Uridine triphosphate pyrimidine synthesis

Figure 20.9 The positions in the pathway for de novo pyrimidine nucleotide synthesis where GLUCOSE provides the ribose molecule and GLUTAMINE provides nitrogen atoms. Glucose forms ribose 5-phosphate, via the pentose phosphate pathway (see chapter 6), which enters the pathway, after phosphorylation, as 5-phospho-ribosyl 1-pyrophosphate. Glutamine provides the nitrogen atom to synthesise carbamoylphos-phate (with formation of glutamate), and also to form cytidine triphosphate (CTP) from uridine triphosphate (UTP), catalysed by the enzyme CTP synthetase. It is the amide nitrogen of glutamine that is the nitrogen atom that is provided in these reactions.

Formation of starting materials for cell wall synthesis begins with two metabolic substances normally found in all life forms N-acetylglucosamine 1-phosphate and the pyrimidine nucleotide uridine triphosphate (UTP) (see Fig. 6-3). Condensation of these two compounds by elimination of pyrophosphate affords uridine-diphospho-N-acetylglucosamine (UDPNAG). Reaction with phosphoenolpyruvic acid (PEP, the activated form of the enol tautomer of pyruvic acid),5 catalyzed by a specific transferase, yields the 3-O-enolic ether. 

Thymidine triphosphate (TTP) depletion. AZT and thymidine (T) compete with each other for phosphorylation by thymidine kinase into AZT-monophosphate (AZT-MP) and thymidine-monophosphate (TMP), respectively (Fig. 13) (Lynx and McKee 2006). AZT can therefore decrease the formation of TMP and TTP, whose relative deficiency can then slow mtDNA replication (Lynx and McKee 2006). Interestingly the administratimi of uridine in animals and perhaps also in humans can prevent AZT, ddC, and d4T toxicity (Walker and Venhoff 2005 Banasch et al. 2006). Uridine administration may provide an alternate route for TTP synthesis, thus preventing TTP depletimi and the impairment of mtDNA replication (Lynx and McKee 2006). Furthermore, the uridine-induced restoration of mtDNA levels and respiratory chain functirm could improve the activity of dihydroorotate dehydrogenase, a key mitochondrial enzyme involved in pyrimidine synthesis. Thus, a virtuous circle is initiated by uridine supplementation (Setzer et al. 2008). 

The Biosynthesis of the Pyrimidine Ring begins with aspartic acid and carbamyl phosphate. The latter is an energy-rich compound which reacts with the former to give carbamylaspartic acid. Ring closure consumes ATP and is in principle an acid amide formation (peptide synthesis). The intermediate dihydro-orotic acid is dehydrogenated to orotic acid, probably by action of a flavoprotein. Orotic acid is the key precursor of pyrimidine nucleotides. It reacts with phosphoribosyl pyrophosphate. The removal of pyrophosphate yields the nucleotide of orotic acid, whose enzymic decarboxylation produces uridine 5 -phosphate. Phosphorylation with ATP yields uridine pyrophosphate and, finally, uridine triphosphate. Beside the above pathway, there is the further possibility of converting free uracil and ribose 1-phosphate to the nucleoside and from there with ATP to the nucleotide. 

While mammahan cells reutilize few free pyrimidines, salvage reactions convert the ribonucleosides uridine and cytidine and the deoxyribonucleosides thymidine and deoxycytidine to their respective nucleotides. ATP-dependent phosphoryltransferases (kinases) catalyze the phosphorylation of the nucleoside diphosphates 2 "-de-oxycytidine, 2 -deoxyguanosine, and 2 -deoxyadenosine to their corresponding nucleoside triphosphates. In addition, orotate phosphoribosyltransferase (reaction 5, Figure 34-7), an enzyme of pyrimidine nucleotide synthesis, salvages orotic acid by converting it to orotidine monophosphate (OMP). 

Figure 4-16 outlines the biosynthesis of pyrimidines and their conversion to the required deoxyribose triphosphates of uridine and cytidine, the necessary building blocks of RNA. The first step involving the condensation of carbamoyl phosphate with aspartic acid is catalyzed by aspartate transcarbamylase. This enzyme is strongly inhibited by the transition-state inhibitor PALA (Chapter 2). Other steps where drug intervention in the scheme can interfere to inhibit DNA synthesis are indicated. 

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