STRUCTURAL AND CHEMICAL ARCHITECTURE OF HOST CELLS 171 



2. Carhoxyl Activation in Model Systems 



The evidence for such a carboxyl activation of free amino acids has been 

 summarized by Borsook (1956), Novelli and DeMoss (1957), and others. 

 A number of systems were studied initially in which the RiCO— NHRg bond 

 was made in nonprotein compounds. Although some differences were ob- 

 served from reaction to reaction, it was evident that in each case the car- 

 boxyl group was activated by a mechanism involving the cleavage of one of 

 the pyrophosphate bonds of ATP. Details of the reaction mechanisms for 

 the following systems have been discussed by Borsook (1956). 



1. In the synthesis of hippuric acid in mannnalian hver the over-aU reac- 

 tion was: 



benzoic acid -f glycine + ATP > benzoyl glycine (bippuric acid) + AMP -j PP 



Dissection revealed an intermediary role for coenzyme A. The mechanism 



of the reaction is considered to involve the following sequence of reactions:^ 



Enzyme (E) + ATP ^ E-AMP + PP (1) 



E-AMP + coASH ^ E-ScoA + AMP (2) 



E-ScoA 4- benzoic acid ^ benzoyl -ScoA -J- E (3) 



Benzoyl-ScoA + glyciiie —^ benzoyl-glycine + coASH (4) 



2. In the synthesis of pantothenic acid by E. coli, the overall reaction is 



pantoic acid + /3-alanine -f- ATP ^ pantothenic acid + AMP + PP 

 In this case, however, coenzyme A is not involved. A ternary complex 

 containing enzyme-bound pantoyl AMP is postulated. 



ATP + pantoate + E % E. pantoyl. AMP + PP (1) 



E. pantoyl. AJVIP + jS-alaniiie > pantothenate + AIVIP + E (2) 



3. In the synthesis of glutamine, ADP and P are products as foUows: 



glutamic acid + NHg + ATP > glutamine + ADP + P 



Some intermediate steps are suggested to be: 



ATP 



/ 

 E 4- ATP + glutamate ^ E (1) 



\ 



glutamate 



ATP 



/ Mg++ 



E + NH3 ^=^ glutamine + ADP 4- P + E (2) 



\ 



glutamate 



^ In the enzymatic synthesis of phenylacetyl glutamine by enzymes of hmnan liver 

 (Moldave and Meister, 1957), phenylacetate and glutamine condense in the presence of 

 ATP and coA. The same mixture of enzymes and cofactors can form hippuric acid from 

 benzoic acid and glycine. The analysis indicated the initial formation of acyl 

 adenylates which exchange with coA to form phenylacetyl-S coA, which then reacts 

 with the amino acid. The hippurate system then in its initial step is perhaps more com- 

 parable to pantoate activation than suggested by the mechanism proposed in the text. 



