METABOLISM OF MICROORGANISMS 



381 



number of papers that are appearing, some information on this problem 

 should soon be forthcoming. 



A second type of direct oxidation that does not involve phosphoryla- 

 tion operates in the metabolism of glucose by certain aerobic bacteria, 

 e.g., Pseudomonas aeruginosa. Oxidation of glucose leads to gluconic 

 acid, 2-ketogluconic acid, pyruvic acid, and the formation of large amounts 

 of a-ketoglutaric acid. Yields of this keto acid up to 0.55 mole per 

 mole of glucose have been obtained. This provides a convenient method 

 for the preparation of a-ketoglutaric acid. 



ANAEROBIC METABOLISM 



Yeast 



Hexose diphosphate and pyruvic acid, so prominent as intermediary 

 products in animal metabolism, were first observed in yeast. The steps 

 in glycolysis are the same as far as pyruvic acid in both animal and 

 yeast metabolism. In yeast fermentation the whole process is anaerobic; 

 the pyruvic acid is decarboxylated to acetaldehyde, and this is then 

 reduced to ethyl alcohol. The hydrogen necessary for the last step 

 comes from the dehydrogenation (oxidation) of phosphoglyceraldehyde 

 to phosphoglyceric acid via DPN • H2 as carrier. If this reduction is 

 blocked, for example, by fixing the acetaldehyde with sulfite, the hydrogen 

 is used to reduce dihydroxyacetone to glycerol. Glycerol production 

 always occurs to a slight extent (3-5 per cent of the glucose), but with 

 the main route of fermentation blocked, the yeast makes the side line 

 a main route. The alternative pathway is a very neat and convenient 

 device for continuing metabolism under adverse conditions. 



If alcoholic fermentation is studied with labeled glucose, it is found 

 that carbon 1 appears in the methyl group, 2 in the carbinol group of 

 ethyl alcohol, and carbon 3 in the carbon dioxide. This accords with 

 the Embden-Meyerhof scheme of intermediary metabolism. (See Chap. 

 13.) 



Bacteria 



The so-called ''mixed" lactic fermentation shows some unexpected 

 departures from the alcoholic fermentation of yeast. Carbon 1 of glucose 

 appears in the carbon dioxide. Carbons 2 and 3 are found in the methyl 

 and carbinol (or carboxyl) groups, respectively, of ethyl alcohol (or 

 acetic acid). Carbon 4 comes out in the carboxyl group of the lactic 

 acid. The two halves of the glucose molecule are metabolized differently. 

 Various intermediary compounds have been found. The first series of 



