G. ROBERT GREENBERG 



does not appear to be a very important intermediate as such, in 

 order to understand how the purine rings are closed. There can 

 be little doubt that these studies have been concerned with a 

 truly artificial system, for not only formate but also the acceptor 

 compound, tetrahydrofolate, appears to be an unnatural sub- 

 strate (13). Nevertheless, these studies and those of Jaenicke 

 (17) have given some ideas of the reaction mechanism. It has 

 been clear that N^°-formyl rather than N^ derivatives (citro- 

 vorum factor is N°-formyltetrahydrofolate) of tetrahydrofolate 

 are involved. In addition ideas about pathways, energy re- 

 quirements, and oxidation-reduction systems have been ob- 

 tained from these studies. 



This is not to suggest that the results from model systems can 

 be accepted without reservation. At the present time one might 

 find it difficult to introduce coenzyme A into the problem of 

 protein synthesis merely because it had been found to be in- 

 volved in forming the peptide linkage in hippuric acid (5). 

 The results of studies on glutathione synthesis (29) may be more 

 to the point. 



COUPLED SYSTEMS 



The detailed treatment of coupled systems has been con- 

 sidered by others and is beyond the scope of this paper (7, 1 1 ) . 

 Classical examples are found in the studies of the glycolytic 

 scheme. 



Equilibrium constants and Eq values soon become a part 

 of the working vocabulary of the investigator entangled in 

 multi-enzyme systems. 



The successful coupling of two individual reactions, of 

 course, requires the proper equilibrium conditions. In oxida- 

 tion-reduction systems the requirements may be expressed in 

 terms of oxidation-reduction potentials. Theoretically we 

 should be able to decide from Eq values and from equilibrium 

 data whether a given system will interact with another, but 

 practically this may not always be the case. For example, the 



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