ENZYMES 165 



Pyruvic carboxylase, whose role in alcoholic fermentation is an important 

 one, contains thiamine pyrophosphate (TPP) as its prosthetic group and 

 the presence of Mg++, or in its absence, of Mn++, is necessary for its action. 

 This enzyme catalyses the reaction 



CH3COCOOH -> CH3CHO + CO2 



It is found in vegetable tissues and in bacteria but not in animal tissues. 

 Nevertheless, in animals and in certain micro-organisms, there is a system 

 for the oxidative decarboxylation of pyruvic acid which requires a series 

 of cofactors (CoA, DPN, TPP and thioctic acid) ; the initial reaction is as 

 follows : 



CH3COCOOH + DPN+ -> CH3CHO + DPNH + CO2 



The amino acid decarboxylases are universally distributed. They catalyse 

 the general reaction 



RCH(NH2)C00H -^ RCH^NHa + CO2 



Each of these enzymes is specific for a definite amino acid, their coenzyme 

 is pyridoxal phosphate attached to the enzyme protein via its phosphate. 

 The ketonic acid decarboxylases fall into two groups, the a-keto- 

 decarboxylases and the /3-ketodecarboxylases. They catalyse respectively 

 the follovking reactions: 



RCOCOOH -> RCHO + CO2 



RCOCH2COOH -> RCOCH3 + CO2 



A further type of carboxylase are the triosephosphate-lyases which, 

 without the intervention of water, decompose hexose phosphates into a 

 molecule of triose phosphate and some other molecule. 



Such, for example, is aldolase or FDP-triosephosphate-lyase. Universally 

 present in the biosphere, it catalyses the reaction 



F-1, 6-PP ^ 3-phosphoglyceraldehyde + phosphodihydroxyacetone 



Enolase is an example of the dehydrase type of enzyme. It is universally 

 distributed and catalyses the reaction 



H H H 



I I I 

 — C— C— ^ — C=C h H2O 



OH OH OH 



Aconitase is another hydrase, widely distributed, catalysing the reaction 



- H2O + H2O 



Citric acid ^ m-aconitic acid ^ L-isocitric acid 

 + H2O - H2O 



