Corpus: Collagen

The collagen family is a heterogeneous group of proteins that make up about a quarter of the total amount of protein in the human organism. Collagen is the most important fibre component of skin, bones, tendons, cartilage, blood vessels and teeth. To date, 25 collagen polypeptides have been described that make up over 28 different types of collagen in the extracellular matrix.

Collagen molecule[Bearbeiten]

The tertiary structure of collagen molecules consists of left-handed helical amino acid chains (procollagen), three of which wind around each other in the form of a right-handed superhelix or triple helix, thus forming fibres known as tropocollagen. The triple helix is stabilised by hydrogen bonds between the individual strands.

The collagen chains have a high proportion of proline and glycine, as well as a number of hydroxylated amino acids such as hydroxyproline and hydroxylysine, which enable cross-linking of the proteins and the formation of a stable collagen matrix.

Although the resulting fibres are helical, they differ structurally from the classic α-helix. Due to the high content of prolines and glycines, the helices of the collagen are stretched longer, resulting in a pitch (the distance that a full turn of the screw takes upwards) of 0.94 nm. In contrast, an α-helix has a pitch of 0.54 nm. The elongated shape of the protein chains means that the triple helix of collagen is very compact and narrow. Depending on its exact type, each collagen helix can be made up of several hundred to several thousand amino acids.

Collagen fibrils[Bearbeiten]

Aggregation of several collagen molecules creates the next higher organisational unit, the collagen fibrils. The collagen molecules are not simply bundled together when the fibrils are formed, but are offset from each other by approx. 1/5 of their length (67 nm). This results in a characteristic transverse striation in the electron microscope image, which is repeated every 67 nm (234 amino acids). This is known as the D-period.

The formation of collagen fibrils from the tropocollagen subunits occurs spontaneously in the extracellular space. The alignment is controlled by fibroblasts, which align fully formed fibrils with the help of so-called fibripositors.

Collagen fibrils have very different diameters in different tissues, ranging from 20 nm to around 500 nm. This range of variation means that the structure of the fibrils can be variably adapted to the requirements of the respective tissue. In the cornea, for example, the fibrils have a very small diameter (25 nm) to enable the cornea to be transparent.

In the organism, collagen is mainly found in connective tissue, where it can make up the majority of the extracellular matrix, depending on its composition. The different properties of the various types of collagen are the basis for a variety of properties of connective and supporting tissue. The tensile strength of ligaments and tendons, the flexibility of bones or the pressure resistance of joint cartilage is largely determined by the collagen predominant in the tissue.

Collagen is the protein with the highest relative proportion (around 25 %) of total human protein.

The synthesis of collagen is ascorbic acid-dependent. A lack of vitamin C causes the clinical picture of scurvy. A large number of genetic diseases are based on defects in collagen or associated proteins (e.g. Ehlers-Danlos syndrome, osteogenesis imperfecta, Stickler syndrome, Alport syndrome).

A number of collagen types have been discovered to date, although their function is still partially unclear.

• Collagen I (fibre-forming in collagen fibres): Skin, tendons, bone, dentin, fibrous cartilage, cornea
• Collagen II (fibre-forming): Hyaline cartilage, fibrous cartilage, elastic cartilage, vitreous humour
• Collagen III (fibre-forming in reticulin fibres): Skin, skeletal muscle, blood vessels
• Collagen IV (reticular): Basal lamina (esp. lamina densa)
• Collagen V (fibre-forming): Fetal tissues, placenta, interstitial connective tissue, cornea
• Collagen VI (fibre-associated, spherical): Connective tissue
• Collagen VII (anchoring fibrils): Anchoring of epithelia to basement membrane and stroma
• Collagen VIII (reticular): Descemet's membrane
• Collagen IX (fibre-associated): Cartilage, corpus vitreum
• Collagen X (reticular): Growth zone of the cartilage
• Collagen XI (fibre-forming): Cartilage
• Collagen XII (fibre-associated): embryonic skin, embryonic tendons
• Collagen XIII: bones, cartilage, skin, striated muscles
• Collagen XIV: mainly in the intraneural connective tissue of the nerves, in the epi- and perimysium, also within the muscle after denervation
• Collagen XV: basal membrane of the muscles, in the kidneys after fibrosis
• Collagen XVI: in internal organs, in the eyes and somewhat in muscle tissue
• Collagen XVII (transmembrane): Association with hemidesmosomes of squamous epithelial cells of the epidermis
• Collagen XVIII: unknown
• Collagen XIX: (fibre-associated): fetal skin, fetal tendons

In the past, "bone glue" was produced from animal collagen denatured by acid or heat. Today, resorbable suture materials and skin grafts are synthesised from collagen fibres. In cosmetic surgery, collagen was used as an injection material for moulding external structures of the body (e.g. lips) and for injecting wrinkles.