EFRAIM RACKER 



There appear to be at least two different kinds of enzyme cata- 

 lyzed exchange reactions ; these will be referred to as ( 7) transfer- 

 exchange reactions, and (2) metabolic exchange reactions. 



( 7) The transfer-exchange reactions represent a special case 

 of a general reaction, usually referred to as a transfer reaction : 



AB + G ' AC + B 



These transfer reactions are catalyzed by enzymes which are 

 widely distributed in nature. Transaminases, transglycosidases, 

 transketolase, and transaldolase are a few examples of this type 

 of reaction. These enzymes are assumed to form a complex with 

 the substrate and then catalyze the transfer of a portion of the 

 substrate to another acceptor molecule. For example, the amino 

 group of glutamate is transferred to oxaloacetate by a trans- 

 aminase. If, in the above equation, free B can substitute for G 

 as an acceptor, we are dealing with a transfer-exchange reaction 

 which results in no synthesis but can be detected by incorporation 

 of a labeled compound. The exchange takes place at the same 

 atom of the donor molecule A, in contrast to the metabolic ex- 

 change reactions discussed below. 



(2) The metabolic exchange reactions occur in the course 

 of a cyclic process and are due to the peculiarity of certain reac- 

 tion mechanisms that illustrate a special aspect of enzymes as 

 reactants. The cyclic process of enzyme catalysis may proceed 

 in such a manner that an exchange of atoms takes place between 

 the substrate and the catalyst or coenzyme. For example, a 

 phosphate exchange reaction is catalyzed by phosphoglucomu- 

 tase (see Figure 2). In experiments with P^^-labeled glucose-1- 

 phosphate, the coenzyme G-l,6-diphosphate becomes labeled 

 (46) because the phosphate is transferred from C-1 of G-1, 6-p to 

 C-6 of the substrate G-l-p, which becomes the coenzyme. An 

 analogous action takes place with oxaloacetate, which acts as a 

 catalyst in oxidation of acetyl CoA in the Krebs cycle. The 

 acetyl group of acetyl GoA condenses with one end of OAA, while 

 the other end is actually oxidized in one turn of the cycle. This 

 is not necessarily an essential feature of the cyclic process but is 



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