Polypeptides keratin

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). 

Perutz, M.F. New x-ray evidence on the configuration of polypeptide chains. Polypeptide chains in poly-g-benzyl-t-glutamate, keratin and haemoglobin. Nature 167 1053-1054, 1951. 

Keratins are alpha-type fibrous polypeptides with a diameter of 7 11 nm. They are important components of the cytoskeleton in almost all epithelial cells as well as in some nonepithelial cell types. Keratins are generally held to be the most ubiquitous markers of epithelial differentiation. So far, 20 distinct types numbered by Moll et al. (1982a, 1990, 1992) have been revealed. Keratins were earlier thought to be separable into hard and soft, or cytokeratins and other keratins, but these designations are now understood to be incorrect. In 2006, a new nomenclature (Schweizer et al. 2006) was adopted for describing keratins which takes this into account (Table 13.1). 

Altmannsberger M, Dirk T, Droese M, Weber K, Osborn M (1986) Keratin polypeptide distribu tion in benign and malignant breast tumors subdivision of ductal carcinomas using monoclo nal antibodies. Virchows Arch B Cell Pathol Incl Mol Pathol 51(3) 265 275 Assefnia S, Jones LP, Torre KM, Furth PA (2006) Expression of p63 in a mouse model of Brcal mutation related breast cancer. Poster presented at the Annual Meeting of the American Association for Cancer Research, Washington, DC (Meeting Abstracts)... 

Moll R, Franke WW, Vole Platzer B, Krepler R (1982b) Different keratin polypeptides in epider mis and other epithelia of human skin a specific cytokeratin of molecular weight 46,000 in epithelia of the pilosebaceous tract and basal cell epitheliomas. J Cell Biol 95(1) 285 295 Moll R, Krepler R, Franke WW (1983) Complex cytokeratin polypeptide patterns observed in certain human carcinomas. Differentiation 23(3) 256 269 Moll R, Dhouailly D, Sun TT (1989) Expression of keratin 5 as a distinctive feature of epithelial and biphasic mesotheliomas. An immunohistochemical study using monoclonal antibody AE14. Virchows Arch B Cell Pathol Incl Mol Pathol 58(2) 129 145 Moll R, Schiller DL, Franke WW (1990) Identification of protein IT of the intestinal cytoskeleton as a novel type I cytokeratin with unusual properties and expression patterns. J Cell Biol 111(2) 567 580... 

An individual polypeptide in the a-keratin coiled coil has a relatively simple tertiary structure, dominated by an a-helical secondary structure with its helical axis twisted in a left-handed superhelix. The intertwining of the two a-helical polypeptides is an example of quaternary structure. Coiled coils of this type are common structural elements in filamentous proteins and in the muscle protein myosin (see Fig. 5-29). The quaternary structure of a-keratin can be quite complex. Many coiled coils can be assembled into large supramolecular complexes, such as the arrangement of a-keratin to form the intermediate filament of hair (Fig. 4-1 lb). 

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. 

Before a polypeptide chain can be degraded it is usually necessary to break any disulfide bridges.218 220 For some proteins such as the keratins of hair, these... 

The sequence of amino acids in the chain of a protein is of critical importance in the biological functioning of the protein, and its determination is very difficult. The chains may be relatively straight, or they may be coiled or helical, In the case of certain types of polypeptides, such as the keratins, they are crosslinked by the disulfide bonds of cystine. Linear polypeptides can be regarded as proteins. See also Amino Acids and Proteins. 

Figure 25-17 Representation of the quaternary structure of a-keratin showing (a) three a-helical polypeptide strands coiled into a rope and (b) eleven units of the three-stranded rope arranged to form one microfibril...

The Structure of the a-Keratins Was Determined with the Help of Molecular Models The fi-Keratins Form Sheetlike Structures with Extended Polypeptide Chains Collagen Forms a Unique Triple-Stranded Structure Globular Protein Structures Are Extremely Varied and Require a More Sophisticated Form of Analysis Folding of Globular Proteins Reveals a Hierarchy of Structural Organization... 

The fi-Keratins Form Sheetlike Structures with Extended Polypeptide Chains... 

Although living organisms contain additional types of fibrous proteins, as well as polysaccharide-based structural motifs, we focused here on the three arrangements that are the most widely distributed. Two of these, the a-keratins and the /3-keratins incorporate polypeptide secondary structures that also commonly occur in globular proteins. Colla-... 

The a-keratins are composed of right-handed helical polypeptide chains in which all the peptide NH and carbonyl groups form intramolecular hydrogen bonds. When these helical coils interact, they form left-handed coiled coils. 

The /3-keratins consist of extended polypeptide chains in which adjacent polypeptides are oriented in either a parallel or an antiparallel fashion. Sheets formed from such extended polypeptide chains may be stacked on top of one another. 

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