Keratin proteins

The leucine zipper DNA-binding proteins, described in Chapter 10, are examples of globular proteins that use coiled coils to form both homo- and heterodimers. A variety of fibrous proteins also have heptad repeats in their sequences and use coiled coils to form oligomers, mainly dimers and trimers. Among these are myosin, fibrinogen, actin cross-linking proteins such as spectrin and dystrophin as well as the intermediate filament proteins keratin, vimentin, desmin, and neurofilament proteins. 

The loss of structure by a protein is called denaturation. This structural change may be a loss of quaternary, tertiary, or secondary structure it may also be degradation of the primary structure by cleavage of the peptide bonds. Even mild heating can cause irreversible denaturation. When we cook an egg, the protein called albumen denatures into a white mass. The permanent waving of hair, which consists primarily of long a helices of the protein keratin, is a result of partial denaturation. 

The hair fibers derived from furry mammals are mainly made up (over 80%) of the structural protein keratin. The distinction between wool and hair is not compositional, but related to size wool fibers are generally fine and short, whereas those of hair are usually thicker and longer. The molecule of keratin consists essentially of a combination of amino acids about 18 amino acids make up the keratin molecule (see Textbox 67). The nature of the amino acids, their relative amounts, and their sequence and arrangement within the molecule of keratin vary from one animal species to another but are characteristic of any variety of wool or hair (Asquith 1977) (see Table 89). 

FIGURE 3-1 Some functions of proteins, (a) The light produced by fireflies is the result of a reaction involving the protein luciferin and ATP, catalyzed by the enzyme luciferase (see Box 13-2). (b) Erythrocytes contain large amounts of the oxygen-transporting protein hemoglobin. (c) The protein keratin, formed by all vertebrates, is the chief structural component of hair, scales, horn, wool, nails, and feath-... 

We have noted previously (see Section 2.1) the role played in biochemistry by the thiol disulfide interconversion, and the disulfide unit is a fundamental feature of the structure of peptides and proteins. The sulfur-sulfur bond between two cysteins link remote parts of a peptide chain or cross-link two such chains. Cleavage of these bonds in the hair protein keratin, followed by reoxidation, gives hair its desired shape. 

WOOL, The natural, highly crimped fiber from sheep, wool is one of the oldest fibers from the standpoint of use in textiles. Minute scales on the surface of the fibers allow them to interlock and are responsible for the ability of the fiber to felt, a phenomenon responsible for felt cloth and mill-finished worsteds. Crimpiness in wool is due to the open formation of the scales. Fine merino wool has 24 crimps per inch ( " 10 per centimeter). Luster of the fiber depends upon the size and smoothness of the scales. The basic wool protein, keratin, comprises molecular chains that are linked with sulfur. When sulfur is fed to sheep in areas deficient of the element, the quality of the wool improves. Wool fibers that fall below 3 inches (7.5 centimeters) in length are known as clothing wool fibers 3-7 inches (7.5-17.8 centimeters) long are referred to as combing wools. The wool-liber diameter ranges from 0.0025 to 0.005 inch (0.06-0.13 millimeter). See also Fibers. 

A Bird feathers are made largely of the protein keratin. 

Abstract The utility of confocal Raman microscopy to study biological events in skin is demonstrated with three examples, (i) monitoring the spatial and structural differences between native and cultured skin, (ii) tracking the permeation and biochemical transformation in skin of a Vitamin E derivative and (iii) tracking the spatial distribution of three major skin proteins (keratin, collagen, and elastin) during wound healing in an explant skin model. 

Epidermis. The epidermis is a 50- to lOO-pm-thick layer made of ker-atinocytes that migrate outward from the basal cell into highly differentiated nondividing cells.46,47 During differentiation, keratinocytes transform from polygonal (cuboidal) cells to spinous (prickly) cells, flattened granular cells, and finally to flattened polyhedral dead corneocytes full of the protein keratin.45,47... 

This maturation and differentiation process is broadly similar to the process for keratinized epithelium, although obviously cells of keratinized epithelium also show increasing amounts of the fibrous protein, keratin, in the upper layers. 

Claws, beaks, horns, hooves, and baleen these materials may seem like strange companions, but they are all composed of basically the same material, the protein keratin. Hair and feathers are also keratin, but are addressed in detail in previous sections. All of these materials form on the outside of an animal s body, in the skin. 

Structure. Collagen is a widespread protein that provides a structural framework of intercellular tissue support in connective tissue, cartilage, bone and other tissues. Elastin is a stretchable support protein. Keratin is the tough protein of fingemails and hair. 

The skin is two discrete tissue layers, both polymeric but differing in protein composition, morphology, and thickness (I, 2) (Figure 1). Epidermis, the outer layer, is cellular and is composed primarily of the intracellular fibrous protein keratin associated with lipids. In contrast,... 

The physicochemical factors that control dmg penetration include the hydration of the stratum corneum, temperature, pFT, drug concentration, and the molecular characteristics of the penetrant and the vehicle. The stratum corneum is a heterogeneous stmcture containing about 40% protein (keratin, a disul-fide-crosslinked linear polypeptide), about... 

Derivation Hydrolysis of protein (keratin), organic synthesis. Occurs as small hexagonal crystals in urine. 

Experimental evidence for the antiparallel-chain jS-sheet structure has been obtained from X-ray diffraction studies of a synthetic polypeptide, (Ala) (Arnott et al., 1967), a fibrous protein, -keratin (Fraser et al.,... 

Cell specializations include keratinized cells and ciliated cells. The keratinized epithelium contains the cytoskeletal protein keratin, and provides a tough impermeable barrier, mainly in the skin. Cihated cells have apical plasma membrane extensions composed of microtubules and are capable of beating rhythmically so as to move mucus or other substances through a duct. CiUa are common in the respiratory system. 

Terminally differentiated keratinocytes of the stratum corneum are known as comeocytes and are largely devoid of normal cellular functions, being predominantly composed of protein (keratin). The ultrastructure of the stratum corneum is described by the brick and mortar model (Elias, 1983 Figure 3). The functional implication of this architecture is that some skin penetrants must diffuse via a long and tortuous route between adjacent comeocytes, thus reducing their rate of absorption. This is known as the intercellular route. In contrast, some chemicals may diffuse equally through both comeocytes and the lipid mortar, resulting in a transcellular route. 

A protein called keratin is insoluble in water but binds to other keratin molecules to form hard structures. Proteins like keratin are known as structural proteins or fibrous proteins. Keratin is the main component in hair and fingernails. A different protein called insulin is soluble in water and achieves a certain shape as an individual molecule. Proteins like insulin are known as globular proteins. Insulin in the bloodstream regulates glucose metabolism. 

DHA works because of a reaction between its carbonyl group and a free amino group (—NH3+) of several amino adds in the skin protein keratin. Amino adds are the building blocks of the biological polymers called proteins (Chapter 19) keratin is just one such protein. The DHA produces brown-colored compounds called melanoids when it bonds to the keratins. These polymeric melanoids are chemically linked to cells of the stratum corneum, the dead, outermost layer of the skin. DHA does not penetrate this outer layer so the chemical reaction that causes tanning only affects the stratum corneum. As this dead skin sloughs off, so does your tan ... 

---