178 



S. S. COHEN 



HCOH- 



I 

 HCOH 



I 

 HOCH 



I 



HCOH 



I . 



HC 



TPN. 

 -2H, 



-o/ 



o=c^ 



I 

 HCOH 



I 

 HOCH 



I 

 HCOH 



I 

 HC 



H2COP03= 

 6 -phosphogluconolactone 



H2COP03= 

 G lucose'- 6- phosphate 



H2C0P03= 

 I 



c=o 



I 



HOCH 



1 

 HCOH 



1 



HCOH 



I 



H2COP03= 



Fructose diphosphate 



H2COH 



c=o 



I 



HOCH 



I 

 HCOH 



, I 

 HCOH 



I 

 H2COP03= 



Fructosa-6-phosphate 



HC=0 



I . 

 HCOH 



HC=:0 



I 



HCOH 



I 

 HCOH 



I 

 H2COP03= 



Erythrose-4-phosphate 



Clyceraldehyde 

 3 -phosphate 



Dihydroxyacetone 

 phosphate 



H2COH 



c=o 



I 



HOCH 



1 

 HCOH 



I 



HCOH 



I 

 HCOH 



I 

 HjCOPOj^ 



Sedoheptulose 

 7-phosphote 



(XXVII) 



Given ribose-5-phosphate via the mechanisms outhned above, this sub- 

 stance may be isomerized to ribose-1 -phosphate in preparation for the 

 condensation with bases to form nucleosides, or pyrophosphorylated in pre- 

 paration for the condensation with bases to form nucleotides. 



Very little is known of the mechanism of deoxyribose formation. Although 

 an enzyme, deoxyribose-5-phosphate aldolase, is known to catalyze the 

 synthesis of this substance from glyceraldehyde-3-phosphate and acetalde- 

 hyde, the properties of the enzyme are such as to suggest that it can be more 



1 The substrate activated by transhetolase to form sedoheptulose-7-phosphate has now 

 been shown to be D-xylulose-5-phosphate, which is formed by epimerization from ribulose- 

 5-phosphate. 



