48 INTERMEDIARY METABOLISM AND GROWTH I 



2. Transketolase-transaldolase pathway 

 [a) Mechanism 



A second pathway of pentose formation exists as a result of which three moles of 

 pentose can be formed from two and one half moles of glucose : 



i) 1/2 Fructose-6-phosphate — >^ glyceraldehyde phosphate 



transketolase 



2) Fructose-6-phosphate + glyceraldehyde-P ^ r xylulose-P + erythrose-P 



transaldolase 



3) Fructose-6-P + erythrose-P ^ >• sedoheptulose-P + glyceraldehyde-P 



transketolase 



4) Sedoheptulose-P + glyceraldehyde-P ^ ^ xylulose-P + ribose-P 



pentose epimerase 



5) 2 Xylulose-P ^ ^ 2 ribulose-P 



pentose isomerase 



6) 2 Ribulose-P ^ ' 2 ribose-P 



Sum: 2 1/2 Friictose-6-P -^ 3 ribose-j-P 



The fructose-6-phosphate is, of course, derived from glucose as a result of the 

 reactions catalyzed by hexokinase and hexose isomerase, while the glycolytic 

 enzymes, phosphofructokinase, aldolase, and triose isomerase, catalyze the forma- 

 tion of glyceraldehyde-P from fructose-6-phosphate. Enzymes catalyzing reactions 

 2 to 6 are widely distributed in nature. Phosphoribose isomerase has been demon- 

 strated in yeast, alfalfa, spinach leaves (Hurwitz et al., 1956) and lymphatic tissues 

 (Villavicencio and Guzman-Barron, 1955); phosphoketopentose epimerase in 

 spinach, yeast (Srere et al., 1955), spleen (Ashwell and Hickman, 1954) muscle, and 

 Lactobacillus pentosus (Horecker et al., 1956a; Stumpfand Horecker, 1956). Trans- 

 ketolase has been crystallized from yeast and both transketolase and transaldolase 

 occur in spinach leaves, yeast, algae, liver and most of the tissues which contain the 

 glucose-6-phosphate and the 6-phosphogluconic acid dehydrogenases (Horecker 

 and Mehler, 1955). Thiamine pyrophosphate is a cofactor for transketolase. The 

 enzymes concerned with pentose formation are not sedimented when the super- 

 natant cytoplasmic proteins of rabbit liver and kidney homogenates are cen- 

 trifuged at 105,000 x^ for 2 h. The enzymes are, however, sedimented after 

 centrifugation at 144,000 x,^' for 16 h. (Newburgh and Cheldelin, 1956). 



{b) '''Active glycoladehyde'' donors and acceptors in transketolase reactions 



A number of substances serve as "active glycolaldehyde'' donors. They include d- 

 sedoheptulose-7-phosphate, D-fructose-6-phosphate, D-xylulose-5-phosphate, l- 

 erythrulose, and hydroxypyruvate. It is noteworthy that the orientation of the 

 hydroxyl group on the third carbon atom of the first three compounds listed above 

 is trans to that of carbon four. "Active glycolaldehyde'' acceptors include: glycolalde- 

 hyde, DL-glyceraldehyde, D-glyceraldehyde-3-phosphate, L-glyceraldehyde-3- 

 phosphate, erythrose-4-phosphate and ribose-5-phosphate (Horecker and Mehler, 

 1955)- 



