A-Keratins

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. 

The central rod domain of a keratin protein is approximately 312 residues in length. What is the length (in A) of the keratin rod domain If this same peptide segment were a true u-helix, how long would it be If the same segment were a /3-sheet, what would its length be ... 

Hayward AF (1976) Ingestion of colloid in a keratinized epithelium and its localization in membrane-coating granules. J Anat 121 313-321... 

Figure 26.5 Immunofluorescent staining for transformed airway epithelial cells with antibodies specific for (A) keratin 18 (a marker of epithelial cells) and (B) ZO-1 (marker of tight junction formation). Both markers indicate that the cells have retained epithelial characteristics after transformation. The staining for the presence of ZO-1 at the periphery of the cells indicates that the cells have not lost their polarity and can form tight monolayers that will generate a transepithelial resistance. This is a particularly attractive feature for the analysis of ion transport and transcellular transport of macromolecules. ZO-1 is also found in the nucleus and can also be detected by the anti-ZO-1 antibody.

In the early 1930s W. T. Astbury and his coworkers observed that the stretched, moist hair showed a drastic change in its X-ray diffraction pattern, compared with the dry, unstretched hair. This was interpreted as two forms of the pol q)eptide chain. One was the extended form, p-keratin, eventually called the P-pleated sheet. The other was the coiled form, a-keratin, eventually called the a-helix. 

The secondary structure is responsible for some of the physical properties of proteins. For example, structural proteins such as a-keratins in skin and hair are fibrous in nature, and have good elastic... 

The a-keratin helix is a right-handed a helix, the same helix found in many other proteins. Francis Crick... 

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). 

FIGURE 4-11 Structure of hair, (a) Hair a-keratin is an elongated a helix with somewhat thicker elements near the amino and carboxyl termini. Pairs of these helices are interwound in a left-handed sense to form two-chain coiled coils. These then combine in higher-order structures called protofilaments and protofibrils. About four protofibrils—32 strands of a-keratin altogether—combine to form an intermediate filament. The individual two-chain coiled coils in the various substructures also appear to be interwound, but the handedness of the interwinding and other structural details are unknown, (b) A hair is an array of many a-keratin filaments, made up of the substructures shown in (a). 

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. 

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