NUCLEOTIDES AND COENZYMES 



transferred, and A, the acceptor. The net reaction is, of 

 course, the sum of (1) and (2) : 



D - G + A > A - G + D (3) 



so that the coenzyme, Hke the enzyme, functions in a catalytic 

 role. Reaction (1) involves the activation of the group on the 

 substrate and transfer to the coenzyme, an energy-requiring 

 reaction, whereas (2) represents the transfer of the group from 

 the coenzyme to a suitable acceptor, an isoenergetic process. 

 Where no coenzyme appears to be involved (i.e., dehydration, 

 transpeptidation, transglycosidation reactions), the over-all 

 reaction is usually characterized by a small change in free energy. 



The groups which undergo transfer include representatives 

 of most of the major metabolites encountered in enzymatic 

 reactions. As yet, the activation of amino acids for peptide 

 synthesis is not well understood, but by analogy with known 

 reactions, such as the condensation of benzoyl CoA with glycine, 

 one might surmise tliat the formation of alanyl glycine, for 

 example, would involve the condensation of alanyl-CoA (or 

 perhaps alanyl-AMP, cf. equation (5)) with glycine. 



It should be noted that the adenosine phosphates are an 

 exception to the rule that the coenzyme acts catalytically. 

 ATP functions irreversibly as a substrate, rather than a coenzyme 

 in the kinase reactions : 



ATP + acceptor > ADP + acceptor phosphate (4) 



and in the activation of acyl groups : 



acetate + ATP + CoA > Acetyl CoA + AMP + PP (5) 



succinate -f ATP + CoA > Succinyl CoA + ADP + P (5') 



glutamate + ATP + NHg > Glutaminc + ADP + P (6) 



In reactions (5) and (5') it is believed that the first step involves 

 the formation of acetyl-AMP or succinyl-ADP (17) followed by 

 an isoenergetic transfer of the acyl group to CoA. Likewise, 

 UDPG is probably formed from a reaction between glucose-1- 

 phosphate and uridine triphosphate (27) {cf. equation (12)), 



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