468 FURTHER EVOLUTION 



6-phosphate to 6-phosphogluconic acid with its subsequent 

 oxidative decarboxylation to pentose-5-phospliate. 



As an example we may cite Microbacterium lacticum 

 which has such an oxidative mechanism. However, in this 

 organism the complex of glycolytic enzymes still predomin- 

 ates to such an extent that even in air the formation of 

 glyceraldehyde phosphate and pyruvic acid mainly follows 

 the scheme for anaerobic fermentation, while the contribu- 

 tion made by the direct oxidation of hexose is relatively 

 small/*^ 



According to V. A. Engelhardt and A. P. Barkhash"" 

 yeasts, on the contrary, switch over definitely to the oxidation 

 of hexose monophosphate under aerobic conditions. Moulds 

 can also oxidise glucose directly and intensively."^ In par- 

 ticular, Aspergillus niger can, under appropriate conditions, 

 transform glucose almost quantitatively into gluconic acid 

 (the so-called ' gluconic acid fermentation '). In this case, 

 however, the oxidation of glucose occurs without its pre- 

 liminary phosphorylation, being mediated by the enzyme 

 glucose oxidase. According to the evidence of P. Kolesnikov"^ 

 an analogous breakdown of hexose without preliminary 

 phosphorylation plays a predominant part in the respiration 

 of unicellular green algae (e.g. Chlorella). 



From our point of view the obligate aerobe Pseudomonas 

 fluorescens is of great interest. Nobody has succeeded in 

 finding in it hexokinase, which brings about the phosphoryla- 

 tion of hexose before its breakdown to triose phosphates, 

 while in addition the aldolase, which catalyses this break- 

 down, is only very weak in these organisms. In Pseudomonas 

 fluorescens, therefore, the glycolytic breakdown of sugars 

 is relegated to the background although glyceraldehyde-3- 

 phosphate and pyruvic acid, which are products of this 

 process, figure in the metabolism of the organisms. They are 

 formed by somewhat different means from those of the classi- 

 cal scheme of glycolysis. W. Wood gives the following scheme 

 for the oxidative breakdown of glucose in this micro-organism 

 (Fig. 41). 



As may be seen from this scheme, the main means of oxida- 

 tive transformation in Pseudomonas fluorescens lies through 

 the direct oxidative dehydrogenation of glucose with its 



