14 JOHN H. NORTHROP 



Specificity of Synthesis 



The preceding outline shows that several mechanisms are known 

 which could result in the synthesis of proteins without coming in con- 

 flict with any of the accepted theories of chemical reactions. Even if 

 some of these overall mechanisms are correct, there still remains the 

 problem of the regulation of the reaction so that the desired protein only 

 is obtained and not simply a random assortment of proteins. 



Nothing is to be gained, in the absence of experimental evidence, by 

 assuming a series of enzymes, each controlling the synthesis of a single 

 protein, since we are then faced with the mechanism of synthesis of the 

 enzymes themselves. These are presumably also proteins and we are 

 therefore back at the beginning again. 



The known proteases hydrolyze practically all (denatured?) proteins 

 and hence must catalyze the synthesis of all proteins, so that these 

 enzymes can hardly account for the formation of any one protein. For 

 instance, horse hemoglobin and pig hemoglobin are both presumably 

 hydrolyzed completely to amino acids by the autolytic enzymes of either 

 the horse or pig. If, therefore, conditions were found in which the re- 

 action were reversed, addition of either of these enzymes would result 

 in the formation of at least two hemoglobins, horse and pig. In the 

 animal, however, this does not occur. Horse hemoglobin only is formed 

 in the horse and pig hemoglobin only is formed in the pig. 



It is possible that the specificity of protein synthesis is not due to the 

 specific action of the enzyme but simply to the chemical nature of the 

 protein itself. Most chemical reactions are specific in that, as a rule, very 

 few and often only one of a large number of possible reaction products 

 actually appear. Thus if the reaction between NaOH and HCl is treated 

 purely statistically, there are a great many possible reaction products. 

 Actually, however, only two products are formed — NaCl and H2O. This 

 "specificity" is not due to any catalyst but simply to the fact that the 

 products, NaCl and HoO, are the only stable ones under the condition 

 of the reaction. It is therefore possible that the formation or synthesis 

 of specific proteins is regulated by the difference in stability of the many 

 structurally different proteins rather than by specific catalysts. 



Autocatalysis 



Autocatalysis is the only known chemical reaction by which indefinite 

 quantities of a required substance may be obtained without assuming 

 the existence of several other substances, the origin of which must also 



