Cross-linking keratin

Keratins - ot-Keratins are the major proteins of hair and fingernails and a compose a major fraction of animal skin, oi-keratins are classified in the broad group of intermediate filament proteins, which play important structural roles in nuclei, cytoplasm, and cell surfaces. Their secondary structure is composed predominantly of -helices. Figure 6.11 shows the coiled-coil structure of the ot-keratin in hair. The chemical composition of the cysteine residues in ot-keratin affects its macromolecular structure and function. For example, hair has relatively few cysteine cross-links, whereas fingernails have many such cross-links, / -keratins, on the other hand, contain much more pleated sheet secondary strucure than ot-keratins and are found in feathers and scales. 

As the skin has evolved to impede the flux of toxins into the body and minimize water loss, it shows a very low permeability to the penetration of foreign molecules [169]. A unique hierarchical structure of lipid-rich matrix with embedded corneocyte in the upper strata (15 pm) of the skin—the stratum corneum (SC)—is essentially responsible for this barrier. The corneocytes, comprising cross-linked keratin fibers, are about 0.2-0.4 pm thick and about 40 pm wide [170]. They are held together by corneodesmosomes, which confer structural stability to the SC. The SC lipids are composed primarily of ceramides, cholesterol, and fatty acids that are assembled into multilamellar bilayers. This unusual extracellular matrix of lipid bilayers serves the primary barrier function of the SC. The layer of lipids immediately adjacent to each corneocyte is covalently bound to the corneocyte and is important in maintaining barrier function. The SC... 

Keratin is the protein of hair and wool. These proteins are insoluble because of the disulfide cross-linking between cystine units. Permanent waving of... 

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). 

Wool, as a keratin, is a highly cross-linked, insoluble proteinaceous fiber, and few animals have developed the specialized digestive systems that aUow them to derive nutrition from the potential protein resource. In nature, these few keratin-digesting animals, principally the larvae of clothes moths and carpet beetles, perform a useful function in scavenging the keratinous parts of dead animals and animal debris (fur, skin, beak, claw, feathers) that ate inaccessible to other animals. It is only when these keratin-digesting animals attack processed wool goods that they are classified as pests. Very often they enter domestic or industrial huildings from natural habitats such as birds nests. 

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. 

Moore, J.F., and Ward, W.H. (1956) Cross-linking of bovine plasma albumin with wool keratin. /. Am. Chem. Soc. 78, 2414. 

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. 

Our fingernails are also composed of alpha-keratin, but keratin with a greater amount of sulfur cross-links giving a more rigid material. In general, for both synthetic and natural polymers, increased cross-linking leads to increased rigidity. 

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. 

We have noted previously (see Section 2.1) the role played in biochemistry by the thiol disulfide interconversion, and the disulfide unit is a fundamental feature of the structure of peptides and proteins. The sulfur-sulfur bond between two cysteins link remote parts of a peptide chain or cross-link two such chains. Cleavage of these bonds in the hair protein keratin, followed by reoxidation, gives hair its desired shape. 

Steinert, P. M., and Marekov, L. N. (1995). The Proteins elafin, filaggrin, keratin intermediate filaments, loricrin and small proline-rich proteins are isodipeptide cross-linked components of the human epidermal comified cell envelope. /. Biol. Chem. 270, 17702-17711. 

In dyeing the hair and the suede portion a number of factors must be allowed for. The keratin of the hair contains basically the same amino acids as the collagen of the skin but in a different ratio. The keratin of the hair includes cysteine, which cross-links the polypeptide chain and imparts stability. The collagen of the skin does not have these substances, and the cross-links are made by the tanning agent. On the other hand, only L-hydroxyproline can be found in the collagen. As a result the thermal stability is different, and in addition the isoelectric points of the two polypeptides diverge. 

Fig. 7.7. Some of the covalent links which occur in structural proteins. ABCD represents one protein, EFB C G another. The N-terminal amino acid A is linked to an o-quinone which is also linked to a lysyl residue F(=quinone tanning) BB represents two tyrosyl residues coupled by a biphenyl linkage CC represents two cysteinyl residues coupled by a cystine linkage (as in keratin) D, E and G are not cross-linked. (After Brunet, 1967.)...

Wool, which is the hair of sheep and goats, contains more than 200 different compounds, of which approximately 80% are keratins (fibrous, helical proteins). Two or three helices are chemically interconnected or cross linked by, for example, S-S bonds to form a protofibril. Eleven... 

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... 

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