BIOSYNTHESIS 



(e) Biosynthesis of Purine Nucleotides 



255 



The biosynthesis of these nucleotides, and consequently, of the purine 

 ring, begins with the formation of 5 '-phosphoribosyl-1 '-pyrophosphate 

 (phosphoribosylpyrophosphate, PRPP) which is the activated pentose 

 required in purine biosynthesis. Figure 73 shows the various ways in 

 which PRPP can be formed. 



G-6-P 



Pentose 

 cycle 



oxidative 

 path 



ribose 



F-6-P 



non-oxidative 

 path 



ribose-5-P 



ATP 



ATP 



kinases 



kinase 



-^PRPP 



phosphomutase 



ribose-1-P 



nuc 



phosphorolysis 



eosides 



Fig. 73 — Pathways of PRPP formation. 



Starting with PRPP, in the presence of glutamine, 5'-phophoribosyl- 

 amine is formed. Addition of glycine and then of formate to the molecule 

 leads to formylglycinaminoribotide. The addition of CO 2 and the amino 

 group of aspartate gives the ribotide of aminoimidazolecarboxamide, 

 then an addition of a C^ fragment followed by closing of the ring gives 

 inosinic acid from which can be derived xanthosine-5 '-phosphate by 

 oxidation and guanosine-5 '-phosphate by oxidation and amination(Fig. 58). 



(/) Biosynthesis of Pyrimidine Nucleotides 



The carbon in the 1 -position and the nitrogen in the 2-position of the 

 pyrimidine ring are derived from NH3 and CO 2 which react in the presence 

 of ATP to form carbamyl phosphate. 



