114 Essays in Biochemistry 



view that peptides are absent from living cells is presented by the 

 problem of the chemical mechanism of the intracellular degradation 

 of proteins. If peptides are not intermediates in this process, is it 

 suggested that protein degradation, like protein synthesis, occurs on a 

 ''template"? 



In the face of the present situation in regard to the question of 

 specificity in the biosynthesis of proteins, it seems profitable to employ 

 model systems as objects of study as has been done in attempts to 

 determine the role of ATP in peptide synthesis. The model systems 

 that have proved to be of special interest have involved tissue pro- 

 teinases as the catalytic agents and, as substrates, peptides or amino 

 acid derivatives of well-defined structure. It has long been customary 

 to speak of proteinases solely as catalysts of hydrolytic reactions, but 

 the work of the past few years has shown clearly that this is too 

 restrictive a formulation since proteolytic enzymes are now known to 

 catalyze replacement (or transfer) reactions of the following type: 



PvCO— NHR' + NH 2 X ^ RCO— NHX + NH 2 R' 



Such reactions have been termed "transamidation" or "transpep- 

 tidation" reactions. 13 Extensive studies of the action of purified pro- 

 teinases (e.g., chymotrypsin, papain, ficin, cathepsin C) in catalyzing 

 the hydrolysis of peptide derivatives have defined many of the struc- 

 tural features required in the substrate for enzymic action. All experi- 

 ments performed thus far on the catalysis of transamidation by these 

 proteinases have shown that the specificity with respect to the struc- 

 ture of the compound containing the sensitive CO-NH bond is the 

 same for transamidation as for hydrolysis. However, in the catalysis 

 of replacement reactions, where water is no longer the common reactant, 

 the nature of the amine that serves as the replacement agent also is 

 important in definining the specificity of enzyme action. For example, 

 in the reaction catalyzed by the plant proteinase papain, in which 

 carbobenzoxyglycinamide is the component having the sensitive CO- 

 NH bond (only the terminal CO-NH 2 bond is susceptible to enzyme 

 action), and glycylglycine, L-leucylglycine, or D-leucylglycine is the 

 replacement agent, only about 13% of the amide reacts with glycyl- 

 glycine under conditions (pH 7.5, 37°) where about 65% of the amide 

 reacts with L-leucylglycine, and about 8% with D-leucylglycine. 14 The 

 extent of transamidation observed in each of these three experiments is 

 a measure of the relative efficiency of the replacement agent in compet- 

 ing with water for reaction with the amide. In the case of the papain- 

 catalyzed interaction of carbobenzoxyglycinamide and L-leucylglycine, 



