COENZYME SYSTEMS 47 



replaced in vitro by coenzyme I isolated from yeast, and (2) the 

 malic dehydrogenase of animal tissues is likewise inactive 

 in the absence of a coenzyme which can be supplied by the 

 autogenous coenzyme I or by partially purified extracts of 

 Esch. coli. Quantitative studies of these relationships leave 

 no doubt but that the coenzyme I of yeast and animal tissues 

 and the L-malic codehydrogenase of Esch. coli are identical. 



Coenzyme II systems also exist in bacteria; for example, 

 the L glutamic acid dehydrogenase of Esch. coli : 



CH2 . CH2 . COOH CH2 . CH2 . COOH 



I + Coenzyme II ^ -^ | 



CHNHo.COOH . C : NH.COOH, 



+ Keduced coenzyme II 



and in this case the dehydrogenase cannot be activated by 

 coenzyme I, although the corresponding L-glutamic acid 

 dehydrogenase of animal tissues is specific for coenzyme I. 

 These coenzyme specificities are worked out with isolated 

 enzymes in vitro, and it is probable that the intact bacterial 

 cell can interconvert coenzymes I and II. The reduced 

 coenzymes cannot react with cytochrome without the inter- 

 vention of the coenzyme dehydrogenases ; little work has been 

 done on the coenzyme dehydrogenases of bacteria, and there 

 is no evidence that these are any different from the similar 

 enzymes of other cells. 



Linked oxidation-reduction reactions: So far in this 

 section we have discussed the oxidation of various substrates. 

 Many of the dehydrogenases are reversible and can carry out 

 the general reaction 



A + 2H > AH2 



in the presence of a suitable hydrogen donator. Keduced 

 coenzyme can act as H-donator in this way and so can act 

 as H-carrier between one dehydrogenase and another. 



AH2 -f Co ^ A + C0H2 C0H2 + B > BH2 + Co. 



In this case AH2 has been oxidised anaerobically by the 



