The biosynthesis of pentoses and their incorporation into mononucleotides 



of two molecules of pentose phosphate. This latter reaction has been studied with 

 highly purified enzymes from liver, spinach (Horecker et aL, 1953) and yeast (Racker 

 et aL, 1953). Several possibilities obviously exist for mechanisms by which sedoheptu- 

 lose phosphate may be formed from pentose phosphates, but conclusive evidence 

 indicates (de la Haba^a/., 1953; Horecker and Smyrniotis, 1953; Racker et aL, 1953) 

 that it is formed by a condensation between a two-carbon compound and a five- 

 carbon compound, and that it is the ribulose-5-phosphate which is donator of the 

 two-carbon compound. The latter, which would be at the oxidation level of glycol 

 aldehyde, then combines with ribose-5-phosphate to form the sedoheptulose-7- 

 phosphate. Free glycolaldehyde, however, is not active nor does it accumulate in 

 any of these reactions. The sedoheptulose phosphate formation has, therefore, been 

 visualized as an acetoin condensation between an activated form of glycolaldehyde 

 and ribose-5-phosphate. This is consistent with the thiamine pyrophosphate require- 

 ment of the reaction which has been formulated as follows : 



Figure 2. The formation of sedoheptulose-'] -phosphate in the transketolase 

 reaction. {From Horecker et al., 1953.) 



Since the enzyme catalyses the transfer of ketol linkages, it has been named 

 transketolase. Thus, in these reactions the keto sugar esters, ribulose-5-phosphate 

 and sedoheptulose-7-phosphate, serve as donors of 'active glycolaldehyde', and the 

 aldo sugar esters, ribose-5-phosphate and glyceraldehyde-3-phosphate, serve as 

 acceptors of 'active glycolaldehyde'. Also several other compounds can serve as 

 substrates in these reactions. Those known at the present time are listed in Table I, 



69 



