ENZYMES 



175 



Another example in which the intervention of a coenzyme is explained, 

 is the case of the conversion of G — 1 — P into G — 6 — P. Here, G — 1, 6 — PP 

 plays the part of coenzyme. Unlike the case of DPN cited previously, this 

 coenzyme does not function as a second substrate, but on the contrary 

 it is constantly reformed from the substrate. In effect a cycle is operating 

 in which G — 6 — P accumulates. 



G— 1, 6— PP + G— 1— P ^ G— 1, 6— PP + G— 6— P 



An exactly analogous mechanism is that in which 2, 3-diphosphoglyceric 

 acid acts as the coenzyme for the phosphoglyceromutase system (trans- 

 formation of 2-phosphoglyceric acid into 3-phosphoglyceric acid or the 

 inverse) (Sutherland, Posternak and Cori, 1949) : 



COOH 



COOH 



O 



o 



COOH 



I 

 HCOH 



HC— O— P— OH HC— O-P— OH 



I + 

 OH 



O 



H2C— O— P— OH H2COH 



OH 



+ 



OH 



2, 3-diphospho- 

 glyceric acid 



2-phospho- 

 glyceric acid 



I II 



HoC-O-P— OH 



OH 



3-phospho- 

 glyceric acid 



COOH 



I 



I o 



I II 



HC-O-P-OH 



I 

 OH 



O 



II 

 H2C-O-P-OH 



I 

 OH 



2, 3-diphospho- 

 glyceric acid 



Another coenzyme whose action has been clearly elucidated is coenzyme 

 A, the coenzyme for acetyl transfer. Its sulphydryl group is capable of being 

 alternately acetylated and deacetylated in the same way as DPN can be 

 alternately oxidized and reduced. 



O 



o 



R— C— OH+HS— CoA ^ R— C 



Acyl 



S— C0A+H2O 



To illustrate the mechanism of CoA intervention, let us take for an 

 example the acetylation of an amine, the energy being supplied by ATP. 

 Thermodynamically, we may have 



ATP + CH3— COOH ;=^ CH,CO--P +ADP 

 CH^CO'-P H- NH.— R -^ CH3CO— NHR + P 



