262 ORIGIN OF STRUCTURES AND FUNCTIONS 



enzymes was based on selection for autocatalysis, the proteins 

 of the present time would have that property to the same 

 extent as they have their specific catalytic effects. But this 

 is not so. An example of the formation of enzymes by simple 

 autocatalysis, which caused a great sensation at one time in 

 the scientific literature, is the formation of trypsin from 

 trypsinogen.^^" In this case there certainly did seem to occur 

 an autocatalytic increase in the number of molecules of the 

 enzyme. If a small amount of trypsin is added to a solution 

 of trypsinogen (which is not proteolytic and may be con- 

 sidered as the nutrient medium in this case) additional 

 amounts of the active enzyme are rapidly formed. One test 

 tube of trypsinogen which has been ' inoculated ' with trypsin 

 may be used for the inoculation of fresh ' media ' and the 

 process may be repeated again and again (as is done in the 

 subculturing of bacteria) with the formation of ever more of 

 the enzyme. 



However, a more careful study of the mechanism whereby 

 this phenomenon is produced shows that, in this case, we 

 are not, in fact, dealing with the synthesis of trypsin de novo. 

 The enzyme is present in trypsinogen in its entirety, but its 

 activity is blocked by a peptide which is combined with it 

 (just as the ignition key of a car is rendered useless for start- 

 ing the engine when it is immobilised in the lock of the 

 door). The proteolytic activity of trypsin depends on the 

 activation of trypsinogen simply by the removal of the key 

 from the lock and has nothing to do with the autocatalytic 

 synthesis of fresh enzyme molecules. ^^^ 



The same may be said of the formation of pepsin by 

 analogous means, from pepsinogen. ^^^ As regards other 

 enzymes, in particular enzymes such as carboxylase or cata- 

 lase, they do not even give a semblance of forming themselves 

 autocatalytically. Like the other proteins of protoplasm, they 

 can only come into being there as a result of a very compli- 

 cated biosynthetic process. 



At present biochemists are only beginning to collect the 

 facts in the field of protein synthesis. The scientific literature 

 concerned with this subject, therefore, contains very few 

 firmly established theories but many more or less plaus- 

 ible hypotheses of various sorts and extremely ingenious 



