Fiber-Forming Proteins

Tubulin is a 110-kDa heterodimeric protein composed of two subunits a- and P- (-450 amino acids each) and it is the basic subunit of MT. MT are hollow cylindrical protein fibers formed by the lateral association of protofilaments, where each protofilament is a linear polymer of tubulin heterodimers that are bound head to tail. The parallel arrangement of protofilament forms the cylindrical MT wall. Typically, MT contain 13 protofilaments in cross-section aligned with the same polarity. The two ends of a MT are not identical. The minus end is crowned by a-tubulin subunits and serves as the site of nucleation at the centrosome, while the plus end is crowned by P-tubulin subunits and faces outward from the nucleus to the plasma membrane. The repeating subunits are held together in the polymers by non-covalent interactions [4-6],... 

Stmctural components of cells and tissues are made of proteins that form fibers. These fibrous proteins are the cables, girders, bricks, and mortar of organisms. 

Complex viruses Some virions are even more complex, being composed of several separate parts, with separate shapes and symmetries. The most complicated viruses in terms of structure are some of the bacterial viruses, which possess not only icosahedral heads but helical tails. In some bacterial viruses, such as the T4 virus of Escherichia coli, the tail itself is a complex structure. For instance, T4 has almost 20 separate proteins in the tail, and the T4 head has several more proteins. In such complex viruses, assembly is also complex. For instance, in T4 the complete tail is formed as a subassembly, and then the tail is added to the DNA-containing head. Finally, tail fibers formed from another protein are added to make the mature, infectious virus particle. 

Reactive dyes contain substituent that, when activated, react with the -OH groups of cellulose (i.e., cotton) or with -NH2 and -SH groups of protein fibers (i.e., wool) forming covalent bonds, making them among the most permanent of dyes. 

In proteins, specific combinations of the dihedral angles c ) and / (see p. 66) are much more common than others. When several successive residues adopt one of these conformations, defined secondary structures arise, which are stabilized by hydrogen bonds either within the peptide chain or between neighboring chains. When a large part of a protein takes on a defined secondary structure, the protein often forms mechanically stable filaments or fibers. Structural proteins of this type (see p. 70) usually have characteristic amino acid compositions. 

The cytoplasm of eukaryotic cells is traversed by three-dimensional scaffolding structures consisting of filaments (long protein fibers), which together form the cytoskeleton. These filaments are divided into three groups, based on their diameters microfilaments (6-8 nm), intermediate filaments (ca. 10 nm), and microtubules (ca. 25 nm). All of these filaments are polymers assembled from protein components. 

Intermolecular - properties utilizing the ability of proteins to form junctions of its own molecules to themselves or to other components including viscosity, thickening, gelation, film formation, foaming, fiber formation, adhesion, cohesion, stickiness, hardness, complex formation, spreading, elasticity, and plasticity. 

C. Note that protein fiber formation is temperature dependent. A continuous fibrous matrix forms at about 130°C. 

Our own skin is made up of specialized cells which become filled with microfibrils of keratin as they move outward and become the relatively dry nonliving external surface (Box 8-F). Internal epithelial cells secrete protein and carbohydrate materials that form a thin basement membrane around the exposed parts of the cells. The connective tissue that lies between organs and which also includes tendons, cartilage, and bone consists of a relatively small number of cells surrounded by a "matrix" consisting of the protein fibers collagen and elastin in a "ground substance" rich in proteoglycans (Chapter 4).616 618 in bone, the calcium phosphate is deposited within this matrix. 

Selenium plays a special role in development and protection of spermatozoa (Chapter 15). Tire selenoprotein phospholipid hydroperoxide glutathione peroxidase (PHGPx Eq. 15-58, Table 15-4) has a high activity in the testis and in spermatids. However, in mature spermatozoa it forms an enzymatically inactive oxidatively crosslinked capsular material around the midpiece of the cell perhaps providing mechanical stability.268 A similar 34-kDa selenoprotein is present in sperm nuclei and may be essential for condensation of DNA.269 Sperm tails contain specialized cytoskele-tal proteins which form "outer dense fibers."270 In contrast to mammalian spermatozoa, nematode sperm move by ameboid motility that depends upon a specialized actin-like molecule.271 Sperm cells are unusually rich in polyamines, most of which are bound to RNA and DNA (Chapter 24). 

Centrifugation is an important step in starch isolation that separates starch from fine fibers, insoluble or soluble protein, and gum or mucilage compounds. Centrifugation (usually 20 to 30 min at 5000 to 8000 x g, 20°C) settles the starch (a white layer) at the bottom of the centrifuge tube (Fig. E2.1.2). The water-insoluble contaminants (e.g., protein, fine fiber) form a dark or brown layer on top of the starch layer. This contaminant layer is normally removed manually by scraping with a spatula, being careful not to scrape away any of the starch layer. 

Leveling dyes are relatively small molecules which form a saltlike bond with the protein fiber. 

Collagen is the name given to a family of structurally related proteins that form strong insoluble fibers. Collagens consist of three polypeptide chains, the identity and distribution of which vary between collagen types. The different types of collagen are found in different locations in the body. 

All this is valid from a theoretical point of view. In practice, foods contain several minor and major constituents they form a special rheological matrix of water, carbohydrates, lipids, proteins, fibers, mineral compounds, and so on, and human metabolism resorts to more than 100 types of enzymes to decompose it. 

Elastic fibers are usually found in tissues rich in smooth muscle or tissues containing fibroblasts possessing some of characteristics of smooth muscle cells (4). There is a recent report, however, that suggests elastin-like proteins may be secreted from chondrocytes (25). When elastin is secreted, it is accompanied by other proteins that appear to be important to its alignment into fibrils. One of these proteins is referred to as microfibrillar protein (cf. Table I, ref. 2). When elastin is secreted, it combines with the microfibrillar protein to form a complex which is initially rich in the microfibrillar protein. 

The foaming and emulsifying capacity of yeast proteins have been studied (83,84). Rha et al. (85) have summarized the limited information on the spinning, i.e. fiber forming ability of proteins from C. utilis. 

Amino Acid Analysis. Protein fibers, such as wool and alpaca, consist of macromolecules that are naturally formed during the growth of the hair. These large molecules are actually copolymers of about 20... 

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