Collagen

Source: Wikipedia, the free encyclopedia.

Tropocollagen molecule: three left-handed procollagens (red, green, blue) join to form a right-handed triple helical tropocollagen.

Collagen (/ˈkɒlədʒən/) is the main structural protein in the extracellular matrix found in the body's various connective tissues. As the main component of connective tissue, it is the most abundant protein in mammals,[1] making up from 25% to 35% of the whole-body protein content. Collagen consists of amino acids bound together to form a triple helix of elongated fibril[2] known as a collagen helix. It is mostly found in connective tissue such as cartilage, bones, tendons, ligaments, and skin. Vitamin C is vital for collagen synthesis, and Vitamin E improves the production of collagen.

P. waltl

Depending upon the degree of

hydrolyzed using heat, basic solutions or weak acids.[5]

Etymology

Collagen

The name collagen comes from the Greek

glue", and suffix -γέν, -gen, denoting "producing".[6][7]

Human types

Over 90% of the collagen in the human body is type I collagen.[8] However, as of 2011, 28 types of human collagen have been identified, described, and divided into several groups according to the structure they form.[9] All of the types contain at least one triple helix.[9] The number of types shows collagen's diverse functionality.[10]

  • Fibrillar (Type I, II, III, V, XI)
  • Non-fibrillar
    • FACIT (Fibril Associated Collagens with Interrupted Triple Helices) (Type IX, XII, XIV, XIX, XXI)
    • Short chain (Type VIII, X)
    • Basement membrane (Type IV)
    • Multiplexin (Multiple Triple Helix domains with Interruptions) (Type XV, XVIII)
    • MACIT (Membrane Associated Collagens with Interrupted Triple Helices) (Type XIII, XVII)
    • Microfibril forming (Type VI)
    • Anchoring fibrils (Type VII)

The five most common types are:[11]

In human biology

Cardiac

The collagenous

cardiac input) and blood out (cardiac output). Pathology of the collagen underpinning of the heart is understood within the category of connective tissue disease.[citation needed
]

Bone grafts

As the skeleton forms the structure of the body, it is vital that it maintains its strength, even after breaks and injuries. Collagen is used in bone grafting as it has a triple helical structure, making it a very strong molecule. It is ideal for use in bones, as it does not compromise the structural integrity of the skeleton. The triple helical structure of collagen prevents it from being broken down by enzymes, it enables adhesiveness of cells and it is important for the proper assembly of the extracellular matrix.[12]

Tissue regeneration

Collagen scaffolds are used in tissue regeneration, whether in sponges,

osteoblasts and fibroblasts, and once inserted, facilitate growth to proceed normally.[17]

Reconstructive surgical uses

Collagens are widely employed in the construction of

burns and wounds.[18][19] These collagens may be derived from bovine, equine, porcine, or even human sources; and are sometimes used in combination with silicones, glycosaminoglycans, fibroblasts, growth factors and other substances.[20]

Wound healing

Collagen is one of the body's key natural resources and a component of skin tissue that can benefit all stages of wound healing.[21] When collagen is made available to the wound bed, closure can occur. Wound deterioration, followed sometimes by procedures such as amputation, can thus be avoided.

Collagen is a natural product and is thus used as a natural wound dressing and has properties that artificial wound dressings do not have. It is resistant against bacteria, which is of vital importance in a wound dressing. It helps to keep the wound sterile, because of its natural ability to fight infection. When collagen is used as a burn dressing, healthy granulation tissue is able to form very quickly over the burn, helping it to heal rapidly.[22]

Throughout the four phases of wound healing, collagen performs the following functions:

  • Guiding function:
    Collagen fibers
    serve to guide fibroblasts. Fibroblasts migrate along a connective tissue matrix.
  • Chemotactic
    properties: The large surface area available on collagen fibers can attract fibrogenic cells which help in healing.
  • Nucleation: Collagen, in the presence of certain neutral salt molecules, can act as a nucleating agent causing formation of fibrillar structures.
  • Hemostatic properties: Blood platelets
    interact with the collagen to make a hemostatic plug.

Basic research

Collagen is used in laboratory studies for cell culture, studying cell behavior and cellular interactions with the extracellular environment.[23] Collagen is also widely used as a bioink for 3D bioprinting and biofabrication of 3D tissue models.

Biology

The collagen protein is composed of a triple helix, which generally consists of two identical chains (α1) and an additional chain that differs slightly in its chemical composition (α2).[24] The amino acid composition of collagen is atypical for proteins, particularly with respect to its high hydroxyproline content. The most common motifs in the amino acid sequence of collagen are glycine-proline-X and glycine-X-hydroxyproline, where X is any amino acid other than glycine, proline or hydroxyproline. The average amino acid composition for fish and mammal skin is given.[25]

Amino acid Abundance in mammal skin
(
residues
/1000)		 Abundance in fish skin
(residues/1000)
Glycine 329 339
Proline 126 108
Alanine 109 114
Hydroxyproline 95 67
Glutamic acid 74 76
Arginine 49 52
Aspartic acid 47 47
Serine 36 46
Lysine 29 26
Leucine 24 23
Valine 22 21
Threonine 19 26
Phenylalanine 13 14
Isoleucine 11 11
Hydroxylysine 6 8
Methionine 6 13
Histidine 5 7
Tyrosine 3 3
Cysteine 1 1
Tryptophan 0 0

Synthesis

First, a three-dimensional stranded structure is assembled, with the amino acids glycine and proline as its principal components. This is not yet collagen but its precursor, procollagen. Procollagen is then modified by the addition of

prolyl 4-hydroxylase[27] and lysyl hydroxylase
. The reaction consumes one ascorbate molecule per hydroxylation. [28] The synthesis of collagen occurs inside and outside of the cell. The formation of collagen which results in fibrillary collagen (most common form) is discussed here. Meshwork collagen, which is often involved in the formation of filtration systems, is the other form of collagen. All types of collagens are triple helices, and the differences lie in the make-up of the alpha peptides created in step 2.

  1. Transcription of mRNA: About 44 genes are associated with collagen formation, each coding for a specific mRNA sequence, and typically have the "COL" prefix. The beginning of collagen synthesis begins with turning on genes which are associated with the formation of a particular alpha peptide (typically alpha 1, 2 or 3).
  2. Pre-pro-peptide formation: Once the final mRNA exits from the cell nucleus and enters into the cytoplasm, it links with the ribosomal subunits and the process of translation occurs. The early/first part of the new peptide is known as the signal sequence. The signal sequence on the
    N-terminal of the peptide is recognized by a signal recognition particle on the endoplasmic reticulum, which will be responsible for directing the pre-pro-peptide
    into the endoplasmic reticulum. Therefore, once the synthesis of new peptide is finished, it goes directly into the endoplasmic reticulum for post-translational processing. It is now known as preprocollagen.
  3. Pre-pro-peptide to pro-collagen: Three modifications of the pre-pro-peptide occur leading to the formation of the alpha peptide:
    1. The signal peptide on the N-terminal is removed, and the molecule is now known as propeptide (not procollagen).
    2. Hydroxylation of lysines and prolines on propeptide by the enzymes 'prolyl hydroxylase' and 'lysyl hydroxylase' (to produce hydroxyproline and hydroxylysine) occurs to aid cross-linking of the alpha peptides. This enzymatic step requires vitamin C as a cofactor. In scurvy, the lack of hydroxylation of prolines and lysines causes a looser triple helix (which is formed by three alpha peptides).
    3. Glycosylation occurs by adding either glucose or galactose monomers onto the hydroxyl groups that were placed onto lysines, but not on prolines.
    4. Once these modifications have taken place, three of the hydroxylated and glycosylated propeptides twist into a triple helix forming procollagen. Procollagen still has unwound ends, which will be later trimmed. At this point, the procollagen is packaged into a transfer vesicle destined for the Golgi apparatus.
  4. Golgi apparatus modification: In the Golgi apparatus, the procollagen goes through one last post-translational modification before being secreted out of the cell. In this step, oligosaccharides (not monosaccharides as in step 3) are added, and then the procollagen is packaged into a secretory vesicle destined for the extracellular space.
  5. Formation of tropocollagen: Once outside the cell, membrane bound enzymes known as collagen peptidases, remove the "loose ends" of the procollagen molecule. What is left is known as tropocollagen. Defects in this step produce one of the many collagenopathies known as Ehlers–Danlos syndrome. This step is absent when synthesizing type III, a type of fibrillar collagen.
  6. Formation of the collagen fibril:
    copper-dependent
    enzyme, produces the final step in the collagen synthesis pathway. This enzyme acts on lysines and hydroxylysines producing aldehyde groups, which will eventually undergo covalent bonding between tropocollagen molecules. This polymer of tropocollagen is known as a collagen fibril.
Action of lysyl oxidase

Amino acids

Collagen has an unusual amino acid composition and sequence:

  • residue
    .
  • Proline makes up about 17% of collagen.
  • Collagen contains two unusual 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
    .

degradation of (skin) collagen into amino acids.[29]

Collagen I formation

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

  1. Inside the cell
    1. Two types of alpha chains – alpha-1 and alpha 2, are formed during
      rough endoplasmic reticulum (RER). These peptide chains known as preprocollagen, have registration peptides on each end and a signal peptide.[30]
    2. Polypeptide chains are released into the lumen of the RER.
    3. Signal peptides are cleaved inside the RER and the chains are now known as pro-alpha chains.
    4. ascorbic acid (vitamin C) as a cofactor
      .
    5. Glycosylation of specific hydroxylysine residues occurs.
    6. Triple alpha helical structure is formed inside the endoplasmic reticulum from two alpha-1 chains and one alpha-2 chain.
    7. Procollagen is shipped to the Golgi apparatus, where it is packaged and secreted into extracellular space by exocytosis
      .
  2. Outside the cell
    1. Registration peptides are cleaved and tropocollagen is formed by procollagen peptidase.
    2. Multiple tropocollagen molecules form collagen fibrils, via covalent cross-linking (aldol reaction) by lysyl oxidase which links hydroxylysine and lysine residues. Multiple collagen fibrils form into collagen fibers.
    3. Collagen may be attached to cell membranes via several types of protein, including fibronectin, laminin, fibulin and integrin.

Synthetic pathogenesis

Vitamin C deficiency causes

Gums deteriorate and bleed, with loss of teeth; skin discolors, and wounds
do not heal. Prior to the 18th century, this condition was notorious among long-duration military, particularly naval, expeditions during which participants were deprived of foods containing vitamin C.

An autoimmune disease such as lupus erythematosus or rheumatoid arthritis[31] may attack healthy collagen fibers.

Many bacteria and viruses secrete virulence factors, such as the enzyme collagenase, which destroys collagen or interferes with its production.

Molecular structure

A single collagen molecule, tropocollagen, is used to make up larger collagen aggregates, such as fibrils. It is approximately 300 

quaternary structure stabilized by many 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 referred to as the collagen microfibril. Each microfibril is interdigitated
with its neighboring microfibrils to a degree that might suggest they are individually unstable, although within collagen fibrils, they are so well ordered as to be crystalline.

polypeptides
coil to form tropocollagen. Many tropocollagens then bind together to form a fibril, and many of these then form a fibre.

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-X or Gly-X-Hyp, where X may be any of various other amino acid residues.[25] Proline or hydroxyproline constitute about 1/6 of the total sequence. With glycine accounting for the 1/3 of the sequence, this means approximately half of the collagen sequence is not glycine, proline or hydroxyproline, a fact often missed due to the distraction of the unusual GX1X2 character of collagen alpha-peptides. The high glycine content of collagen is important with respect to stabilization of the collagen helix as this allows the very close association of the collagen fibers within the molecule, facilitating hydrogen bonding and the formation of intermolecular cross-links.[25] This kind of regular repetition and high glycine content is found in only a few other fibrous proteins, such as silk fibroin.

Collagen is not only 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

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

poikilotherm animals leads to their collagen having a lower thermal stability than mammalian collagen.[25] This lower thermal stability means that gelatin
derived from fish collagen is not suitable for many food and industrial applications.

The tropocollagen

extracellular spaces of tissues.[32][33] Additional assembly of fibrils is guided by fibroblasts, which deposit fully formed fibrils from fibripositors. In the fibrillar collagens, molecules are staggered to adjacent molecules by about 67 nm (a unit that is referred to as 'D' and changes depending upon the hydration state of the aggregate). In each D-period repeat of the microfibril, there is a part containing five molecules in cross-section, called the "overlap", and a part containing only four molecules, called the "gap".[34] These overlap and gap regions are retained as microfibrils assemble into fibrils, and are thus viewable using electron microscopy. The triple helical tropocollagens in the microfibrils are arranged in a quasihexagonal packing pattern.[34][35]

The D-period of collagen fibrils results in visible 67nm bands when observed by electron microscopy.

There is some

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 (i.e. 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.[37] 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.[38][39] For example, using AFM–based nanoindentation it has been shown that a single collagen fibril is a heterogeneous material along its axial direction with significantly different mechanical properties in its gap and overlap regions, correlating with its different molecular organizations in these two regions.[40]

Collagen fibrils/aggregates are arranged in different combinations and concentrations in various tissues to provide varying tissue properties. In bone, entire collagen triple helices lie in a parallel, staggered array. 40 nm gaps between the ends of the tropocollagen subunits (approximately equal to the gap region) probably serve as nucleation sites for the deposition of long, hard, fine crystals of the mineral component, which is hydroxylapatite (approximately) Ca10(OH)2(PO4)6.

tensile strength
.

Associated disorders

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

Genetic defects of collagen genes
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, cornea, the endomysium surrounding muscle fibers, fibrocartilage, and the organic part of bones and teeth.
COL1A2
 Osteogenesis imperfecta, Ehlers–Danlos syndrome, infantile cortical hyperostosis a.k.a. Caffey's disease
II
Vitreous humour
of the eye.
COL2A1
 Collagenopathy, types II and XI
III
 This is the collagen of granulation tissue and is produced quickly by young fibroblasts before the tougher type I collagen is synthesized. Reticular fiber. Also found in artery walls, skin, intestines and the uterus
COL3A1
 Ehlers–Danlos syndrome, Dupuytren's contracture
IV
capillaries and the glomeruli of nephron in the kidney
.
COL4A6
Goodpasture's syndrome
V Most interstitial tissue, assoc. with type I, associated with placenta
COL5A3
 Ehlers–Danlos syndrome (classical)
VI Most interstitial tissue, assoc. with type I
COL6A5
Ulrich myopathy, Bethlem myopathy, atopic dermatitis[42]
VII Forms
anchoring fibrils in dermoepidermal junctions
COL7A1
 Epidermolysis bullosa dystrophica
VIII Some endothelial cells
COL8A2
Posterior polymorphous corneal dystrophy 2
IX FACIT collagen, cartilage, assoc. with type II and XI fibrils
COL9A3
EDM3
X
mineralizing
cartilage
COL10A1
Schmid metaphyseal dysplasia
XI Cartilage
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, also known as undulin
COL14A1
 –
XV –
COL15A1
 –
XVI FACIT collagen
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 FACIT collagen
COL22A1
 –
XXIII MACIT collagen
COL23A1
 –
XXIV – COL24A1 –
XXV –
COL25A1
 –
XXVI – EMID2 –
XXVII –
COL27A1
 –
XXVIII –
COL28A1
 –
XXIX Epidermal collagen
COL29A1
 Atopic dermatitis[43]

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

Diseases

One thousand mutations have been identified in 12 out of more than 20 types of collagen. These mutations can lead to various diseases at the tissue level.[44]

Osteogenesis imperfecta – Caused by a mutation in type 1 collagen, dominant autosomal disorder, results in weak bones and irregular connective tissue, some cases can be mild while others can be lethal. Mild cases have lowered levels of collagen type 1 while severe cases have structural defects in collagen.[45]

Chondrodysplasias – Skeletal disorder believed to be caused by a mutation in type 2 collagen, further research is being conducted to confirm this.[46]

Ehlers–Danlos syndrome – Thirteen different types of this disorder, which lead to deformities in connective tissue, are known.[47] Some of the rarer types can be lethal, leading to the rupture of arteries. Each syndrome is caused by a different mutation. For example, the vascular type (vEDS) of this disorder is caused by a mutation in collagen type 3.[48]

Alport syndrome – Can be passed on genetically, usually as X-linked dominant, but also as both an autosomal dominant and autosomal recessive disorder, those with the condition have problems with their kidneys and eyes, loss of hearing can also develop during the childhood or adolescent years.[49]

COL18A1 gene that codes for the production of collagen XVIII. Patients present with protrusion of the brain tissue and degeneration of the retina; an individual who has family members with the disorder is at an increased risk of developing it themselves since there is a hereditary link.[44]

Characteristics

Collagen is one of the long,

tissue development. It is present in the cornea and lens of the eye in crystalline form. It may be one of the most abundant proteins in the fossil record, given that it appears to fossilize frequently, even in bones from the Mesozoic and Paleozoic.[53]

Uses

A salami and the collagen casing (below) it came in

Collagen has a wide variety of applications, from food to medical.

casings for sausages
.

If collagen is subject to sufficient denaturation, such as by heating, the three tropocollagen strands separate partially or completely into globular domains, containing a different secondary structure to the normal collagen polyproline II (PPII) of random coils. This process describes the formation of gelatin, which is used in many foods, including flavored gelatin desserts. Besides food, gelatin has been used in pharmaceutical, cosmetic, and photography industries. It is also used as a dietary supplement, and has been advertised as a potential remedy against the ageing process.[55][56][57]

From the Greek for glue, kolla, the word collagen means "

human skulls.[58] Collagen normally converts to gelatin, but survived due to dry conditions. Animal glues are thermoplastic, softening again upon reheating, 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.[59]

Cosmetics

dermal fillers for aesthetic correction of wrinkles and skin aging.[60] Collagen cremes are also widely sold even though collagen cannot penetrate the skin because its fibers are too large.[61] Most research on collagen supplements has been funded by industries that could benefit from a positive study result.[61]

History

The molecular and packing structures of collagen eluded scientists over decades of research. The first evidence that it possesses a regular structure at the molecular level was presented in the mid-1930s.

quaternary structure in collagen.[64][65][66][67][68] This model was supported by further studies of higher resolution in the late 20th century.[69][70][71][72]

The packing structure of collagen has not been defined to the same degree outside of the

electron microscopy in the late 20th century and early 21st century.[77][78][79] The microfibrillar structure of rat tail tendon was modeled as being closest to the observed structure, although it oversimplified the topological progression of neighboring collagen molecules, and so did not predict the correct conformation of the discontinuous D-periodic pentameric arrangement termed microfibril.[34][80][81]

See also

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