MACRO MOLECULES 



97 



(a) Molecular weights 



The molecular weights of globular proteins lie between 10,000 and 

 several million. Several attempts to arrange these molecular weights into 

 groups (Svedberg; Bergmann and Neumann) have not stood the test of 

 time, and one is forced to admit that the laws governing the molecular 

 weights of proteins are still unknown to us. 



Fig. 10 (Pauling and Corey) — Forms in which 

 a-helical polypeptide chains may be twisted to- 

 gether. An ABg bundle (six chains rolled around 

 a seventh : this is the structure of the keratin 

 found in hair and nails) or a D3 cord (three 

 rolled chains). 



50A 



(a) 



(b) 



(b) Shape 



This is sometimes visible in the electron microscope, as is the case for 

 haemocyanin (Fig. 11). In other cases, measurement of physical constants 

 permits the dimensions of the protein molecule to be calculated approxi- 

 mately. Thus, beef insulin (M.W. = 12 X 10^) is a right prism 44 A long, 

 26 A wide and 20 A thick, whilst tobacco mosaic virus (M.W. = 4 X 10') 

 is a rod 2980 A long by 150 A in diameter. 



(c) Structure 



From the sum total of the evidence it appears that the globular proteins 

 have a structure similar to the a form of the k-m-e-f group of the fibrous 

 proteins, that is, a helical twist with 3-6 residues per turn or 18 residues per 

 5 turns. However, this does not explain the compactness of the globular 

 proteins. It appears that the polypeptide ribbons are bunched into compact 

 globules and held by lateral linkages between the chains. 



H 



