Keratin amino acid composition

The composition of keratins. Amino acid composition of hair, wool, horn, and other eukeratins. Ibid., 128, 181 (1939). With cooperation of D. Bolling, F. C. Brand, and A. Schein. 

The amino acid composition of keratin, the protein of hair and wool, includes a greater-than-average proportion of the sulphur-containing amino acid, cystine. Since this is the least soluble of the protein amino acids it can readily be isolated after carefully neutralising an acid hydrolysate of hair (Expt 5.187). Protein hydrolysis is usually effected by boiling for about 10-20 hours with 20 per cent hydrochloric acid. The hydrolysis of hair for the isolation of cystine is, however, best achieved using a mixture of hydrochloric and formic acids. 

Although a helices are abundant in proteins, the average length is fairly short, that is, 17 A long containing about 11 amino acids or three turns. Therefore, a helices are typically found in short domains within proteins and not as continuous stretches. Keratins that make up hair and the most superficial layer of skin contain a central domain with an a helical component of 310 amino acids or about 46.5 nm in length. Keratin is an example of a protein with a fairly long a helix. The amino acid composition that favors a helix formation is fairly broad with the exception of proline and serine. 

The amino acid composition of keratins. Composition of gorgonin, spongin, turtle scutes, and other keratins. J. Biol. Chem., 127, 685 (1939). With D. Bolling. 

Keratinized tissues contain several types of cells and these cells in turn contain many protein constituents. It is possible therefore for keratins to differ in amino acid composition either because the constituent proteins differ or because of differences in the relative amounts of identical proteins. It is rarely possible therefore to relate differences in over-all amino acid composition to differences in character or content of a particular protein constituent. 

Amino Acid Composition of Shell Keratins from Hens Eggs... 

The most important discovery in recent years has been the clear evidence obtained by Filshie and Rogers (1961) of an organized protofibrillar substructure within the microfibril. This substructure is illustrated in Fig. 13 together with an example of the less definite evidence for longitudinal substructure. As far as is known the size and substructure of the microfibril seem to be a constant feature of a-keratins from a variety of animals. Thus the microfibril may be regarded as the fibrillar unit of structure, although it is to be anticipated that some species-to-species variation in detail will occur in view of the known variability in amino acid composition (Section III,R,1). 

It is probable that Bailey s first interest in the muscle field, in which lies his greatest contribution, was aroused by the work of Astbury and Dickinson who showed that fibers of denatured myosin behaved in ways similar to keratin so far as their elastic properties were concerned and their structures were revealed by X-ray analysis. At this time the Chibnall group was much interested in the amino acid composition of proteins. The obvious similarities in fibrous behavior between keratin and myosin despite their differences in amino acid composition, particularly in cystine content, stimulated Bailey to make a comparative study of the composition of some of the then recognized muscle proteins. This was Bailey s first paper on muscle and extension of the... 

The morphological location of the fibrous protein principally responsible for the deformation and viscoelastic behavior is uncertain. Both the cell membrane and intracellular regions are composed of fibrous proteins which differ considerably in amino acid composition. Since the alpha-keratin within the cells shows few orientation properties until high elongations, it has been suggested that the membrane proteins determine the viscoelastic behavior at low deformations (84). 

Table 6.2 lists the amino acid compositions of four different fibrous proteins, namely ot-keratin, fibroin, collagen, and elastin. Notice the relatively high abundance of amino acids with non-bulky side chains, such as glycine, alanine, serine, glutamate, and glutamine. A notable exception to this is the quite high amount of proline in collagen and, to a lesser extent, in elastin. Each of the proteins in Table 6.2, however, has a unique amino acid composition, because each is comprised of a unique sequence of amino acids. 

The data for cuticle analysis are based on the work of Bradbury et al. [16] who analyzed cuticle and whole fiber from several keratin sources, including human hair, merino wool, mohair, and alpaca. These scientists concluded that there is very nearly the same difference between the amino acid composition of the cuticle and each of these fibers from which it was derived. They listed the average percentage differences used in these calculations. More recent analyses of cuticle and whole fiber of human hair [68, 69] are in general agreement with these data [18]. 

Another method, the method of Alexander and Earland [115], consists of oxidation of the disulfide bonds of the keratin to sulfonic acid groups, using aqueous peracetic acid solution, and separation of the oxidized proteins, generally by means of differences in solubilities of the different components of the mixture. The first three fractions in this separation are called keratoses. The amino acid composition of these three fractions isolated from merino wool has been reported by Corfield et al. [116]. 

Table 9.6.2 Amino acids composition of a-keratin and of fibroin and...

The extraction method depends on the extraction of amino acids from the acid hyrolysates of proteins. Although the method was commercialized for amino acid production (proline, serine, tyrosine, etc), the availability of raw materials like hair, feather, keratin, blood meal, etc is usually a limiting factor. Moreover, the yield of amino acids depends on the amino acid composition of the raw material. 

The amino acid composition of wool fibres is given in Table 4.10. For different animal fibres, the cystine content of the keratin varies, but it is higher than in any other protein. There are differences in the structural positions of the amino acids in the keratin between the hair from different animal species, and also along the fibres. 

Table IL Amino Acid Composition of Wool Keratin (Molecular Weight 68,000 kD) (18,19)...

Silk is a natural protein fiber excreted by the moth larva Boinhvx mori. better known as the common silkworm. Silk is a fine continuous monofilament fiber of high luster and strength and is highly valued as a prestige fiber. Because of its high cost, it finds very limited use in textiles. A minor amount of wild tussah silk is produced for specialty items. Attempts have been made to commercialize silk from spiders over the years, but all ventures have met with failure. Domestic and wild silks are essentially uncrosslinked and relatively simple in amino acid composition compared to the keratin fibers. The properties for silk listed here are for silk formed by Bombvx mori moth larvae. 

The properties of keratins depend on their self-assembled structure (see also Chapter 7). They are ultimately defined by the properties of the individual polypeptide polymer chains, and the amino acid composition and sequence. The a-helix and P-sheet motifs, and disulfide bridges, are very important in the conformations of globular, functional... 

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