94 



UNITY AND DIVERSITY IN BIOCHEMISTRY 



and more or less parallel to the axis of the fibre. The structural proteins fall 

 into this class. Their molecular weights are very high but hard to define 

 since they are generally insoluble. 



By contrast, the molecular weights of the globular proteins are definite 

 (between 10,000 and several million) and their molecules are more or less 

 spherical. Most often they may be crystallized and they are soluble in 

 aqueous solvents (water or aqueous solutions of salts, acids, bases or 

 alcohols, depending on the particular protein). They may be denatured. 

 Active substances like enzymes, hormones, etc., belong to this class. 



Fig. 7 (Springall) — Polypeptide ribbon in zig-zag form (^ form) 



The fibrous proteins have many similarities to the synthetic polymers. 

 The latter are not very soluble and give infra-red absorption spectra and 

 X-ray diffraction spectra very similar to those obtained from the fibrous 

 proteins. The globular proteins do not resemble synthetic polypeptides. 

 However, complete denaturation transforms them into substances similar 

 to polypeptides. 



(a) Fibrous Proteins 



Organisms often employ fibrous proteins for supporting material. This 

 is most particularly the case in animals, for plants delegate the same 

 function preferentially to polysaccharides. Among the fibrous proteins are 



