INTERGONVERSION OF SUGARS 



trehalose phosphate. The biosynthesis of UDPG has been ob- 

 served to be carried out by an enzyme which catalyzes the reac- 

 tion (12): 



UTP + glucose-1 -phosphate , ' 



UDPG + inorganic pyrophosphate (6) 



And UTP (urichne triphosphate), in turn, has been found 

 to be formed by phosphorylation of UDP (uridine diphosphate) 

 by adenosinetriphosphate or phosphopyruvate. 



Still one more substance of the nucleotide-sugar family has 

 been isolated from yeast. The structure is like that of UDPG, 

 but instead of uridine it contains guanine, and the glucose residue 

 is replaced by mannose. Its role is unknown at present. 



Transfer of Two-Carbon Units 



An enzyme which has been named transketolase (9) has 

 been obtained from yeast, spinach, and liver. When suitably 



CH9OH 



I ' 

 G=0 



HCOH 



HCOH 

 I 

 CHsOPOg— 



(VI) 

 D-Ribulose-5-phosphate 



CHO 



HCOH 



I 

 HCOH ; 



I 

 HCOH 



CH2OPO3— 



(VH) 

 D-Ribose-5-phosphate 



CH2OH 



c=o 



I 

 HOCH 

 I 

 HCOH 



HCOH 



I 

 HCOH 



I 

 CHsOPOg— 



(vni) 



Sedoheptulose-7-phosphate 



(7) 



CHO 



I 

 HCOH 



I 

 CH2OPO3- 



(IX) 



D -GIyceraldehyde-3-phosphate 



593 



