THE SYNTHESIS OF PROTEINS I7 



such marked specificity. Therefore cross-reactions between the pro- 

 teinogen and the native proteins derived from it would not be expected. 



It is probably unnecessary to assume that the molecular weight of the 

 proteinogen is equal to or greater than that of the largest molecules 

 which are to be formed from it since Svedberg (1937) and others have 

 found that very large protein molecules like the hemocyanins dissociate 

 very easily and reversibly. The formation of these very large molecules 

 from molecules the size of normal proteins, therefore, requires little or 

 no energy (Northrop, 1938, p. 363). 



It is also unnecessary to assume that any appreciable quantity of this 

 protein exists at any one time. It could be decomposed as rapidly as 

 formed and still act as an intermediate between the energy-requiring 

 step and the specific step in protein synthesis. 



In the second step the individual proteins are formed by a catalytic 

 or an autocatalytic reaction from this proteinogen. This reaction does 

 not require energy and may occur anywhere. 



The formation of an enzyme from its precursor is an example of 

 such a reaction. It is the only mechanism so far discovered whereby 

 proteins may actually be produced in vitro. The reactions are specific 

 and are adequate to account for the formation of proteins in general, 

 provided the precursor is present. The exact chemical changes involved 

 in these reactions are not known. 



The proposed mechanism accounts for the group specificity of pro- 

 teins from the same organ and species. Assumption of one (or a few) 

 synthetic reactions is simpler than assumption of a separate synthetic 

 reaction for each individual protein. It has the further advantage that 

 it is in keeping with the gradual development of the cell, since, once the 

 proteinogen is formed, other proteins can readily be derived from it 

 (cf. Troland, 191 7). If each protein must be synthesized individually, 

 each one requires an energy source and the evolution of a series of re- 

 lated proteins is more difficult. 



Madden and Whipple (1940) have pointed out that partial synthesis 

 of all proteins in one organ (the liver?) would greatly simplify the 

 mechanism of tissue metabolism since it would then be unnecessary for 

 every cell of every tissue to contain the complete protein-synthesizing 

 system. According to the present mechanism, the proteinogen could well 

 be synthesized in one organ and distributed from there to the various 

 other organs and tissues where it would be modified into the specific 

 proteins needed. 



It may be noted in this connection that many autocatalytic and many 



