ATP AND RELATED NUCLEOTIDES 



41 



NH. 



-CHo—O- 



O 



II 



OH 



P— OH 

 OH 

 Fig. 15. "Active sulfate' 



E. The Activation of Sulfate 



(Table 4, reactions 29 and 68, pp. 34, 35). Liver and yeast contain soluble enzymes 

 which catalyze the activation of sulfate and the transfer of the "active sulfate" to 

 nitrophenol or to m-aminophenol (Hilz and Lipmann, 1955; Segal, 1956; Wilson 

 and Bandurski, 1 956) . In the presence of the liver enzymes, 3.3-3.8 moles of inorganic 

 phosphate are liberated per mole of sulfate ester formed, showing that more than 



one mole of ATP is involved in the 

 reactions. A sulfate activating en- 

 zyme is also present in Neurospora 

 extracts but in this case, the en- 

 zyme needed for the esterification 

 of nitrophenol is absent (De Meio 

 'SOaHa and Witzerkanuik, 1956). "Active 

 sulfate" has been shown to be ade- 

 nosine -3'- phosphate -5'-phospho- 

 sulfate (Robbins and Lipmann, 

 1956), Fig. 15. 



The sulfate activating enzyme 

 catalyzes a sulfate dependent ex- 

 change of •^2p_32p ^it;h ATP. Pyrophosphate inhibits the esterification of 

 w-aminophenol by sulfate. The following mechanism is consistent with these 

 observations : 



Sulfate + 2ATP ' — r adenosine-3'-phosphate-5'-phosphosulfate + PP + ADP 



I OT-aminophenol 



m-aminophenylsulfate 



"Active sulfate" is probably utilized by various transferring enzymes for the syn- 

 thesis of the sulfate esters of amino sugars, and steroid phenols and alcohols. 



F. CO 2 Fixation 



Reactions illustrating the role of high energy phosphate bonds in the fixation 

 of CO2 into organic linkage are shown in Table 4, reactions 34, 38, 69, 91, and 98. 

 These include the reactions which lead to the synthesis of the tricarboxylic acid 

 cycle catalysts, oxalacetate and succinate, of carbamyl phosphate, and of 5-amino- 

 4-imidazole-N-succinylo-carboxamide ribotide (Jones et al., 1955; Marshall et al., 

 1955; Lowenstein and Cohen, 1954). Carbamyl phosphate is required for the 

 synthesis of the amino acids, citrulline and arginine, and for the intermediate in 

 pyrimidine synthesis, ureidosuccinic acid, while the carboxamide ribotide deriva- 

 tive is an intermediate in purine biosynthesis. 



Oxalacetic carboxylase, the enzyme which catalyzes reaction 34, Table 4(p. 34), 

 occurs in liver mitochondria and has also been observed in rat kidney (Bandurski 

 and Lipmann, 1956; Utter and Kurahashi, 1954a, 1954b; \]iitr et al., 1954). The 

 related enzyme "malic enzyme" also catalyzes the formation of a tricarboxylic 



Literature p. 134 



