258 UNITY AND DIVERSITY IN BIOCHEMISTRY 

 ATP ^^ ^ COj + NHs 



O 



^ ^ ^^ II 



ADP-f ^HaN — C - O — PO(OH)2 



carbamyl phosphate 



The carbamyl phosphate is condensed with a molecule of aspartate giving 

 ureidosuccinic acid, from which orotic acid is formed by cyclization and 

 oxidation. In the presence of PRPP and a pyrophosphorylase this acid 

 forms a ribotide and decarboxylation yields uridine monophosphate. The 

 decarboxylation of the product of amination of orotidine phosphate gives 

 cytidine monophosphate (Fig. 75). It can be seen that the pentose inter- 

 mediate in pyrimidine nucleotide biosynthesis is PRPP, the same as for 

 purine nucleotide biosynthesis. 



(^) Biosynthesis of Nucleoside Diphosphates and 

 Triphosphates 



This biosynthesis operates through the purine and pyrimidine mono- 

 nucleotides as follows: 



nucleoside-P + nucleoside-PPP ^ nucleoside-PP + nucleoside-PP 

 2 nucleoside-PP ^ nucleoside-P + nucleoside-PPP 

 2 nucleoside-PP ^ nucleoside-P + nucleoside-PPP 



(nucleoside and nucleoside designating nucleosides differing in the nature 

 of the base). 



(/z) Biosynthesis of Dinucleotides 



1. DPNandTPN 



Nicotinamide can be formed by animal cells from tryptophan but is 

 usually present in the diet. Starting from ribose-1 -phosphate and niacin 

 (nicotinamide), DPN is probably synthesized as shown in Fig. 76. TPN 

 results from the phosphorylation of DPN in the presence of a specific 

 phosphokinase. TPN can be converted to DPN by a phosphatase. 



2. FAD 



Figure 77 depicts the formation of FMN, in the presence of a phos- 

 phokinase, from ATP and riboflavin, the latter a substance which is not 

 synthesized by the cells of the body and consequently must be obtained 

 from the diet. FMN is converted to FAD by stages analogous to those 

 governing the biosynthesis of DPN. 



