REACTION ■ CHAINS 27 



by a specific enzyme the distribution of which is by no means 

 universal. 



The nature of enzyme action means that if a reaction is a 

 reversible one, then the enzyme-catalysed reaction is also 

 reversible and the back reaction will be catalysed to the same 

 extent as the forward reaction so that the final equilibrium 

 mixture is the same whether the whole reaction is catalysed 

 or not. This means that the degree to which a given meta- 

 bolite in a chain of reactions will accumulate will depend upon 

 the velocity of the forward and backward reactions of the 

 various steps in the chain, and all the substances in one chain 

 may be side-tracked into another chain by an alteration in 

 the conditions governing the various equilibria. Where a 

 given molecule may be dehydrogenated in organism A, it 

 may be hydrolysed in organism B with the production of a 

 different chain of reactions. Taking the above series of 

 reactions as example again, Esch. coli possesses an enzyme 

 " aspartase " which brings about a reaction between fumaric 

 acid and ammonia to produce aspartic acid without the other 

 intermediate steps: 



CH.COOH Aspartase CH^.COOH 



II +NH3 ^=^ I 



CH.COOH CHNH2.COOH 



Other organisms possess an oxalacetic decarboxylase which 

 decarboxylates oxalacetic acid to pyruvic acid and that opens 

 up an enormous number of possible reaction chains (see 

 Chap. VII). 



CHg . COOH Oxalacetic CH3 



I - CO2 > I 



CO. COOH decarboxylase CO. COOH 



The final product of succinic acid metabolism in an organism 

 will depend therefore upon, first, the enzymes present in the 

 organism and, second, upon the various side reactions occurring 

 at the particular moment studied. The chemical variety of 

 bacterial action is therefore based upon the permutations and 



