The Biosynthesis of Peptide Bonds 117 



thai rapidly catalyzes a series of successive peptide syntheses without 

 releasing into the cellular fluids appreciable amounts of peptide inter- 

 mediates, and if the nature of this assembly is genetically controlled, 

 then much of the difference in opinion becomes a difference in language. 



The significance of the proteinase-catalyzed transamidation reac- 

 tions that have been studied thus far does not lie in demonstrating 

 processes involving peptide derivatives known to occur in nature, but 

 rather in suggesting new approaches to the study of the energetics 

 and specificity of the enzymic synthesis of peptides. To convert amino 

 acids into a form that is capable of reacting with other amino acid 

 residues, the participation of ATP is required, as has clearly been 

 shown in the studies on the biosynthesis of peptides such as glutathione. 

 On the other hand, once the y-glutamylcysteine bond of glutathione 

 has been formed, it is capable of reacting with other amino acids or 

 peptides in transpeptidation reactions, as has been shown by Hanes 17 

 and by Waelsch. 3 - Similarly, in the synthesis of glutamine, ATP is 

 required, but, once formed, the CO-NEP bond can react, in the presence 

 of suitable enzyme preparations, with amines such as hydroxylamine 

 or amino acids (Speck, Elliott, Thorne, and Williams). The most 

 plausible interpretation of these findings has been the suggestion that 

 a "y-glutamyl enzyme" is an intermediate both in the synthesis of 

 glutamine from glutamic acid and in the transamidation reactions of 

 glutamine. 



In the light of these important studies on the intimate relation 

 between the direct synthesis of CO-NH bonds and transamidation 

 reactions in which they may participate, it seems justifiable to extend 

 the hypothesis drawn from the reactions of y-glutamylcysteinc and of 

 glutamine to include the action of intracellular proteinases. This 

 would involve the assumption that, in the catalysis by papain of the 

 reactions of carbobenzoxyglycinamide, there is formed as an "acti- 

 vated" intermediate, a "carbobenzoxyglycylpapain," that can react 

 either with water or with an amine such as L-leucylglycine. Similar 

 "acvl enzyme" compounds could be invoked in the case of other trans- 

 amidation reactions studied. It may be expected that such "acyl 

 enzyme" compounds would be formed more readily when the reactive 

 carboxyl group is joined in a bond that has a relatively high free energy 

 of hydrolysis. This appears to be the case with the a-amides of 

 acylamino acids and peptides. 



As noted earlier in this essay, little is known about the enzymic 

 mechanisms whereby free at-amino acids are "activated" in reactions 

 involving ATP. Until direct evidence for such mechanisms is avail- 



