162 GENERAL BIOCHEMISTRY 



-H03POCH2CHOHCOOP03= + ADP -^ 



1 ,3-diphospho-D-glycerate 



-HO3POCH2CHOHCOO- + ATP 



3-pliospho-D-gly cerate 



ADP or adenosine diphosphate has a structure like that of ATP 

 except that it contains one less phosphate group. For the present 

 purpose the structure might be abbreviated to 



Adenosine— OPO3PO3H 



ADP 



The reaction is catalyzed by a specific enzyme in the presence of 

 magnesium ion and shifts a part of the free energy inherent in the 

 1,3-diphosphoglycerate to the resulting ATP when the phosphate 

 group is transferred. The 3-phosphoglycerate undergoes the subse- 

 quent reactions of the pathway leading to compounds of lower free 

 energy. However, the free energy stored in the ATP is used to drive 

 various reactions employed for synthetic purposes in the cell or to 

 modify foodstuffs for digestion and to get them into a form suitable 

 for metabolism. Ramifications of a reaction already mentioned 

 illustrate the biochemical effect of both this driving energy and certain 

 subsequent reactions. 



Mg+ + 



Glucose + ATP \ > glucose-6-phosphate + ADP 



glucokinase 

 several 



Glucose-6-phosphate > 1,3-diphosphoglycerate, etc. 



reactions 



Glucose-6-phosphate > glucose-1 -phosphate 



Glucose-1 -phosphate + fructose > sucrose + HP04^ 



Glucose-1 -phosphate + glucose > (glucose) „ + HP04^ 



The last three reactions illustrate the synthetic role and result in the 

 reserve carbohydrates made by plants and animals and used later in 

 times of need. Enzymes and cof actors are required. 



Coenzyme A shown on page 154 also plays a key part in energy 

 transfer. A part of this compound resembles ADP and ATP and, 

 like them, would show a large free energy on hydrolysis. However, 

 during the energy transfers mediated by coenzyme A (CoA) the 

 phosphate bonds are not directly involved. Rather the energy transfer 

 is associated with transfers of acyl groups to and from the terminal 

 thiol ( — SH) group of the molecule. The importance of this group 

 is emphasized by abbreviating the name and structure to CoA — SH 

 and that of acetyl coenzyme A to CH;{CO — S — C^oA. The free energy 

 of the latter may be used as in 



