Keratin, a ubiquitous biological material, constitutes a group of insoluble, high-sulfur content, and filament-forming proteins. It is the key structural material making up scales, hair, nails, feathers, horns, claws, hooves, and the outer layer of skin in tetrapod vertebrates. Keratin also protects epithelial cells from damage or stress. These keratinous materials are formed by cells filled with keratin and are considered 'dead tissues'. Nevertheless, they are among the toughest biological materials, serving a wide variety of interesting functions, e.g. scales to armor body, horns to combat aggressors, hagfish slime as defense against predators, nails and claws to increase prehension, hair and fur to protect against the environment. The vivid inspiring examples can offer useful solutions to design new structural and functional materials.
Keratin, a fibrous structural protein, is a defining component of the integumentary structures of vertebrates. These structures, ranging from the pliable epidermis to the rigid horns and claws, showcase the diverse functionality arising from variations in keratin composition, structure, and organization. This article explores the structure and properties of keratin, its presence in biological organisms, and efforts at bioinspiration.
Keratins can be classified as α- and β-types. Both show a characteristic filament-matrix structure: 7 nm diameter intermediate filaments for α-keratin, and 3 nm diameter filaments for β-keratin. Both are embedded in an amorphous keratin matrix. The molecular unit of intermediate filaments is a coiled-coil heterodimer and that of β-keratin filament is a pleated sheet. The mechanical response of α-keratin has been extensively studied and shows linear Hookean, yield and post-yield regions, and in some cases, a high reversible elastic deformation. Thus, they can be also be considered 'biopolymers'. On the other hand, β-keratin has not been investigated as comprehensively.
Alpha-keratins (α-keratins) are found in all vertebrates. They form the hair (including wool), the outer layer of skin, horns, nails, claws and hooves of mammals, and the slime threads of hagfish. The baleen plates of filter-feeding whales are also made of keratin. 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 harder beta-keratins (β-keratins) are found only in the sauropsids, i.e., all living reptiles and birds. They are found in the nails, scales, and claws of reptiles, in some reptile shells (Testudines), and in the feathers, beaks, and claws of birds. These keratins are formed primarily in beta sheets. Recent scholarship has shown that sauropsid β-keratins are fundamentally different from α-keratins at a genetic and structural level.
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Keratins (also described as cytokeratins) are polymers of type I and type II intermediate filaments that have been found only in chordates (vertebrates, amphioxi, urochordates). The human genome encodes 54 functional keratin genes, located in two clusters on chromosomes 12 and 17. Protein sequence alignment of human keratin 1, 2A, 3,4, 5, 6A, 7, and 8 (KRT1 - KRT8). Only the first rod domain is shown above. Limited interior space is the reason why the triple helix of the (unrelated) structural protein collagen, found in skin, cartilage and bone, likewise has a high percentage of glycine. The connective tissue protein elastin also has a high percentage of both glycine and alanine.
In addition to intra- and intermolecular hydrogen bonds, the distinguishing feature of keratins is the presence of 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-in much the same way that non-protein sulfur bridges stabilize vulcanized rubber. Human hair is approximately 14% cysteine. The pungent smells of burning hair and skin are due to the volatile sulfur compounds formed. The more flexible and elastic keratins of hair have fewer interchain disulfide bridges than the keratins in mammalian fingernails, hooves and claws (homologous structures), which are harder and more like their analogs in other vertebrate classes.
Keratinous materials exhibit a complex hierarchical structure: polypeptide chains and filament-matrix structures at the nanoscale, organization of keratinized cells into lamellar, tubular-intertubular, fiber or layered structures at the microscale, and solid, compact sheaths over porous core, sandwich or threads at the macroscale. These produce a wide range of mechanical properties: the Young's modulus ranges from 10 MPa in stratum corneum to about 2.5 GPa in feathers, and the tensile strength varies from 2 MPa in stratum corneum to 530 MPa in dry hagfish slime threads. Therefore, they are able to serve various functions including diffusion barrier, buffering external attack, energy-absorption, impact-resistance, piercing opponents, withstanding repeated stress and aerodynamic forces, and resisting buckling and penetration.
Keratinous materials are strain-rate sensitive, and the effect of hydration is significant. The length of keratin fibers depends on their water content. They can bind approximately 16 percent of water; this hydration is accompanied by an increase in the length of the fibers of 10 to 12 percent.
During the process of epithelial differentiation, cells become cornified as keratin protein is incorporated into longer keratin intermediate filaments. Eventually the nucleus and cytoplasmic organelles disappear, metabolism ceases and cells undergo a programmed death as they become fully keratinized. In many other cell types, such as cells of the dermis, keratin filaments and other intermediate filaments function as part of the cytoskeleton to mechanically stabilize the cell against physical stress. 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 thickening of the outer, cornified layer of the epidermis and form protective calluses, which are useful for athletes and on the fingertips of musicians who play stringed instruments. These hard, integumentary structures are formed by intercellular cementing of fibers formed from the dead, cornified cells generated by specialized beds deep within the skin.
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The processes of keratinization and cornification in skin modifications are different especially with respect to the keratins that are produced. Future research in keratins will provide a better understanding of the processes of keratinization and cornification of stratified epithelia, including those of skin modifications, of the adaptability of epithelia in general, of skin diseases, and of the changes in structure and function of epithelia in the course of evolution.
Keratin is extremely insoluble in water and organic solvents. Keratin is completely insoluble in cold or hot water; it is not attacked by proteolytic enzymes (i.e., enzymes that break apart, or lyse, protein molecules), and therefore cannot replace proteins in the diet. The great stability of keratin results from the numerous disulfide bonds of cystine. The amino acid composition of keratin differs from that of collagen. Cystine may account for 24 percent of the total amino acids. Reduction of the disulfide bonds to sulfhydryl groups results in dissociation of the peptide chains, the molecular weight of which is 25,000 to 28,000 each.
The formation of permanent waves in the beauty treatment of hair is based on partial reduction of the disulfide bonds of hair keratin by thioglycol, or some other mild reducing agent, and subsequent oxidation of the sulfhydryl groups (―SH) in the reoriented hair to disulfide bonds (―S―S―) by exposure to the oxygen of the air.
Keratin filaments are abundant in keratinocytes in the hornified layer of the epidermis; these are proteins which have undergone keratinization. They are also present in epithelial cells in general. For example, mouse thymic epithelial cells react with antibodies for keratin 5, keratin 8, and keratin 14. Keratins that form intermediate filaments are expressed exclusively in epithelial cells sensu lato, regardless of the germ layer origin of these cells. In non-epithelial cells, there are various types of intracellular intermediate filaments, including desmin (in myogenic cells), vimentin (in fibroblasts and fibrocytes), lamin (in cell nuclei) and neurofilaments (in neuronal cells).
All stratified epithelia start as simple epithelia and stratify as well as differentiate during the embryonic, fetal development and even postnatal development. The structures of epithelia reflect their various functions, e.g. semipermeable or protective barrier.
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In the process of soft cornification, the KFAPs form a filament-matrix complex and a proteinaceous cellular envelope on the inside of the cell membrane. The superficial cornified cells of stratified non-modified soft-cornified epithelia (e.g. epidermis) desquamate continuously and readily, whereas those of modified stratified soft-cornified epithelia.
Keratin expression is helpful in determining epithelial origin in anaplastic cancers. Tumors that express keratin include carcinomas, thymomas, sarcomas and trophoblastic neoplasms. Furthermore, the precise expression-pattern of keratin subtypes allows prediction of the origin of the primary tumor when assessing metastases.
A fascinating part of the new frontier of materials study is the development of bioinspired materials and designs. A comprehensive understanding of the biochemistry, structure and mechanical properties of keratins and keratinous materials is of great importance for keratin-based bioinspired materials and designs. Current bioinspired efforts including the manufacturing of quill-inspired aluminum composites, animal horn-inspired SiC composites, and feather-inspired interlayered composites are presented and novel avenues for research are discussed. The first inroads into molecular-based biomimicry are being currently made, and it is hoped that this approach will yield novel biopolymers through recombinant DNA and self-assembly.
Corneous, or horny, tissues have a long history of interest due to their economic, practical and emotional value. For example, the sheaths of horns have been fashioned into drinking vessels; mammalian fur has been used for clothing; the skin of reptiles has been manufactured into leather for clothing and pouches; mammalian hair has been used to make felt or to spin yarn for weaving and knitting; feathers have been used for various bedding materials and clothing; baleen has been used as whalebone in the fashion industry; ‘tortoise shell’ has been used for making combs and decorative objects; and hooves of farm animals have been used as slowly decaying fertilizers. Keratin-rich tissues are studied for their economic importance in the wool industry, for cosmetics and dermatology. Furthermore, the health of the hooves of farm and draft animals is of crucial economic importance to large animal producers and forms the basis of a longstanding interest in veterinary medicine concerning the structure and function of keratinized and cornified tissues. Recently, environmental problems arising from keratins as a byproduct of mass-produced poultry have been addressed.
tags: #keratin #structure #properties
