Protein fibrous

The fibrous proteins are naturally occurring axially ordered systems, with the cross-links imposed on the ordered structure. Therefore, according to theory,(4) 

Charlesby, A., Atomic Radiation and Polymers, Pergamon Press (1960). 

Radiation Chemistry of Macromolecules, Academic Press (1972). 

Principles of Polymer Chemistry, Cornell University Press (1953) p. 464. Treloar, L. R. G., The Physics of Rubber Elasticity, Oxford University Press (1947). 

Shibayama, M., H. Takahashi, H. Yamaguchi, S. Sakurai and S. Nomura, Polymer, 

Most fibrous proteins have regular extended structures representing a structural complexity intermediate between pure secondary and tertiary structures of globular proteins. This conformational regularity is derived from regularities in their amino acid sequences. Table 5.9 lists some of their structural elements. 

Two representative membrane folds are illustrated for (A) all a-helices of rhodopsin (lL9H.pdb) with seven membrane traversing a-helical segments and (B) 16-stranded 3-barrel structure of porin (2POR.pdb). 

Amphipathic a-heUces associate via their hydrophobic faces in a knobs-into-holes side-chain arrangement, a-Hehces are characterized by heptad repeats (repeated seven residues, a-g, within heUx). The helices interact via hydrophobic residues between residues a and d to form an apolar stripe along one side and an electrostatic interaction between residues e and g on the other side of each helix. 

Right-handed P-rolls are formed by stacked cods of two or more P-strands built of short repeats. The silkworm fibroin consists of antiparallel P-sheets with 50 repeated - (GA)2GSGAAG-(SGAG)gY-sequences. The P-sheets are stacked on top of each other with Gly tending in contact with each other alternating with contacting Ala and Ser to give close-packed sheets. 

Three polypeptide chains of collagen are coiled together, each with a slightly twisted, left-handed, three-fold helical formation. Collagen polypeptides are distinctive in their repetitive sequences, -Gly-X-Y- with a preponderance of Pro as X or Y in which many Pro and Lys at Y are hydroxylated. These hydroxyPro and hydroxyLys form hydrogen bonds to stabilized the triplet heUx. 

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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. 

Residues 50-64 of the GAL4 fragment fold into an amphipathic a helix and the dimer interface is formed by the packing of these helices into a coiled coil, like those found in fibrous proteins (Chapters 3 and 14) and also in the leucine zipper families of transcription factors to be described later. The fragment of GAL4 comprising only residues 1-65 does not dimerize in the absence of DNA, but the intact GAL4 molecule does, because in the complete molecule residues between 65 and iOO also contribute to dimer interactions. 

Proteins are usually separated into two distinct functional classes passive structural materials, which are built up from long fibers, and active components of cellular machinery in which the protein chains are arranged in small compact domains, as we have discussed in earlier chapters. In spite of their differences in structure and function, both these classes of proteins contain a helices and/or p sheets separated by regions of irregular structure. In most cases the fibrous proteins contain specific repetitive amino acid sequences that are necessary for their specific three-dimensional structure. 

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 coiled-coil fibrous proteins have heptad repeats in their amino acid sequence and form oligomers—usually dimers or trimers—through their coiled coils. These oligomeric units then assemble into fibers. 

As described in Chapter 2, the first complete protein structure to be determined was the globular protein myoglobin. However, the a helix that was recognized in this structure, and which has emerged as a persistent structural motif in the many hundreds of globular proteins determined subsequently, was first observed in x-ray diffraction studies of fibrous proteins. 

Biological fibers, such as can be formed by DNA and fibrous proteins, may contain crystallites of highly ordered molecules whose structure can in principle be solved to atomic resolution by x-ray crystallography. In practice, however, these crystallites are rarely as ordered as true crystals, and in order to locate individual atoms it is necessary to introduce stereochemical constraints in the x-ray analysis so that the structure can be refined by molecular modeling. 

With a knowledge of the methodology in hand, let s review the results of amino acid composition and sequence studies on proteins. Table 5.8 lists the relative frequencies of the amino acids in various proteins. It is very unusual for a globular protein to have an amino acid composition that deviates substantially from these values. Apparently, these abundances reflect a distribution of amino acid polarities that is optimal for protein stability in an aqueous milieu. Membrane proteins have relatively more hydrophobic and fewer ionic amino acids, a condition consistent with their location. Fibrous proteins may show compositions that are atypical with respect to these norms, indicating an underlying relationship between the composition and the structure of these proteins. 

Collagen is a rigid, inextensible fibrous protein that is a principal constituent of connective tissue in animals, including tendons, cartilage, bones, teeth, skin, and blood vessels. The high tensile strength of collagen fibers in these struc-... 

Fibrous proteins, although interesting for their structural properties, represent only a small percentage of the proteins found in nature. Globular proteins, so named for their approximately spherical shape, are far more numerous. 

Fibrous protein (Section 26.9) A protein that consists of polypeptide chains arranged side by side in long threads. Such proteins are tough, insoluble in water, and used in nature for structural materials such as hair, hooves, and fingernails. 

The natural polymers known as proteins make up about 15% by mass of our bodies. They serve many functions. Fibrous proteins are the main components of hair, muscle, and skin. Other proteins found in body fluids transport oxygen, fats, and other substances needed for metabolism. Still others, such as insulin and vasopressin, are hormones. Enzymes, which catalyze reactions in the body, are chiefly protein. 

Collagen, the principal fibrous protein in mammalian tissue, has a tertiary structure made up of twisted a-helices. Three polypeptide chains, each of which is a left-handed helix, are twisted into a right-handed super helix to form an extremely strong tertiary structure. It has remarkable tensile strength, which makes it important in the structure of bones, tendons, teeth, and cartilage. 

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