Information AboutKeratin |
| CATEGORIES ABOUT KERATIN | |
| keratins | |
| cytoskeleton | |
|
Keratins are a family of Fibrous Structural Proteins ; tough and insoluble, they form the hard but Nonmineralized structures found in Reptile s, Bird s and Mammal s. The Baleen plates of filter-feeding Whale s are made of them. Keratins are also found in the Gastrointestinal Tract s of many animals, including Roundworm s. They are rivaled in Biological toughness only by Chitin , a Cellulose -like Polymer of Glucosamine and the main constituent of the Exoskeleton s of Arthropod s. There are various types of keratins, even within a single Animal . Some Infectious Fungi , such as those which cause Athlete's Foot and Ringworm , feed on keratin. The Silk fibroins produced by Insect s and Spider s are often classified as keratins, though it is unclear whether they are phylogenetically related to vertebrate keratins. KERATIN IN THE KINGDOM ANIMALIA Cells in the Epidermis contain a structural matrix of keratin which makes this outermost layer of the Skin almost waterproof, and along with Collagen and Elastin , gives skin its strength. Rubbing and pressure cause keratin to proliferate with the formation of protective Callus es — useful for athletes and on the fingertips of musicians who play stringed instruments. Keratinized epidermal cells are constantly shed and replaced (see Dandruff ). In mammals there are soft Epithelial keratins, the Cytokeratin s, and harder Hair Keratin s. As certain skin cells Differentiate and Cornify , pre-keratin Polypeptide s are incorporated into Intermediate Filament s. Eventually the Nucleus and Cytoplasm ic Organelle s disappear, Metabolism ceases, and cells undergo a Programmed Death as they become fully keratinized. Keratins are the main constituent of structures that grow from the skin: the α-keratins in the Hair (including Wool ), Horns , Nails , claws and hooves of mammals; also the harder β-keratins in the Scales and Claw s of reptiles, and their Shells ( Tortoise s), and in the Feather s, Beak s, and claws of birds. These hard, Integument ary structures are formed by Intercellular cementing of fibers formed from the dead, cornified cells generated by Specialized Beds deep within the skin. Hair grows continuously and feathers Moult and regenerate. The constituent Protein s may be Phylogenetically Homologous but differ somewhat in Chemical structure and super Molecular organization. The Evolution ary relationships are complex and only partially known. Multiple Gene s have been identified for the β-keratins in feathers, and this is probably characteristic of all keratins. MOLECULAR BIOLOGY AND BIOCHEMISTRY The properties which make structural proteins like keratins useful depend on their supermolecular aggregation. These depend on the properties of the individual Polypeptide strands, which depend in turn on their Amino Acid composition and sequence. The α-helix and β-sheet motifs, and disulfide bridges, are crucial to the Conformations of Globular, Functional Proteins like Enzymes , many of which operate semi-independently, but they take on a completely dominant role in the architecture and aggregation of keratins. Keratins contain a high proportion of the smallest of the 20 amino acids, Glycine , whose " Side Group " is a single Hydrogen Atom ; also the next smallest, Alanine , with a small and uncharged Methyl Group . In the case of β-sheets, this allows Sterically-unhindered Hydrogen Bond ing between the Amino and Carboxyl Group s of Peptide Bond s on adjacent protein chains, facilitating their close alignment and strong binding. Fibrous keratin molecules can twist around each other to form Helical intermediate filaments. Limited interior space is the reason why the triple helix of the (unrelated) structural protein collagen, found in skin, side groups is characteristic of structural proteins, for which H-bonded close packing is more important than Chemical Specificity . In addition to intra- and intermolecular hydrogen bonds, keratins have large amounts of the Sulfur -containing amino acid Cysteine , required for the Disulfide Bridges that confer additional strength and rigidity by permanent, thermally-stable Crosslinking — a role sulfur bridges also play in Vulcanized Rubber . Human hair is approximately 14% cysteine. The pungent smells of burning hair and rubber are due to the sulfur compounds formed. Extensive disulfide bonding contributes to the in Solubility of keratins, except in Dissociating or Reducing agents such as Urea . The more flexible and elastic keratins of hair have fewer interchain disulfide bridges than the keratins in mammalian Fingernail s, hooves and claws (homologous structures), which are harder and more like their analogs in other vertebrate classes. Hair and other α-keratins consist of α-helically -coiled single protein strands (with regular intra-chain H-bonding ), which are then further twisted into superhelical ropes that may be further coiled. The β-keratins of reptiles and birds have β-pleated sheets twisted together, then stabilized and hardened by disulfide bridges. Silk found in insect Polymer s such as Nylon , developed as a silk substitute. Silk from the Hornet Cocoon contains doublets about 10 µm across, with cores and coating, and may be arranged in up to 10 layers; also in plaques of variable shape. Adult hornets also use silk as a Glue , as do spiders. SEE ALSO REFERENCES
EXTERNAL LINKS |
|
|