112 PROBLEMS OF PHOTOSYNTHESIS 



reducing fermen- 

 2. pyruvic acid + DPNH + H+ < ^ lactic acid + DPN + 



tation enzyme 



§ 45 Oxidative Decarboxylation of Pyruvic Acid 



The opinion used to be held that the further oxidation of carbohydrates 

 was continued immediately after glycolysis. The lactic acid produced was 

 considered to be the primary substance for further breakdown. This is cer- 

 tainly not the case. In muscle cells of some animals some of the lactic acid 

 produced can be used for resynthesizing glycogen as all steps in glycolysis 

 are reversible. However, further oxidation of lactic acid is out of the ques- 

 tion. The primary substance for the continued breakdown of the carbo- 

 hydrates is not lactic acid but pyruvic acid. When lactic acid is produced in 

 cell metabolism, it must be dehydrogenated to pyruvic acid again before any 

 further oxidation is possible. This dehydrogenation reaction is the reversal 

 of the lactic acid dehydrogenase reaction discussed in the preceding paragraph 



COOH-CHOHCHs + DPN+ -> COOHCOCHs + DPNH + H + 



It follows from this that lactic acid production is a "blind alley" in the re- 

 action series of carbohydrate breakdown. It only serves for the back oxi- 

 dation of DPNH formed during the oxidation of 3-phosphoglyceraldehyde 

 (see the reaction equation at the end of § 44). Under aerobic conditions, 

 this back oxidation of DPNH is effected in quite a different manner, namely 

 in the normal way via the respiratory chain. 



Pyruvic acid dehydrogenase catalyses the oxidative decarboxylation of py- 

 ruvic acid 



CH3COCOOH + HoO + DPN+ <=± CH.3COOH + CO2 + DPNH + H + 



However, free acetic acid does not occur and, as a matter of fact, this re- 

 action is far more complicated than would be supposed from the above equa- 

 tion. Recent investigations have shown that at least five cofactors are indis- 

 pensable. They are Mg ions, thiamine diphosphate (TPP+), DPN+, co- 

 enzyme A (A — S— H) and lipoic acid.* Gunsalus (9) proposed the following 

 four reactions 



1 . Formation of a hypothetical intermediate product with TPP+ 



CH3COCOOH + TPP+^CH3-CO---TPP + H+ + COo 



Lipoic or thioctic acid has the formula 



CH, 



/\ , /^ 



H2C CH-(CH,)4-COOH abbreviated as L 



\i 



S S 



For Uterature on lipoic acid, see Reed (28). 



