54 BACTERIAL ENZYMES 



remove COg from other keto-acids. Azotobacter and Micro- 

 coccus lysodeikticus contain an enzyme which decarboxylates 

 oxalacetic acid to pyruvic acid: 



Oxalacetic 



HOOC . CHg . CO . COOH > CH3 . CO . COOH + COg. 



decarboxylase 



CI. acetohutylicum possesses an enzyme which decarboxylates 

 acetoacetic acid to acetone: 



Acetoacetic 



CH3 . CO . CH2 . COOH > CH3 . CO . CH3 + CO2. 



decarboxylase 



These enzymes have all been studied in a cell-free state and 

 do not appear to involve thiamindiphosphate. 



Some bacteria also carry out a decarboxylation of certain 

 amino-acids to the corresponding amines. 



R . CHNH2 . COOH > R . CH2 . NH2 + CO2. 



The amino-acid decarboxylases are specific for a single amino- 

 acid and, so far, enzymes have been isolated in a cell-free 

 state which are specific for the natural isomers of lysine, 

 arginine, histidine, ornithine, tyrosine, and glutamic acid. All 

 these amino-acids have a polar group other than the a-NH2 

 and the 1-COOH groups, and it has been found that 

 substitution of the third polar group, i.e. the second — NHg 

 group in lysine, ornithine, or arginine, the — OH in tyrosine, 

 or the second — COOH group in glutamic acid, results in 

 complete inhibition of the decarboxylation. This suggests 

 that the enzyme and substrate must combine through at 

 least two polar groups — other than the — COOH attacked — 

 before decarboxylation can occur. The product of the 

 decarboxylation is the corresponding amine or, in the case 

 of glutamic acid, y-amino-butyric acid. The decarboxylases 

 of lysine, arginine, ornithine, tyrosine, and glutamic acid 

 consist of a protein portion and. a prosthetic group which can 

 be replaced in vitro by pyridoxal phosphate (see p. 33). 



