PENTOSE FERMENTATION AND OXIDATION 22 1 



The importance of the oxidation of free glucose is difficult to assess. 

 A relatively large number of fungi form gluconic acid (Chapter 6) and 

 we may reasonably assume that the same reaction is involved in all. 

 However, the known glucose aerodehydrogenase has been found only 

 in the fungi mentioned above and in Penicillium glaucum, apparently 

 not occurring in Aspergillus jumigalus, Mucor racemosus, Rhizopus 

 nigricans, or Dematium pullulans (92). As mentioned earlier, the 

 availability of the energy of the reaction to the cell is uncertain; the 

 hydrogen peroxide formed could, in the presence of catalase, serve as 

 an oxidizing agent for other cell metabolites (157, 294). The further 

 metabolism of gluconic acid is similarly obscure; the likeliest possibili- 

 ties are that the gluconate is phosphorylated and enters the hexose 

 monophosphate pathway, or that there is a continued non-phosphoryla- 

 tive metabolism after the pattern of the bacterial genera Pseudomonas 

 and Acetobacter (297, 329); discovery of 2-keto-gluconic acid in small 

 amounts in the culture fluid of Penicillium brevi-compactum (Chapter 

 6) suggests that the bacterial pathway may be worth investigation. 



8. PENTOSE FERMENTATION AND OXIDATION 



The fermentation of pentoses by fungi has been studied primarily 

 as it occurs in the genus Fusarium, although other fungi can carry 

 out at least a slow fermentation of pentose (221). Early results and 

 theories on the reaction are reviewed by Foster (84), and are of 

 historical interest only. 



Resting cells of Fusarium lini ferment xylose in accordance with the 

 equation: 



C 5 H 10 O, -> C 2 H 5 OH + CH3COOH + C0 2 (10) 



Xylose Ethanol Acetate 



With xylose- 1-C 14 as substrate, virtually all of the label appears in the 

 methyl carbon of acetate (98). This system resembles that of Lacto- 

 bacillus spp. (169), with the difference that the lactic acid character- 

 istic of the bacterial fermentation is replaced by ethanol and carbon 

 dioxide. In Lactobacillus pentosus preparations the pentose chain 

 is split between carbons 2 and 3, yielding triose phosphate and acetyl 

 phosphate (35, 123). Presumably in F. lini the triose phosphate is 

 then metabolized to ethanol and carbon dioxide — the enzymes neces- 

 sary for this transformation are all present (62). 



The pentose fermentation by F. lini is striking in another connec- 

 tion: there does not seem to be any conversion of pentose to hexose by 

 the non-oxidative reactions of the phosphogluconate oxidation path- 



