212 UNITY AND DIVERSITY IN BIOCHEMISTRY 



In the absence of catalase, the hydrogen peroxide oxidizes the ketonic 

 acid with formation of the aUphatic acid having one carbon atom less and 

 the overall reaction becomes 



R R 



NH2— CH— COOH+Oo-^COOH+COj+NH, 



2. Non-oxidative deaminations 



Enzymes also exist, in the case of serine, threonine and homoserine, 

 which can catalyse a non-oxidative deamination commencing with a 

 dehydration of the substrate. 



In the case of serine for example, the mechanism is as follows : 



— H2O 

 HOCH2CHCOOH > CH2=CCOOH<=^CH3C-COOH 



^^^ serine dehydrase ^^^ ^^ 



serine a — aminoacrylic acid a — iminopropionic acid 



H2O 



CH3CCOOH >CH,COCOOH + NH3 



II 



NH hydrolysis 



a — iminopropionic acid 



{c) Deamidations 



The deamidation of glutamine and asparagine have already been con- 

 sidered with the enzymes catalysing the reaction (p. 156). 



{d) Decarboxylation of the Ketonic Acids formed by Deamination 



of Amino Acids 



These reactions are brought about by four types of decarboxylases. 



1. a-ketodecarboxylases 



The a-ketonic acids are decarboxylated in the presence of these enzymes 

 with formation of the aldehyde having one carbon atom less and liberation 

 of CO2. The coenzyme is DPT. The carboxylase is formed by the union of 

 the coenzyme and the specific protein. This protein appears to be present 

 only in plants and micro-organisms, whilst DPT is present in animal 

 tissues also. The decarboxylation of pyruvic acid to form acetaldehyde 

 during alcoholic fermentation is due to an a-ketodecarboxylase. 



2. Oxidative a-ketodecarboxylases 



One example has already been described at the point of entry of pyruvic 

 acid into the tricarboxylic acid cycle. 



