The Biosynthesis of Peptide Bonds 115 



R R' 



I I 



Cbzo-NHCH 2 CO— NH 2 + NH 2 CHCO— NHCHCOOH ^± 



R R' 



Cbzo-NHCH 2 CO—NHCHCO— NHCHCOOH + NH 3 



the replacement agent is so much more effective than water at pH 7.5, 

 despite its lower molar concentration, that the predominant reaction is 

 one of replacement rather than hydrolysis. On the other hand, the 

 enantiomer D-leucylglycine is much less effective in this competition, as 

 is glycylglycine. Results of this kind show clearly that, in transamida- 

 tion reactions, the specificity of enzyme action is more exacting than 

 in hydrolysis. 



Studies on the mechanism of transamidation reactions have demon- 

 strated that it is the uncharged amino group of the replacement agent 

 that is reactive; the extent of transamidation at a given pH depends, 

 therefore, on the pK' of the corresponding cation. 15 For this reason, 

 the extent of transamidation observed with the above dipeptides (pK'2 

 ca. 8 1 is much greater at pH 7.5 than at pH 5, the optimum for the 

 hydrolytic action of papain. 



The reaction between carbobenzoxyglycinamide and L-leucylglycine 

 illustrates the replacement of a small group ( — NH 2 ) by a dipeptide 

 unit, thus effecting the elongation of the peptide chain from the car- 

 boxyl end of the sensitive substrate. In general, transamidation reac- 

 tions in which an a-amide is converted to a peptide appear to be 

 exergonic reactions, as suggested by the thermodynamic data presented 

 in Table 1. The exergonic nature of enzyme-catalyzed elongation of 

 peptide chains may be further illustrated by results obtained with the 

 intracellular proteinase cathepsin C, purified from beef spleen. This 

 enzyme only acts on derivatives of dipeptides composed of two a-amino 

 acid residues (X may be either NH 2 , as in an amide, or OCHo, as in 

 an ester). If cathepsin C is allowed to act on a dipeptide amide such 



R R' 



I I I 



NH 2 CHCO— NHCHCO— X 



as L-alanyl-L-phenylalaninamide at pH 7.5, the extent of hydrolysis 

 is negligible and there separates from the solution a precipitate which, 

 on chemical analysis, was found to be the amide of the hexapeptide 



( 1. 1 Ala. ( l 1 Phe. ( li Ala. (D Phe. (l I Ala. (l) Phe. lfl 



