380 METABOLISM OF MICROORGANISMS 



SYSTEMS OF INTERMEDIARY METABOLISM 



Aerobic metabolism 



Microorganisms that use oxygen have a glycolysis and oxidizing system 

 such as animals possess and metabolize glucose via pyruvic acid and the 

 citric acid cycle to carbon dioxide and water. However, another route, 

 by which glycolysis is side-stepped and possibly also the citric acid cycle, 

 appears to function in yeast and bacteria, and to some extent in liver. 

 This route lias been known for a long time but has not received much 

 attention until recently. It has been called "the hexose monophosphate 

 shunt," but this is a poor name since the route appears to be more than 

 a detour around glycolysis. It is a direct oxidation of glucose in which 

 a number of entirely new compounds appear as intermediates in tiie 

 following sequence: 



Glucose ■ -^ glucose-G-PO* • > gluconic acid-G-POi 



(4) 

 -^ 2-ketogluconic acid-6-P04 ^ ribulose-5-P04 



(6) 



C. compound (X 2) *■ tetiose| 



(tn 



(7) 



j^l " >^-^ sedoheptulose-7-P04 



fiose-3-P04 *- ' 



(9) 



(glucose-6-P04 *■ recycled 



(8) 



+ triose-3-P04 '^ 1 ^ , _,^ (lO)^ „ , . o T,n _l rn 



( tetrose-4-P04 *" ? triose-S-PO^ + COj 



The nature of the Co compound in step 5 is still unknown; it does not 

 appear to be a glycolic aldehyde. The ketopentose, ribulose-5-P04, is 

 in equilibrium with the aldopentose, ribose-5-P04, but the predominant 

 form seems to be the ketose. The occurrence of a C7 sugar, sedoheptulose, 

 in the metabolism of a Cq sugar is an unexpected and noteworthy phenom- 

 enon. The disposition of the tetrose-4-P04 (step 10) is still uncertain, 

 but it appears to go to a triose-phosphate and a one carbon compound, 

 which may be carbon dioxide. All details of the direct oxidation pathway 

 have not yet been worked out, but the main outlines of the route are 

 evident. 



Since some cells are equipped with both the glycolysis-citric acid cycle 

 mechanism and the direct oxidation system, the question naturally arises 

 as to the relative importance of the two systems. Investigators are 

 cautious about expressing an opinion, because sufficient data are not 

 yet available for answering this question. However, judging from the 



