STRUCTURAL AND CHEMICAL ARCHITECTURE OF HOST CELLS 157 



Enzymes have been found for converting two moles of nucleoside 

 diphosphate to one mole of triphosphate and one mole of monophosphate. 

 The first of these was myokinase which catalyzes the reversible reaction 

 2 ADP ^ AJVIP + ATP. It can be seen that the forward reaction is merely the 

 reversal of a typical kinase-catalyzed reaction in which ATP is a phosphoryl 

 donor. Similar reactions are known for UDP, guanosine diphosphate (GDP), 

 and cytidine diphosphate (CDP), as in the system: 2 UDP ^UMP+ UTP. 

 Thus, the triphosphates of each ribose nucleotide can be formed and in turn 

 act as a phosphoryl donor in kinase reactions. Deoxynucleoside monophos- 

 phates can also be converted to di- and triphosphates in enzyme systems 

 contauiing ATP as a primary phosphoryl donor. The mechanisms of the 

 latter reactions have not yet been determined in detail. 



In each of the cases indicated above, the terminal phosphoryl group of the 

 triphosphate has been transferred. In some instances a pyrophosphoryl group 

 is itself transferred, as in the reaction: 



ribose-5-phosphate -\- ATP ^ 1 -pyrophosphoryl ribose-5-phosphate-|-AMP 



in which the activated sugar derivative essential to several routes of nucleo- 

 tide synthesis is generated. The role of this compound in the scavenging 

 recovery of ingested bases has been recorded earlier; other aspects of its 

 essential role in de novo metabohc paths wiU be described below. 



3. Pyrophosphorylases 



In the formation of the coenzymes, a reaction occurs in which the nucleo- 

 tide monophosphate is transferred from a triphosphate to form a new 

 pyrophosphate hnkage and inorganic pyrophosphate. The followhig exem- 

 plify this reaction: 



nicotinamide mononucleotide + ATP ^ diphosphopjTidine nucleotide (DPN) + 



pjTophospliate (PP) 



riboflavin phosphate -|- ATP ^flavin adenine dinucleotide (FAD) -)- PP 



phosphopantotheine + ATP ^ dephosphocoenzyme A + PP 



glucose -1 -phosphate -f- UTP ^ uridine diphosphosphoglucose (UDPG) + PP 



mannose-1 -phosphate + GTP ^ guanine diphosphomannose (GDP) mannose + PP 



phosphoryl choline + CTP ^ cytidine diphosphocholine (CDP choline) + PP. 



It can readily be seen in these instances that the reversal of these reactions 

 constitutes a pyrophosphorylytic cleavage of a pyrophosphate to form a 

 triphosphate and a monophosphate metabohte. All four ribosenucleoside 

 triphosphates participate in these reactions and are relatively specific for 

 particular acceptors. Comparable reactions to form mixed phosphoanhy- 

 drides are now known for fatty acids, amino acids, sulfuric acid, and carbonic 

 acid, as foUows: 



