PRIMING REACTIONS 



213 



3. ^-ketodecarboxylases 



An example of this type of decarboxylation is the action of oxaloacetic 

 decarboxylase which is present in animal tissues and in many micro- 

 organisms. 



COOH COOH 



c=o 



I 

 CH., 



I 

 CH, 



4- CO, 



COOH 



4. Oxidative ^-decarboxylases 



The oxidation of a ^-hydroxyacid with decarboxylation of the j8-ketonic 

 acid formed has already been described above in the case of the passage 

 of isocitric acid into oxalosuccinic acid and then to a-ketoglutaric acid 

 during the tricarboxylic acid cycle. 



Another enzyme of the same type has been discovered by Ochoa in 

 animal tissues : it is known as "malic enzyme" and catalyses the oxidation 

 of malic acid to pyruvic acid and COg. 



COOH 



TPN+ + CHOH ^ TPNH + 



CH^ 



I 

 COOH 



malic acid 



COOH 



I 



c=o 



CH, 



COOH 



oxaloacetic acid 



COOH 



I 

 ^ C=0 + CO, 



Mn+ + 



CH, 



pyruvic acid 



The equilibrium constant favours the reaction occurring from left to 

 right but the reverse reaction occurs if the TPN formed is continuously 

 removed. The oxaloacetic acid is shown in brackets because it does not 

 appear in the free form during the reaction. Under the action of the enzyme 

 it is decarboxylated straight away; the enzyme must not be confused with 

 malic dehydrogenase which requires DPN as its coenzyme. 



{e) Tracts aminations 



All the naturally occurring amino acids can in vivo participate in trans- 

 amination reactions catalysed by tra?isaminases. The reactions are universal 

 in the biosphere in which they play an important role. They form important 

 metabolic links between aspartate, glutamate and alanine on the one hand, 

 and their corresponding a-keto acids in the tricarboxylic acid cycle on the 



