DECAEBOXYLASES 53 



R . CHNH2 . COOH ±=:^ R . C : NH . COOH + 2H (-> coenzyme) 



JH,0 

 E.CO.COOH + NH3. 



Deamination of amino-acids can also take place by reduction, 

 desaturation, or hydrolysis (Chap. IX), but in the majority 

 of these cases the intermediate steps, if any, are not known. 



6. Decarboxylation 



The removal of CO2 from the molecule has been observed 

 with two types of compound, keto-acids and amino-acids. 

 In yeasts a-keto-acids are decarboxylated by the enzyme 

 carboxylase which has been isolated in a cell-free condition 

 and studied in a highly purified state. The enzyme consists 

 of a protein and a loosely attached coenzyme or prosthetic 

 group identified as thiamindiphosphate (see p. 33). Carbo- 

 xylase attacks a-keto-acids, decarboxylating them to the 

 corresponding aldehydes, 



Carboxylase 



R. CO. COOH >R.CHO-f CO2. 



Pyruvic acid, CH3.CO.COOH, is attacked more rapidly than 

 other acids of this group and, in general, the longer the carbon 

 chain of the R group, the slower the rate of attack by the 

 enzyme. Although thiamindiphosphate would seem to bear the 

 same relation to carboxylase that coenzyme I does to L-malic 

 dehydrogenase, we have as yet no definite knowledge of the way 

 in which it functions in the decarboxylation of keto-acids. Carbo- 

 xylase itself has not been found in the enzyme constitution of 

 organisms such as Esch. coli, and it is possible that it does not 

 enter into bacterial metabolism. The breakdown of pyruvic 

 acid by bacteria is not by simple decarboxylation to acetalde- 

 hyde, but involves other mechanisms which also, however, 

 require the presence of thiamindiphosphate (see Chap. VII). 



Yeast carboxylase is specific for the decarboxylation of 

 a-keto-acids, but in some bacteria we find enzymes which 



