PRIMING REACTIONS 



193 



important metabolic pathway is due to Racker and to Horecker and their 

 co-workers. If ribose-5 -phosphate is present to act as an aldehyde acceptor, 

 the xylulose-5 -phosphate is rapidly converted by an enzyme in the yeast 

 into triose phosphate. 



By means of the cycle can be explained the formation of sedoheptulose- 

 7-P (thus named because of its accumulation in the leaves of Sedum which 

 lacks the enzyme system for its further transformation). 



G-6-P 



2H 



O = C — O 



I 

 HCOH 



I 



Q't-V'dehydrogenose 

 TPN 



HOCH 

 HCOH 



H,0 



ghtconolactonase 



COOH 



I 



HCOH 



I 



HOCH 



I 



HCOH 



— CO, 



— 2H 



CHjOH 

 C = O 



HCOH 

 HCOH 



H,COPO(OH), 

 S-gIuconolactone-6-P 



phosphoglyconic , 



HCOH acid dehydrogenase CH,0 — PO(OH), 



HsCOPO(OH), 



Ru-5-P 



6-phosphog!uconic acid 



3-phosphogIyceraldehyde 

 1 



Fig. 38 — Pentose Cycle 

 Xu = xylulose, the ketose corresponding to xylose. 



The next step is the action of a transaldolase which transfers the 

 dihydroxyacetone group of sedoheptulose to an acceptor aldehyde 

 (3-phosphoglyceraldehyde) forming fructose-6-P and leaving a phos- 

 photetrose, D-erythrose-4-P. 



A further molecule of ribulose-5-P furnishes an "active glycolaldehyde" 

 to the tetrose, forming a new molecule of F — 6 — P. 



The overall reaction is the following : 



3G— 6— P + 3O2 -^ 2F— 6— P + 1 Triose-P + 3CO2 + 3H2O 



Thus in the course of the complete cycle, one molecule of glucose 

 is broken down to 3 molecules of COg, 3 molecules of water and a molecule 

 of triose ; a molecule of triose is oxidized completely. 



o 



