PROTEINS 



365 



3.5i 



Fig. 181. \^olume of an amino acid residue. 



Keratin-myosin group. In extended crystallizing polypeptide chains 

 the space needed by an amino acid residue in the direction of the 

 chain axis is 3.5 A. This spacing 

 is called main chain spacing; the 

 fibre period found by X-rays is 

 usually a multiple of this value. 

 The lateral distance of the main 

 chains in the direction perpen- 

 dicular to the plane of the side 

 chains, termed backbone spacing 



by AsTBURY, is 4.5 to 4.6 A. In the third direction, the side chain 

 spacing depends on the length of the radicals R of the amino acids 

 involved. With the exception of silk fibroin, this spacing is astonishingly 

 constant, indicating an average length of the amino acid residues of 

 about 10 A. Therefore, the average volume of an amino acid residue 

 is roughly 3.5 A X 4.6 A X 10 A = 161 (A)=^ (Fig. 181). 



In Mammals ectodermal formations such as hairs, feathers, epi- 

 dermis, nails, horns and the mesodermal proteins muscle myosin, 

 blood fibrinogen and fibrin correspond to this type. Astbury (1947) 

 has therefore called it keratin-myosin-epidermis-fibrinogen (k-m-e-f) 

 group. 



The special interest of this group is the possibility of the ^^^a 

 transformation of its polypeptide chains (Fig. 171, p. 339), whereby 

 the main chain spacing of three amino acid residues is reduced from 

 10.5 A to 5.1 A. Many natural fibrous proteins exist in this folded 

 form (Table XXXII). As shown by Astbury, the a-chains can be 

 reversibly transformed into the ^^-configuration ; hence they display 

 an inherent elasticity and potential contractility. The muscle protein 

 of Invertebrates is also of this type (clam muscle of Molluscs) and 

 even the bacterial flagella of Proteus vulgaris show the characteristic 

 spacing of 5.1 A (Astbury and Weibull, 1949). 



Silk fibroin differs from the fibrous proteins of the k-m-e-f group 

 not only by its short side chain spacing but also by its lack of any 

 /S ^ a transformation. 



The specificity of the different proteins in the k-m-e-f group is due 

 to the special share of the different amino acids and their arrangement 

 along the polypeptide chain. It is thought that bonds between the 

 side chains stabilize the chain lattice both of the ^- and of the a-form. 



