J. S. FRUTON 



the heterogeneous character of the protein hydrolyzates employed by 

 previous workers, has permitted a closer study of various factors which 

 play a role in the synthesis of peptide bonds by proteolytic enzymes. 

 The most important of these factors are: (a) the specificity of the 

 enzyme action; (b) the role of activators in the enzyme action; and 

 (c) the energy relationships involved in peptide synthesis. 



With respect to the specificity of peptide synthesis, it may be 

 sufRcient, at this point, to recall that the action of a proteolytic enzyme 

 is to catalyze the attainment of equilibrium between a peptide and 

 its hydrolytic products. Consequently, one should expect the speci- 

 ficity of synthesis to be the same as that of hydrolysis. Indeed, it has 

 been found experimentally that, if the chemical nature of one of the 

 groups near a peptide linkage is altered so that a given enzyme no 

 longer is able to hydrolyze that linkage, then a similar structural 

 change in the components for the enzymic synthesis will also prevent 

 the formation of the peptide. 



As is well known, several of the intracellular proteolytic enzymes 

 of animals and plants require, for their full catalytic activity, the 

 addition of sulfhydryl compounds as activators. The view was ex- 

 pressed some years ago (26) that, by oxidation of the sulfhydryl groups 

 of a proteolytic enzyme into disulfide groups, the enzyme would cause 

 peptide synthesis instead of hydrolysis. Experiments with model 

 substrates soon showed, however, that the activation requirements 

 were the same for the synthetic as for the hydrolytic reaction (7). 



Turning now to the energy relationships involved in peptide 

 synthesis, we should recall that the hydrolysis of peptide bonds in 

 proteins and peptides proceeds spontaneously in the presence of a 

 suitable enzyme and that the equilibrium which is established is very 

 far on the side of hydrolysis. Thus, in order to reverse the hydrolytic 

 reaction, energy is required. Borsook (14) has calculated, from 

 thermal data, that the energy needed for the synthesis of a peptide 

 bond is approximately 3000 calories per mole. This energy may be 

 obtained in a variety of ways. In the case of the synthesis of benzoyl- 

 leucylleucinanilide, mentioned earlier, the driving force for synthesis 

 comes from the removal, by crystallization, of the synthetic product 

 from the solution. In order to restore the balance of the equilibrium 

 reaction, synthesis occurs, which, in turn, causes more of the synthetic 

 product to crystallize. 



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