108 Essays in Biochemistry 



in condensation reactions that are "pulled" by subsequent processes, 

 among which some may be strongly exergonic. 2 This type of energetic 

 coupling has been clearly illustrated in model experiments. For exam- 

 ple, in the synthesis of benzoyl-L-tyrosylglycinamide from benzoyl-L- 

 tyrosine (0.025 M) and glycinamide (0.025 M), catalyzed by chymo- 

 trypsin, equilibrium is attained when only about Y/c of the reactants 

 has undergone condensation. On the other hand, if glycinamide is 

 replaced by glycinanilide, the resulting benzoyl-L-tyrosylglycinanilide 

 crystallizes from the solution in a yield of about 65%. The driving 

 force in the formation of the anilide is its removal from solution, 

 because, unlike the amide, the anilide has a solubility lower than 

 2.5 X 10~ 4 M. Thus, the endergonic synthesis of the peptide bond 

 is coupled to the exergonic process of the removal of the peptide 

 derivative from supersaturated solution. Model experiments of this 

 kind, conducted with intracellular proteinases as catalysts, provide the 

 principal support for the view that, in the synthesis of the peptide 

 bonds of proteins, such "pull" mechanisms are operative. It should 

 be emphasized, however, that there is no evidence from biochemical 

 studies with intact cells or organisms that this type of energetic 

 coupling is important in protein synthesis, although it must also be 

 noted that no investigations have yet been conducted that permit an 

 objective decision on this question. 



As mentioned before, the studies on the energy changes in the 

 hydrolysis and synthesis of peptide bonds have called attention to 

 the difference in the AF° values for condensation reactions involving 

 peptides, as compared with condensation reactions involving free amino 

 acids. Since the latter reactions are, in general, more endergonic in 

 character, it has seemed plausible to assume that, instead of a "pull" 

 type of coupled reaction, the biosynthesis of a CO-NH bond between 

 two amino acid residues is "pushed" by an exergonic reaction, such as 

 the cleavage of a pyrophosphate bond of ATP. In some of the recent 

 discussion of possible mechanisms of protein synthesis it has been 

 implied that these two types of coupling are mutually exclusive. In 

 the face of the available experimental knowledge, it would appear 

 more profitable to consider the working hypothesis that both types 

 of mechanism are involved in the biosynthesis of proteins from amino 

 acids, but at different stages of the overall process. It is implicit in 

 this hypothesis that peptides are intermediates in protein synthesis, 

 a view that has been challenged recently on the basis of experimental 

 findings to be discussed later in this essay. 



