Fibers polypeptide chains

The differences in the amino acid chemistry of the hide coUagen and the hair keratin are the basis of the lime-sulfide unhairing system. Hair contains the amino acid cystine. This sulfur-containing amino acid cross-links the polypeptide chains of mature hair proteins. In modem production of bovine leathers the quantity of sulfide, as Na2S or NaSH, is normally 2—4% based on the weight of the hides. The lime is essentially an unhmited supply of alkah buffered to pH 12—12.5. The sulfide breaks the polypeptide S—S cross-links by reduction. Unhairing without sulfide may take several days or weeks. The keratin can be easily hydrolyzed once there is a breakdown in the hair fiber stmcture and the hair can be removed mechanically. The coUagen hydrolysis is not affected by the presence of the sulfides (1—4,7). 

As a result, leather is made up of interlaced bundles of coUagen fibers (Fig. 1). A schematic model of coUagen bundles in leather is shown in Figure 2 (4). A coUagen bundle (about 80 )Tm in diameter) is made up of coUagen fibers (1—4 pm), composed of microfibrils (0.08—0.1 pm). Furthermore, a microfibril consists of many protofibrils (about 1.5 nm), which consist of several bundles of polypeptide chains. 

Progress in deducing more structural details of these fibers has instead been achieved using NMR, electron microscopy and electron diffraction. These studies reveal that the fibers contain small microcrystals of ordered regions of the polypeptide chains interspersed in a matrix of less ordered or disordered regions of the chains (Eigure 14.9). The microcrystals comprise about 30% of the protein in the fibers, are arranged in p sheets, are 70 to 100 nanometers in size, and contain trace amounts of calcium ions. It is not yet established if the p sheets are planar or twisted as proposed for the amyloid fibril discussed in the previous section. 

Figure 14.9 Spicier fibers are composite materials formed by large silk fibroin polypeptide chains with repetitive sequences that form p sheets. Some regions of the chains participate in forming 100-nm crystals, while other regions are part of a less-ordered mesh-work in which the crystals are embedded. The diagram shows a model of the current concepts of how these fibers are built up, which probably will be modified and extended as new knowledge is gained. (Adapted from F. Vollrath, Sci. Am. p. 54-58, March 1992 and A.H. Simmons, Science 271 84-87, 1996. Photograph courtesy of Science Photo Library.)...

Silk fibers, which have incredible strength, comprise well-ordered microcrystals of P-sheets that make up about 30% of the protein mass, interspersed in a matrix of polypeptide chains without order. The p strands of the sheets are oriented parallel to the fiber axis. 

X-ray diffraction reveals that the polypeptide chains in the fibrillar assemblies formed by polyAn, polyAi3, and polyA2o are in the cross-// arrangement, that is, where the //-strands run perpendicular to the fiber direction (Fig. 8) (Shinchuk et al, 2005). The X-ray patterns show broad,... 

The structure of proteins determines their function and can be described on four levels, illustrated on page 447. The primary structure is the sequence of amino acids in the polypeptide chain. The secondary structure describes how various short portions of a chain are either wrapped into a coil called an alpha helix or folded into a thin pleated sheet. The tertiary structure is the way in which an entire polypeptide chain may either twist into a long fiber or bend into a globular clump. The quaternary structure describes how separate proteins may join to form one larger complex. Each level of structure is determined by the level before it, which means that ultimately it is the sequence of amino acids that creates the overall protein shape. Fhis final shape is maintained both by chemical bonds and by weaker molecular attractions between amino acid side groups. 

The P structure is one of the most important secondary structures in proteins. It occurs in about 80% of the soluble globular proteins whose structures have been determined. In many cases almost the entire protein is made up of P structure. Single strands of extended polypeptide chain are sometimes present within globular proteins but more often a chain folds back on itself to form a hairpin loop. A second fold may be added to form an antiparallel "P meander"102 and additional folds to form P sheets. Beta structures are found in silk fibers (Box 2-B) as well as in soluble proteins. 

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. 

In contrast with synthetic polymers, proteins are characterized by very high levels of structural order. Unlike synthetic polymers, proteins are characterized by absolutely uniform chain lengths and well-defined monomer sequences (primary structure) [3]. These features are two of the requirements that enable folding of linear polypeptide chains into structurally well-defined and functional proteins. Proteins play an important role in numerous processes in biology, e.g. as carriers for small molecules and ions (examples are presented in Chapter 2.2), as catalysts, or as muscle fibers, and their exquisite properties are closely related to their well-defined three-dimensional structure [3]. 

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