156 



S. S. COHEN 



H OH 



\/ 



c — o 



I 



HCOH 



I 



HOCH 



I 



HCOH 



I 



HC 



p O ■ o ■ 



// // \ // 



A-R-O-P-O-P-OfP-OH 

 OH OH '' OH 



H OH 

 \/ 



c — o 



I 



HCOH 



HOCH 



HCOH 



HC- 



O O 



// // 



+ A-R-0-P-0-P-OH 

 I I 

 OH OH 



H^COH H2COPO3H2 



glucose ATP glucose-6-phospliate ADP 



(XIII) 



As a result of this reaction, glucose is converted to a charged derivative, a 

 form in which association with enzymes is thereby facihtated. In this case, 

 ATP is a phosphoryl donor to an alcohol and fulfills a role comparable to the 

 phosphorylation of adenosine to adenylic acid, mentioned earlier. Compounds 

 in which the phorphoryl group is combined to alcohols or hemiacetals, as in 

 the formation of sugar- 1 -phosphates, e.g., galactose-1-phosphate, are con- 

 sidered to possess relatively low group -transfer potentials. Their formation 

 from ATP therefore involves relatively irreversible reactions. 



The reactions of ATP to form pyrophosphates, enol phosphates, and 

 carboxyl phosphates are readily reversible, a fact which relates to the rela- 

 tively high group -transfer potentials of the latter compounds. In some cases 

 the reversibihty is more easily demonstrated by the addition of coupled 

 reactions. Examples of the initial reactions are given in formula (XIV). 



UMP 



kinase 

 uridine-5'-P (UMP) -f ATP ^ uridine diphosphate (UDP) -|- ADP 



CH, 



=0 -I- ATP 



Pyruvo — kinase 



COOH 

 Pyruvate 



CH2 



i 



C OPO3H2 -1- ADP 



COOH 



Phosphoenol pyruvate 



o=c- 



HC- 



-OH 



-OH + ATP 



Phosphoglyceryl- 

 kinase 



H2COPO3H2 

 3-phosphoglycerate 



(XIV) 



0=C OPO3H2 



I 



HCOH -t- ADP 



H2COPO3H2 

 1,3- diphosphoglycerate 



