STRUCTURAL AND CHEMICAL ARCHITECTURE OF HOST CELLS 



161 



H p 



\/ I 



C O OH 



o p 



/ / 



p — O — P — OH 



OH 



HCOH 



I 



HCOH 



I 

 HC 



+ Clutamine 



-PP 



->- Clutamate 



H NH2 



\/ 

 C O 



I 



HCOH 



I 

 HCOH 



I 



HC 



H2COPO3H2 



H2COPO3H2 



5- Phosphoribosy lamine 



1. Glycine 



2. Clutamine 



3. CO2 



4. Aspartate 



I etc. 

 Purine ribotides 



(XIX) 



It is evident then that inorganic pyrophosphate is an important inter- 

 mediary metabohte. Pyrophosphatases exist to cleave pyrophosphate to 

 inorganic phosphate. However, the conservation of the energy of this Hnkage 

 is desirable and a number of reactions may exist to fulfiU this requirement. 

 For example: PPP + AMP ^ PP + ADP ^ Pi + ATP. The equihb- 

 rium would shift to the right as ATP is used in other metabolic events. 



During the respiration of rat liver homogenates, uiorganic pjo-ophosphate 

 is produced from orthophosphate (Cori et al., 1951). The PP formed does not 

 appear to be a direct product of oxidative phosphorylation. The PP-forming 

 system is primarily m the microsomal fraction and derives the pyrophosphate 

 from ATP and other nucleoside triphosphates (Kenney et al., 1957). It has 

 been suggested that in this system the primary product is the nucleoside 

 monophosphate: ATP > AMP + PP. 



This microsomal system is clearly different from the system postulated by 

 Kornberg (1957b) as a major source of PP. He has supposed that this meta- 

 bolite may arise mainly from the reaction of ATP with the coenzyme-formmg 

 pyrophosphorylases. However, these enzymes are mainly concentrated in 

 nuclei. It is possible that the ATP essential for these systems is generated 

 almost entirely \aa anaerobic glycolysis rather than from oxidative phos- 

 phorylation. 



Of particular interest to our discussion is the recent discovery of the in 

 vitro synthesis of DNA by a p}Tophosphorylase (Kornberg et al., 1956; 

 Kornberg, 1957a,b). This reaction, to be discussed m greater detail below, 

 may be represented as foUows: 



DNA 



n deoxynucleoside triphosphates > (deoxynucleoside-P)„ -|- nPP 



4. Phosphorylases 



The Coris (1936) discovered the conversion of glycogen to a-D-glucose-1- 

 phosphate by muscle phosphorylase in the presence of inorganic phosphate. 



