Cornification also known as keratinization is the foundation of how keratin in formed. The outer cells on the epidermis lose their function in this process, the cells organelles such as the nucleus and mitochondria disappear and metabolism ceases as the cells become replaced by keratin. This new layer is incapable of sensory perception and is classified as dead. Keratin covers the width of a cell enabling it to connect indirectly with other keratin in adjoining cell to form junctions called desmosomes2, this makes the outer most layer of non-porous, almost water proof.
There are many types of keratin found in the body and in mammals, each type structured to suit its function. The most common form of keratin found in most parts of the body is alpha keratin3. They have long fibrous strands that super coil into alpha helixes, the folds and the short repeating units along the polypeptide chain are what make up their composition. The alpha helix is formed from an alpha keratin polypeptide, two of these twists around each other to form a coil. The hydrophobic strips on one helix associates itself with the other. This allows the side chains to interlock. The combination of the two coils is known as a dimmer. This is the basis of the protofilament of which 2 make up a protofibril. Four protofibrils make up a microfibril. The associations of many microfibrils form a macrofibril4. Hair for example consists of layers of dead cells that are packed with macrofibrils.
The diagram below shows a general idea of how the higher order of alpha keratin is structured.
A key feature of it’s structure is that is contains cysteine disulfide, this makes up for almost 24% of the amino acid structure5, it allows it to form disulphide bridges which forms a helix shape that is extremely strong. As the sulphur atoms bond covalently to each other across the helix, it forms a fibrous matrix, which is highly stable. High percentages of glycine and alanine aid to the formation of hydrogen bonds between amino and carboxyl groups of adjacent peptide bonds6. The disulphide bridges and hydrogen bonding cause insolubility in things like water. This structural feature is vital in mammals. The long filaments present in the structure provide support for the epithelial cells that contain keratin such as skin. It protects the body from damage by allowing other tissues to form around it and stabilize.
Collagen is one of other types of scleroprotein, as with keratin it is an important part in majority of the body’s structures. It is the main component in connective tissue. It makes up for roughly 25% of the body’s protein7. Its strength and insolubility make it the most abundant protein in mammals. Being a connective protein it is seen in body tissues such as the skin, muscles, tendon and cartilage. There are about 25 different types of collagen that occur in the body. Collagen widely works with the other scleroproteins supporting the body’s tissues. It does this by providing support, firmness and strength. Collagen may be seen in different forms such as endomysium and fibrous tissue8, different types being made for different parts of the body such as the cornea, bone, blood vessels as well as the gut. It is able to withstand intense pulling and stretching. It makes up the major stress bearing parts of connective tissue4. One of the most important qualities of collagen is strength; it has the tensile strength of steel and is used to strengthen bone similarly to how metal rods reinforce concrete. It has been seen as a form of ‘glue ‘ that hold the body together; with out it the human body would not be one complete structure.
Collagen appears to be able to do this as a result of features within its structure. For a long period of time scientists have struggled to identify the exact structure of collagen, this is because of its unusual amino acid composition.
Collagen has a super-coiled helical shape, it is long, stringy, strong, and resembles a rope. Its primary structure consists of a repeating sequence of glycine and two other amino acids, often proline and hydroxyproline; this forms its sturdy structure. These amino acids cause the chain not to gain the normal alpha- helix or beta- sheet structure; instead they form the long separate chains that allow the collagen triple helix to form9.
A single strand of collagen is composed of three chains of polypeptides each being about 1000 amino acids long, the three strands are arranged parallel to each other and are wound up into a triple helix. The bundling of the fiber together and hydrogen bonding between the lengths of the polypeptide chain give it strength.
The diagram below gives a general idea as to how a collagen fiber is shaped.
Collagen’s tightly wound matrix of fibers in its structure is very vital in its function, being that majority of the body comprises of it, it need be very strong. For instance collagen is a major component of tendons, which connect muscles to bones, and ligaments, which connect bones to joints. The variance of collagen in these two parts is very little, but remain specialised for both areas. Both parts require strength and flexibility, things that are provided and adjusted from the compactions of the collagen fibers. As mentioned collagen varies in strength and flexibility depending on where its located and its function as connective tissue.
Structural defects within the structures of vital proteins such as collagen and keratin can be responsible for major metabolic related diseases. Osteogenesis imperfecta also known as brittle bone disease is an example of one of these conditions that affect collagen. Osteogenesis imperfecta (OI) is a genetic disorder that causes extremely fragile bones. As a result people with the condition tend to have many fractures within a lifetime. It is an inherited disease; it is caused by mutations within the genes of type 1 collagen (the most prevalent collagen in the body). The deficiency stems from the substitution of the amino acid glycine, it should normally be at every third position for the triple helix to form properly, mutations that result in amino acids other than glycine in that position cause an issue within the collagen complex which produce unstable helices. The body’s lack of response to the dysfunctional collagen structure allows it to be hydrolyzed. When unstable helices form in the collagen associated with tendons and bones, such as Type I collagen, severely weak bone formation results10. There are 8 types of OI ranging form type I- VIII. The later being the worst case of it. The severity the condition depends on the specific gene defect. OI is typically hard to generalize as it varies from person to person, even when two people in the same family have the same form of it, the characteristics may be completely different. Symptoms of the condition vary; they may include blue sclera and early hearing loss. Milder forms of OI result in bowed arms and legs, scoliosis and kyphosis11.
Currently there aren’t any cures for the condition. Treatment is directed toward preventing or controlling the symptoms, maximizing independent mobility, and developing optimal bone mass and muscle strength. Various forms of therapy are in place to reduce suffering for individuals with the condition. In extremely severe cases a surgical procedure called rodding is used. This involves inserting metal rods through the length of the long bones to strengthen them and prevent and/or correct deformities12.
Research into various medication and other treatments are being explored, some include growth hormones, gene therapy and other multiple types of drugs.
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