n\nLinking mutations to disease and designing biomaterials are the practical goals of much of this work. A missense mutation in the C-propeptide of the α1(I) chain is notable because it interferes with incorporation of a normal but genetically distinct chain into the trimer. Mutations in the genes for type I collagen that cause Ehlers–Danlos syndrome VII have also been shown to disrupt the structure of procollagen, slow its cleavage by N-proteinase, and produce the \"cauliflower\" appearance of collagen fibrils. On the materials side, achieving precise control over self-assembled states, to the degree achieved with α-helical coiled coils, remains elusive, and algorithms such as AlphaFold2 and Rosetta, which are highly effective for α-helical assemblies, struggle to predict collagen oligomer formation. Designing biomaterials that reproduce native behavior therefore depends on understanding sequence, chain selection and stagger together."
    }
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  "license": "https://creativecommons.org/licenses/by/4.0/"
}