THE PHOSPHOGLUCON ATE OXIDATION PATHWAY 217 



no enzymes are present which can anaerobically metabolize pyruvate 

 and regenerate diphosphopyridine nucleotide from its reduced form 

 (61). This biochemical lesion occurs also in certain of the bacteria 

 (132). These data suggest that failure of certain of the true fungi, 

 e.g., Myrothecium verrucaria, to ferment glucose may ultimately be 

 traced to the same enzymatic deficiency. 



In summary, the cumulative evidence for the Embden-Meyerhof 

 pathway in at least some of the filamentous fungi is impressive. There 

 are, however, several different variations of the pattern. Fusarium spp. 

 and, probably, Aspergillus spp. appear to resemble yeast most closely. 

 The enzymatic basis of the two types of lactic acid formation in the 

 phycomycetes remains unknown, and at present it is only on the basis 

 of comparative biochemistry that we regard these as subtypes of the 

 Embden-Meyerhof sequence. Finally, it is possible that non-fermenta- 

 tive forms can utilize the pathway by coupling it to an aerobic process 

 in which reduced coenzyme is regenerated by an oxidase system and 

 pyruvate is metabolized via the citric acid cycle. 



6. THE PHOSPHOGLUCONATE OXIDATION PATHWAY 



Studies carried out during the decade 1930-1940 by Dickens, 

 Lipmann, and Warburg and more recently by Horecker, Racker, and 

 many others have established the occurrence of a second sequence of 

 respiratory reactions, variously known as the "hexosemonophosphate 

 shunt," the "direct oxidative pathway," and the "pentose phosphate 

 cycle." The system appears to be widespread in nature, occurring in 

 animals, plants, and microorganisms (238). 



For convenience and to avoid unnecessary detail, we may, following 

 Racker (236), divide the pathway into two phases. The first is an 

 oxidative phase, in which glucose-6-phosphate is transformed to 

 pentose phosphate via 6-phosphogluconic acid; carbon atom 1 of 

 glucose, the aldehyde carbon, is converted to carbon dioxide in the 

 process: 



Glucose-6-phosphate + TPN+ -» 6-phosphogluconic acid 



+ TPNH + H+ (6) 



6-Phosphogluconic acid + TPN+ — » pentose phosphate 



+ TPNH + H+ + C0 2 (7) 



The key enzymes of this phase are glucose-6-phosphate dehydrogenase 

 (Equation 6) and 6-phosphogluconic acid dehydrogenase (Equation 

 7); both are linked to triphosphopyridine nucleotide (TPN). Reac- 



