IV. BIOCHEMICAL SYSTEMS 499 



H 

 



HsC— C— C— OH + DPNHo ;:± DPN + HjC— C— C— OH 



I 

 H 



It should be noted that this reaction could provide a mechanism to re- 

 oxidize the DPN which was reduced in the conversion of phosphoglyceral- 

 dehyde to phosphoglyceric acid. 



Also, under anaerobic conditions pyruvic acid may undergo /3-carboxyla- 

 tion to yield oxalacetic acid,'-" which in turn can be converted by malic 

 dehydrogenase, plus DPNH2 as coenzyme, to malic acid as depicted below. 



H 

 OHOO OHOO 



II I II II II I I II 



HO— C— C— C— C— OH + DPNHo ±^ DPN + HO— C— C— C— C— OH 



H H H 



Pyruvic acid may be converted by reductive fixation of carbon dioxide 

 to malic acid'-^ in the presence of "malic" enzyme, TPNH2, and Mn++ as 

 follows. 



H 

 OHOO 



II II Mn++ 11 I I II 



COo + H3C— C— C— OH + TPNH2 ;=— HO— c— C— C— C— OH 



I I 



H H 



It should be noted that this reaction provides a mechanism for carbon 

 dioxide fixation (see p. 503). 



A fourth type of anaerobic utilization of pyruvic acid (in yeast) involves 

 decarboxylation to acetaldehyde which can be converted to ethanol. 



O H 



II I 



H3C— C— H + DPNH2 ;=i DPN + H3C— C— OH 



I 

 H 



Pyruvic acid may be split to yield other 2-carbon compounds, i.e., acetic 

 acid, or other 2-carbon acetyl fragments (activated acetate'-^) which are 

 now known to be combined with coenzyme A (acetyl coenzyme A). Under 

 aerobic conditions pyruvate may undergo oxidative decarboxylation to 



'20 H. G. Wood and C. H. Werkman, Biochem. J. 32, 1262 (1938). 



1" S. Ochoa, A. H. Mehler, and A. Kornberg, J. Biol. Chem. 174, 979 (1948). 



1" F. Lipmann, Advances in Emymol. 6, 231 (1946). 



