Amino acid composition, wool keratin

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. 

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 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]. 

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)...

The hair fibers derived from furry mammals are mainly made up (over 80%) of the structural protein keratin. The distinction between wool and hair is not compositional, but related to size wool fibers are generally fine and short, whereas those of hair are usually thicker and longer. The molecule of keratin consists essentially of a combination of amino acids about 18 amino acids make up the keratin molecule (see Textbox 67). The nature of the amino acids, their relative amounts, and their sequence and arrangement within the molecule of keratin vary from one animal species to another but are characteristic of any variety of wool or hair (Asquith 1977) (see Table 89). 

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