CHAPTER V 



Molecular and Macromolecular Structure 



The present status of the chemical structure of the keratins 



The chemical composition and constitution of a protein may be con- 

 sidered established when the following are known: (a) the number of 

 separate polypeptides composing the molecule and the nature of any 

 covalent cross-linkages uniting them ; (b) the amino acid sequence in each 

 of the polypeptides; (c) if a prosthetic group is present, its relation to the 

 polypeptide moiety. 



Complete solutions to (a) and (b) are available for three proteins; for the 

 keratins, which are far more complex, there is no immediate prospect of 

 even partial solutions. There is no evidence, however, to show that there 

 is anything in the nature of a prosthetic group to complicate the position 

 further. Essential information concerning a protein is provided by a 

 knowledge of its total amino acid composition and its end-group com- 

 position, i.e. the groups which terminate the main polypeptide chains. 

 For many keratins we have adequate, although not complete, deter- 

 minations of both end-group and total amino acid composition from which 

 may be inferred a general picture of the overall chemical reactivity of the 

 molecular complex (Table 7). For purposes of reference, in Tables 1, 2 and 

 3 (Chapter I) will be found the amino acid composition from a number of 

 determinations as reported by various authorities. The most studied 

 material is wool and the data relating to it are considered to be of a high 

 order of accuracy although, it is clear that precise agreement between 

 independent analysts has not been reached. Table 7, devised by Ward and 

 Lundgren (1954), presents a summary of the amounts and kinds of chemical 

 groups to be found in several keratins as calculated from their amino acid 

 composition. Assuming all these are accessible to reagents the Table 

 enables a fair prediction to be made of chemical behaviour. Similar 

 Tables will be found in Tristram's extensive compilation (1953) from 

 which the data for Fig. 2 (Chapter I) were taken. 



End Groups 



Information concerning molecular weight, the minimum number of 

 polypeptides, the presence of branched or cyclic chains and amino acid 

 composition may be obtained by determining the amino acids which form 



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