194 



UNITY AND DIVERSITY IN BIOCHEMISTRY 



D. The Terminal Stages of Glycolysis 



The glycolysis process (Embden-Meyerhof pathway) leads, as we have 

 said, to pyruvate. In the case of yeast fermenting glucose, the acetaldehyde 

 resulting from the decarboxylation of pyruvate serves as an acceptor of 

 electrons borne by the DPNH from the dehydrogenation of phospho- 

 glyceraldehyde at the time of the first oxido-reduction. Ethanol is formed. 

 However, the formation of acetaldehyde from pyruvic acid is not a general 



CHjOH 

 I 

 CO 



HOCH 

 I 

 HCOH 



I 

 HCOH 



I 

 HCOH 

 I 

 CH20P0(0H)j 



Sedoheptulose — 7 — P 



transaldolase 



CH2OH 

 I 

 CO 



HCOH 



I 

 HCOH 



I 

 CH20P0(0H)j 



Ribulose-J-P 



CHoOH » 



I 

 CO 



I 



CH20H 



Dihydroxyacetone 

 + 



CHO 



I 

 HCOH S9 



I 

 HCOH 

 I 

 CH20PO(OH)2 



D-erythose-4-P 



CHO Diose 

 I 

 CHoOH »— 



+ 



CHO 



I 



CH2OH 

 I 

 ^ CO 



I 



HCOH 



I 

 HCOH 



I 

 - HCOH 



I 

 CHaOPO(OH)2 



2F-6-P 



HCOH 

 1 

 CH20PO(OH)2 



3 -phosphoglyceraldehyde s» 



Fig. 39 (Dickens) — Formation of F — 6 — P in the hexosemonophosphate shunt. 



phenomenon. It is a phenomenon peculiar to certain bacteria, yeasts and 

 plants which because of a particular specialization contain carboxylase. The 

 general phenomenon is the presence of a hydrogen acceptor more highly 

 oxidizing than phosphodihydroxyacetone. This oxidizing substance is 

 pyruvic acid in animals and in certain bacteria provided with lactic de- 

 hydrogenase. This enzyme in the presence of DPNH converts pyruvate 

 to lactate, which is the product of anaerobic carbohydrate catabolism in 

 animals and some bacteria. In addition, certain bacteria are specialized 

 to accept the same hydrogen in various other ways, either by organic 

 compounds such as oxalo-acetic acid, or by inorganic substances. 



