Hair a-keratin

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 assembly of hair a-keratin from one a helix to a protofibril, to a microfibril, and finally, to a single hair. (Illustration copyright by Irving Geis. Reprinted by permission.)... 

Structural These proteins provide strength and elasticity to, for example, bone (collagen), hair (a-keratins) and connective tissue (elastin). 

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... 

In the tanning process hides are first washed or soaked, hair and keratinous debris are removed, bated (enzymes are used to break down non-collagenous components, which are washed out) and the hide is acid-pickled to prepare for the addition of the chromium salt. Contemporary processes are exclusively based on one-bath procedures and utilize chromium(III). The older two-bath process is now obsolete, mainly because it involved the in situ reduction of chromate, a major environmental and toxicological hazard (cf. chromate toxicity p. 947) to chromium(III) on the hide. A useful review of the history of chromium tannage processes is available.1205... 

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). 

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. 

In many cases there are important interactions between protein molecules that may lead to highly organized structures such as the pleated sheet of silk fibroin (Figure 25-13) or the coiling of a helices, as found in a-keratins, the fibrous proteins of hair, horn, and muscles (Figure 25-17). This sort of organization of protein molecules is called quaternary structure and is an important feature of many proteins that associate into dimers, tetramers, and so on. The tetrameric structure of hemoglobin is an important example. 

Hair keratin has historically been associated with the a-helical structure. For that reason, it is termed a-keratin. And indeed the basic keratin polypeptides are a-helical except for their N- and C-terminal domains. These are believed to be involved in head-to-tail condensation to form keratin polymers. When hair keratin is stretched, the resulting secondary structure is the parallel pleated sheet (see Chapter 4). Stretched keratin is referred to as /3-keratin to emphasize its secondary structure. 

As we have mentioned, there are various types of keratins, and this brings us to one final aspect of sulfur cross-link bridges. The a-keratins can be classified as soft or hard by their sulfur content The low sulfur content keratins of wool and hair are much more flexible than the hard, high sulfur... 

Hair is a keratinous material, quite strong and totally flexible. Long hair was required for fashioning, for example a bracelet needed hair that was at least 60 centimetres long. Before it was worked the hair was boiled. It was then plaited or woven like lace around a mould or firm base, and boiled again to keep the shape. 

Both the myosin in muscle and a-keratin in hair are proteins composed almost entirely of a-helices. 

Owing to the economic importance of wool most investigators have used this material as a convenient source of a-keratin. When parallel studies have been made on hairs from other animals and on nails, claws, hoofs, and quills it has been found that conclusions reached by studying wool proteins apply, with only minor qualifications, to other keratinized tissues. Feathers are only of slight economic value and correspondingly less attention has been devoted to their chemistry, despite the fact that feather proteins are more readily solubilized and purified and that feather rachis yields X-ray diffraction patterns of excellent quality. 

---