Fibrous proteins keratin

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. 

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,... 

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.,... 

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. 

The hydrophilic and biodegradable c-poly(glutamic acid) has been used to modify chitosan matrices, and the resulting cytocompatible composite biomaterial showed to be suitable for tissue engineering applications [56]. Another potential skin replacement blend has been prepared using chitosan and the cysteine-rich major structural fibrous protein keratin that supported fibroblast attachment and proliferation, demonstrating to be a good substrate for mammalian cell culture [137]. 

The fibrous proteins (keratin) are likewise highly organized see Figure 6.44... 

Proteins can be broadly classified into fibrous and globular. Many fibrous proteins serve a stmctural role (11). CC-Keratin has been described. Fibroin, the primary protein in silk, has -sheets packed one on top of another. CoUagen, found in connective tissue, has a triple-hehcal stmcture. Other fibrous proteins have a motile function. Skeletal muscle fibers are made up of thick filaments consisting of the protein myosin, and thin filaments consisting of actin, troponin, and tropomyosin. Muscle contraction is achieved when these filaments sHde past each other. Microtubules and flagellin are proteins responsible for the motion of ciUa and bacterial dageUa. 

Alpha helices are sufficiently versatile to produce many very different classes of structures. In membrane-bound proteins, the regions inside the membranes are frequently a helices whose surfaces are covered by hydrophobic side chains suitable for the hydrophobic environment inside the membranes. Membrane-bound proteins are described in Chapter 12. Alpha helices are also frequently used to produce structural and motile proteins with various different properties and functions. These can be typical fibrous proteins such as keratin, which is present in skin, hair, and feathers, or parts of the cellular machinery such as fibrinogen or the muscle proteins myosin and dystrophin. These a-helical proteins will be discussed in Chapter 14. 

Fibrous proteins can serve as structural materials for the same reason that other polymers do they are long-chain molecules. By cross-linking, interleaving and intertwining the proper combination of individual long-chain molecules, bulk properties are obtained that can serve many different functions. Fibrous proteins are usually divided in three different groups dependent on the secondary structure of the individual molecules coiled-coil a helices present in keratin and myosin, the triple helix in collagen, and P sheets in amyloid fibers and silks. 

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. 

Fibrous proteins are long-chain polymers that are used as structural materials. Most contain specific repetitive amino acid sequences and fall into one of three groups coiled-coil a helices as in keratin and myosin triple helices as in collagen and p sheets as in silk and amyloid fibrils. 

The strength of fibrous proteins is enhanced by covalent cross-links between polypeptide chains within the multihelical ropes and between adjacent chains in a supramolecular assembly. In a-keratins, the cross-links stabilizing quaternary structure are disulfide bonds (Box 4-2). In the hardest and toughest a-keratins, such as those of rhinoceros horn, up to 18% of the residues are cysteines involved in disulfide bonds. 

In many cases there are important interactions between protein molecules that may lead to highly organized structures such as the pleated sheet of silk fibroin (Figure 25-13) or the coiling of a helices, as found in a-keratins, the fibrous proteins of hair, horn, and muscles (Figure 25-17). This sort of organization of protein molecules is called quaternary structure and is an important feature of many proteins that associate into dimers, tetramers, and so on. The tetrameric structure of hemoglobin is an important example. 

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