Keratins Quill, Wool

Keratin is another group of fibrous proteins. These compounds are major components of hair, skin, wool, horns, hooves, fingernails, claws, and feather quills. These proteins contain a lot of cysteine, one of the few amino acids that has sulfur in it. 

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. 

Following the work on fibrin, the next protein to be extensively studied as far as the isopeptide was concerned was the heterogeneous structural protein keratin. In their work, Asquith et al. ( ) identified and isolated G-L from native wool keratin and subsequently isolated the aspartyl analogue e-(-B-ASPARTIC) LYSINE (A-L) from the same source (26). Other workers went on to identify the isopeptides in human hair, guinea pig hair, porcupine quills and protein from hair follicles (27, ). Other tissues where isopeptides have been located include avian and mouse muscle ( ), human stratum corneum ( ), erthyrocyte membranes... 

The group of proteins commonly known as high-tyrosine proteins (Mr less than 10,000) vary widely in their distribution and relative proportions across the various mammalian fiber types and other keratins. These proteins range from virtual absence in Lincoln wool and human hair to about 30-40% in echidna quill [240]. 

Fibrous proteins, being insoluble in water often have a structural or protective function. The most familiar fibrous proteins are the keratins and collagen. a-Keratin (Figure 25.18) is based on the a-helix secondary structure and is the protein structural component of hair, wool, nails, claws, quills, horns, and the outer layer of skin. P-Keratin is based on the P-sheet secondary structure and occurs in silk as fibroin. L-Cysteine is especially abundant in keratins, where it can account for more than 20% of the amino acids present. Collagen occurs mainly in connective tissue (cartilage and tendons) and has a triple helix structure. 

Alpha keratin fibers occur in hairs, wool, quills, and together with fibroin fibers such as silks and spiders webs are all highly extensible fibrous protein while collagen is a relatively inextensible fibrous protein. Because of their commercial applications and the relatively complex structure at the molecular and near molecular level, the interpretation of the physical properties of a-keratin fibers has been object of recent studies [188]. 

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