phosphate) are transferred during a reaction similar to that 

 catalyzed by transketolase (45,46) to an aldo-sugar phosphate, 

 producing a new ketose phosphate, two carbon atoms longer 

 than the starting aldose. Other enzymes have been found in 

 nonphotosynthetic organisms which convert the carbon 

 atoms number 1 and 2 of a ketose phosphate to acetyl phos- 

 phate, leaving the remainder of the sugar as an aldose phos- 

 phate. One of these is phosphoketolase (47), which is specific 

 for xylulose-5-phosphate, while another is fructose-6-phos- 

 phate ketolase (48), which can act on either fructose-6-phos- 

 phate or xylulose-5-phosphate. These enzymes require thia- 

 mine pyrophosphate, inorganic phosphate and, in some cases, 

 Mg+ + . Stimulation by Mn++ or Ca++ in place of Mg+ + 

 could sometimes be observed, whereas levels of Mn above 

 10~^ were inhibitory. 



Breslow has proposed a mechanism for the role of thi- 

 amine pyrophosphate in these reactions (49,50). In his mech- 

 anism, some of which forms the basis for part of Figure 7, 

 the hydrogen at position 2 of the thiazole ring is an active 

 hydrogen which can dissociate from the acidic carbon at that 

 position to give a carbanion. This carbanion adds to the Qar- 

 bonyl carbon of the ketose (somewhat analogous to cyan- 

 hydrin addition). The bond between carbons 2 and 3 of the 

 ketose breaks, with the electron pair going to the reduction 

 of carbon 2 of the ketose, to give a glycolaldehyde-thiamine 

 pyrophosphate. The remainder of the sugar becomes an 

 aldose. Reversal of this reaction path, with a different aldose, 

 completes the transketolase reaction. 



Alternatively, glycolaldehyde-thiamine pyrophosphate 

 may eliminate the elements of water (OH~ from the beta 

 carbon and H+ from the alpha carbon of the glycolaldehyde 

 moiety) to give the enol form and thence the keto form of 

 acetyl-ThPP. This compound can undergo phosphoroclastic 

 cleavage to give acetyl phosphate and thiamine pyrophos- 

 phate (ThPP). 



The mechanisms find support in the demonstration by 

 Breslow that the hydrogen atom on the number 2 position 



41 



