BIOSYNTHESIS OF PENTOSES 253 



mediate product in the formation of hexosemonophosphate from pentose 

 phosphate/' '^^'^^ following the initial discovery of Benson et al.^^ that this 

 is one of the first phosphorylated products to be formed in photosynthesis. 

 Using a purified pentose phosphate-splitting enzyme from liver together 

 with crystalUne muscle aldolase to catalyze the condensation of the cleavage 

 products, Horecker and Smyrniotis-^ obtained approximately one mole 

 of sedoheptulose phosphate from every two moles of pentose phosphate 

 that disappeared. Ketoheptose was detected by paper chromatography" 

 and by the characteristic absorption band at 600 m/n in the Bial reaction 

 and was characterized by the preparation of sedoheptulosan tetraben- 

 zoate.** The rate of acid hydrolysis suggested that it was sedoheptulose-7- 

 phosphate. A similar conversion of pentose phosphate into sedoheptulose- 

 7-phosphate was also found with a spinach enzyme.^' This enzyme was 

 found to contain firmly bound thiamine pyrophosphate which was shown 

 to be essential for the synthesis of sedoheptulose-7-phosphate. The same 

 enzyme preparation catalyzed the condensation of L-erythrulose with 

 D-gIyceraldehyde-3-phosphate to form a mixture of pentose phosphate and 

 heptulose phosphate, and the following sequence of reactions was suggested 

 to account for the formation of hexosemonophosphate from pentose phos- 

 phate :^» 



(1) D-ribulose-5-P ^ 2 D-gh'ceraldehyde-3-P + L-eiythrulose 



(2) L-erythrulose + u-glyceraldehyde-3-P ^ sedoheptulose-7-P 



(3) sedoheptulose-7-P -f D-glyceraldehyde-3-P-> fructose-6-P + tetrose P 



Reaction (3) is catalyzed by enzymes from brewer's yeast and liver and the 

 reaction mechanism has been clarified by the use of C'^-labeled triose 

 phosphate."*^ The dihydroxyacetone group is transferred from sedoheptu- 

 lose-7-phosphate to glyceraldehyde-3-phosphate with the formation of 

 fructose-6-phosphate and tetrose phosphate (presumably erythrose-4- 

 phosphate), as shown in Fig. 3. The enzyme catalyzing this reaction has 

 been called "transaldolase" since it promotes the transfer of aldol linkages 

 rather than their hydrolytic cleavage. The fate of the residual tetrose 

 phosphate has not been satisfactorily explained. 



" B. L. Horecker, Ahslr. 2iid Intern. Congr. Biochem., Paris p. 292 (1952). 



'» Z. Dische and E. Pollaczek, Abstr. 2nd Intern. Congr. Biochem,. Paris p. 289 (1952). 



36 A. A. Benson, J. A. Bassham, and M. Calvin. /. Am. Chem. Soc. 73, 2970 (1951). 



" R. Klevstrand and A. Xordal, Acta Chem. Scand. 4, 1320 (1950). 



38 W. T. Haskins, R. M. Hann, and C. S. Hudson, J. Am. Chem. Soc. 74, 2198 (1952). 



" B. L. Horecker and P. Z. Smyrniotis, /. Am. Chem. Soc. 75, 1009 (1953). 



" B. L. Horecker, P. Z. Smyrniotis, and H. Klenow, Federation Proc. 12, 219 (1953). 



^1 B. L. Horecker and P. Z. Smyrniotis, J. Am. Chem. Soc. 75, 2021 (1953). 



