Keratin disulfide bonding

Cystine, which contains a disulfide bond, is reported to be the most numerous and reactive amino acid present in hair keratin. Disulfide bonds in cystine are reduced by mercaptans and phosphines, and oxidized by perborates, bromates, and bleach. These reactions result in structural rearrangements within keratin which may affect the physiochemical properties of hair, since disulfide bonds in cystine contribute to the stability of hair. For example, hydrogen peroxide bleaching of hair is an oxidative process which occurs readily in an alkaline medium. This results in the formation of perhydroxy anions which have been proposed to react with cystine to form cysteic acid residues. The process of bleaching results in the loss of approximately 15% of the cystine bonds originally present in keratin and may explain the increased permeability of bleached hair to chemicals. - ... 

Cysteine disulfide formation is one of the most important posttranslational modifications involved in protein structure. Disulfides play a crucial role in maintaining the structure of many proteins including insulin, keratin, and many other structurally important proteins. While the cytoplasm and nucleus are reducing microenvironments, the Golgi and other organelles can have oxidizing environments and process proteins to contain disulfide bonds (Scheme 5). 

Similar keratin filaments are found in hair. In a single wool fiber with a diameter of about 20 pm, millions of filaments are bundled together within dead cells. The individual keratin helices are cross-linked and stabilized by numerous disulfide bonds (see p. 72). This fact is exploited in the perming of hair. Initially, the disulfide bonds of hair keratin are disrupted by reduction with thiol compounds (see p. 8). The hair is then styled in the desired shape and heat-dried. In the process, new disulfide bonds are formed by oxidation, which maintain the hairstyle for some time. 

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. 

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. 

Wang, H., Parry, D. A. D., Jones, L. N., Idler, W. W., Marekov, L. N., and Steinert, P. M. (2000). In vitro assembly and structure of trichocyte keratin intermediate filaments A novel role for stabilization by disulfide bonding./. Cell Biol. 151, 1459-1468. 

The keratin structure is destroyed through the use of proteolytic enzymes such as pronase and proteinase K. They are often used with chemical agents such as urea and thioglycolic to cleave the disulfide bonds and increase the dissolution rate of enzyme activity. The extracting procedures, using enzymatic digestion, last about 4-6 h and must be conducted at constant temperature and pH for providing maximum enzyme activity [155],... 

Fibrous proteins also gain strength due to interactions between the side chains of the residues. The alpha-keratin polypeptides, for example, have a large number of cystine residues, which can form disulfide bonds. 

Hair is composed of approximately 65 to 95% protein, 1 to 9% lipid, and small quantities of trace elements, polysaccharides, and water. - - The majority of hair protein is often referred to as keratin, which is a general term used to describe aggregates of protein with low or high sulfur content. These proteins are synthesized in the keratogenous zone of the hair follicle as matrix cells move upward from the hair bulb to form layers of the hair shaft. The cuticle, cortex, and medulla are comprised largely of keratin, although it is structurally different in each layer. Keratin in the exocuticle contains a high concentration of cysteine, which forms disulfide bonds which link the A-layer to the exocuticle, and this makes the cuticle... 

Disulfide bonds present in the keratin of hair are potential binding sites for many nucleophilic molecules. Tolgyesi and Fang investigated the structural changes which occur in keratin as a result of alkaline treatments of hair. They reported that hydroxide ions initiated a p-ehmination reaction, resulting in cleavage of the disulfide bonds of cystine to produce dehydroalanine intermediates. Cysteine and lysine may react with dehydroalanine to form new cross links in keratin. Nucleophilic amines such as ethylamine and n-pentylamine may also react with dehydroalanine to form... 

Reduction of the disulfide bonds of keratin has been much studied not only because of its use for preparation of soluble derivatives (Section... 

The curves relating residual disulfide of /S-methylated fibers and strain at the end of the yield region are identical within experimental error for a number of different keratin fibers. This suggests that disulfide bonds in both high-sulfur and low-sulfur proteins contribute to the properties of the fiber in the post-yield region. 

Permanent waves. The shape of hair is determined in part by the pattern of disulfide bonds in keratin, its major protein. How can curls be induced ... 

Animal hair fibers consist of a protein known as keratin, It has a composition similar to human hair. Keratin proteins are actually crystalline copolymers of nylon, where the repeating units are amino acids. The fibrous proteins form crystals. They also crosslink through disulfide bonds present in the cystine amino acid. 

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