II ENERGY-RICH PHOSPHATE BONDS 37 



The guanidine-phosphate bond of creatine phosphate is also a "high- energy" 

 phosphate bond since approximately 14 kcal of free energy are liberated upon the 

 hydrolysis of one mole of phosphocreatine. Creatine phosphate is a major storage 

 form of 'iiigh-energy phosphate", especially in muscle and liver of animal tissues. 

 When the concentration of ATP declines, reaction i) is reversed and ATP is 

 regenerated from the creatine phosphate. In invertebrates, phosphoarginine 

 replaces phosphocreatine as the storage compound for high-energy phosphate 

 bonds. 



III. THE UTILIZATION OF ATP AND RELATED NUCLEOTIDES 



High-energy phosphate bonds of ATP are required in a variety of chemical 

 reactions which are of significance in biosynthetic processes. Table 4 lists some of 

 these reactions involving ATP and other compounds containing "energy-rich" 

 phosphate bonds. The reactions have been grouped into seven classes. 



Group I and Group II include those reactions in which there is a transfer of the terminal 

 phosphate of ATP, GTP, ITP, or UTP; the corresponding diphosphate is the product of 

 the reaction. The two reactions of Group III illustrate the transfer of pyrophosphate to 

 acceptor compounds; in this case adenylic acid is a product. The reactions of Group IV 

 and probably, Group V involve the transfer of an adenylic acid, guanylic acid, cytidylic 

 acid, or uridylic acid moiety to an acceptor compound; here, pyrophosphate is a product 

 of the reaction. The reactions of Group VI have been placed in a separate category since 

 the mechanism has not been precisely defined. It is likely that the Group VI reactions 

 involve at least two steps and that these reactions will later turn out to belong to either 

 Groups I, II, or III. 



Group VII includes the growing list of reactions requiring 5-phosphoribose pyro- 

 phosphate rather than ATP. They are of interest in that 5-phosphoribose is transferred 

 to an acceptor compound while pyrophosphate is a product of the reaction. The poly- 

 nucleotide phosphorylase reactions shown in Group VIII require nucleotide diphosphates 

 or triphosphates and as in the case of the Group IV reactions, nucleotide monophosphate 

 groups are transferred to acceptor compounds. 



A. Group I Reactions 



Let us now inquire as to the function of the reactions of Table 4. Many of the 

 reactions of Group I are catalyzed by kinase enzymes and result in the formation 

 of phosphate ester bonds. It will be noted that a number of the latter are sugar 

 phosphate compounds. A phosphorylation of a sugar to the sugar phosphate is in 

 almost every case a necessary preliminary step for the further metabolism of the 

 sugar. Likewise, the phosphorylations of glycerol, glyceric acid, or gluconic acid 

 convert these compounds to substances which are readily metabolized by cells. 



The kinase reactions shown in Group I also include steps in the synthesis of 

 coenzymes and nucleotide intermediates of nucleic acid synthesis. The phosphory- 

 lation of nicotinamide riboside, thiamine, pyridoxal, or pantetheine are examples 

 of the former while the phosphorylation of thymidine, adenosine, or uridine are 

 examples of the latter. Reactions which are of importance in the synthesis of 

 polynucleotides and of coenzymes are also to be found in Groups II to VIII. The 



Literature p. X24 



