Biosynthesis of Pyrimidine Nucleotides

Mechanism of action of S-adenosylhomoeysteine hydrolase (enzyme) and its inhibition by deoxyadenosine. (a) The enzyme uses enzyme-bound NAD+ to temporarily oxidize substrate and eventually hydrolyze it to adenosine and homocysteine. [Reproduced with permission from R. H. Abeles, Suicide enzyme inactivators. Chem. Eng. News 61(38), 55 (September 19, 1983). 1983 by the American Chemical Society.] (b) Deoxyadenosine, a suicide substrate, is also oxidized by the enzyme with the formation of a ketosugar, which undergoes decomposition, with the product dissociating from the enzyme and leaving the enzyme in the reduced state (NADH). 

Purine nucleotide. The cycle plays an important role in energy production in skeletal muscle during exercise. 

Ila (fast-twitch oxidative) and type I (slow-twitch oxidative) muscle fibers have greater oxidative capacity and are less dependent on the cycle than type Ilb (fast-twitch glycolytic) fibers. Thus, gradual exercise programs that lead to production of a greater proportion of type Ila and type I fibers might improve exercise tolerance in AMP deaminase deficiency. 

This autosomal recessive trait results in hypouricemia and in increased urinary excretion of hypoxanthine and xanthine. Patients frequently have xanthine stones. 

Pyrimidine nucleotides, in common with purine nucleotides, are required for the synthesis of DNA and RNA. They also participate in intermediary metabolism. For example, pyrimidine nucleotides are involved in the biosynthesis of glycogen (Chapter 15) and of phospholipids 

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The pyrimidine antagonists inhibit the biosynthesis of pyrimidine nucleotides or interfere with vital cellular functions, such as the synthesis or function of nucleic acids. The analogues of deoxycytidine and thymidine that are used are inhibitors of DNA synthesis while 5-fluorouracil (5-FU) an analogue of uracil, is an inhibitor of both RNA function and of the synthesis of thymidylate (see Fig. 2). PALA (N-phosphonoacetyl-L-aspartate), an inhibitor of as-... 

Allosteric enzymes are generally larger and more complex than nonallosteric enzymes. Most have two or more subunits. Aspartate transcarbamoylase, which catalyzes an early reaction in the biosynthesis of pyrimidine nucleotides (see Fig. 22-36), has 12 polypeptide chains organized into catalytic and regulatory subunits. Figure 6-27 shows the quaternary structure of this enzyme, deduced from x-ray analysis. 

Scheme 4 Biosynthesis of pyrimidine nucleotides (P = P03H2, PP = P206H3) (11a) ribosyl-5- ...

Treatment of HCN-polymer with 6.0NHC1 afforded adenine 1, AICN, 3,4-dihydroxypyrimidine 9 and 5-hydroxyuracil 10, while 1, AICN and orotic acid 11 were recovered after reaction with sodium hydroxide (Scheme 5). A reaction mechanism involving the formation of different aminopyridines as intermediates and reduction steps was proposed to explain the distribution of the obtained products. In accordance with the chemomimetic concept, orotic acid is a key intermediate in the current biosynthesis of pyrimidine nucleotides [62],... 

The atoms of the pyrimidine ring are derived from carbamoyl phosphate and aspartate, as shown in Fig. 15-14. The de novo biosynthesis of pyrimidine nucleotides is shown in Fig. 15-15. The first completely formed pyrimidine ring is that of dihydroorotate. Only after oxidation to orotate is the ribose attached to produce orotidylate. The compound 5-phosphoribosyl 1-pyrophosphate (P-Rib-PP) provides the ribose phosphate. L-Glutamine is used as a substrate donating nitrogen atoms at reactions 1 and 9, catalyzed by carbamoyl phosphate synthetase II and CTP synthetase, respectively a second... 

There are two multifunctional proteins in the pathway for de novo biosynthesis of pyrimidine nucleotides. A trifunctional protein, called dihydroorotate synthetase (or CAD, where the letters are the initials of the three enzymatic activities), catalyzes reactions 1, 2 and 3 of the pathway (HCC>5"- CAP— CA-asp—> DHO Fig. 15-15). The enzymatic activities of carbamoyl phosphate synthetase, aspartate transcarbamoylase and dihydroorotase, are contained in discrete globular domains of a single polypeptide chain of 243 kDa, where they are covalently connected by segments of polypeptide chain whch are susceptible to digestion by proteases such as trypsin. A bifunctional enzyme, UMP synthase, catalyzes reactions 5 and 6 of the pyrimidine pathway (orotate— OMP—> UMP Fig. 15-15). Two enzymatic activities, those of orotate phosphoribosyltransferase and OMP decarboxylase, are contained in a single protein of 51.5 kDa which associates as a dimer. 

In this experiment we will examine some of the properties of the aspartate transcarbamylase of Escherichia coli, which is typical of many enzymes subject to feedback inhibition and which has been studied extensively. Aspartate transcarbamylase (ATCase) catalyzes the first reaction unique to the biosynthesis of pyrimidine nucleotides. ATCase is subject to specific inhibition by quite low concentrations of one of its end products, cytidine 5 -triphosphate (CTP). This relationship and two other regulatory interactions important to the control of pyrimidine biosynthesis are summarized in Figure 9-1. 

Minic Z, Simon V, Penverne B, Gaill F, Herve G. Contribution 64. of the bacterial endosymbiont to the biosynthesis of pyrimidine nucleotides in the deep-sea tube worm Riftia pachyptila. J. Biol. 

Chapter 19). Biosynthesis of pyrimidine nucleotides can occur by a de novo pathway or by the reutilization of preformed pyrimidine bases or ribonucleosides (salvage pathway). 

The biosynthesis of pyrimidine nucleotides may be conveniently considered in two stages the formation of uridine monophosphate (UMP) and the conversion of UMP to other pyrimidine nucleotides. 

In mammals, carbamoyl phosphate synthetase II is the key regulatory enzyme in the biosynthesis of pyrimidine nucleotides. The enzyme is inhibited by UTP, the product of the pathway, and stimulated by purine nucleotides. In many bacteria, aspartate carbamoyl transferase is the key regulatory enzyme. It is inhibited by CTP and stimulated by ATP. 

C] aspartie and orotic acids for the biosynthesis of pyrimidine nucleotides. 

K45 Koreshkova, N. A., Matveenko, V. N. and Fedorov, N. A. Comparative effectiveness of the biosynthesis of pyrimidine nucleotides by de novo and salvage pathways in a rat bone marrow culture. Mater. Resp. S ezda Gematol. Trans-fuziologov Beloruss., (Pub. 1973) 42-45 (Russ.)... 

Glycine, glutamine, and aspartic acid are involved in purine nucleotide biosynthesis. Aspartic acid and glycine play a role in the biosynthesis of pyrimidine nucleotides. Protein biosynthesis has already been discussed. Glutathione is a tripeptide composed of glutamine, cysteine, and glycine its biosynthesis involves the formation of two peptide bonds. The tripeptide... 

Citric acid (1.8 g/1) is the predominant organic acid in milk. During storage it disappears rapidly as a result of the action of bacteria. Other acids (lactic, acetic) are degradation products of lactose. The occurrence of orotic acid (73 mg/1), an intermediary product in biosynthesis of pyrimidine nucleotides, is specific for milk ... 

Phosphoribosyl-l-pyrophosphate (PRPP) may be considered a precursor in the de novo sjmthetic reactions of purines, since this ribose derivative was required for the formation of 5-phosphoribosylamine (PRA). PRA was the precursor of nitrogen 9, ribose, and phosphate of the completed purine nucleotide structure (Section II, B, 1). PRPP was also a key substance in the biosynthesis of pyrimidine nucleotides. This compound was formed from ribose 5-phosphate and ATP by a pyrophosphorylation of carbon 1 of ribose 5-phosphate (78-80). This was an unusual kinase reaction in that pyrophosphate was transferred rather than phosphate as was the case with most kinases. The ribose 5-phosphate required for the syntheas of PRPP probably originated from glucose, and was formed either by an oxidative pathway from glucose 6-phosphate via 6-pho hogluconate and ribulose 5-phosphate (81) or anaerobically from fructose 6-pho hate (88). The formation of PRPP is shown in Fig. 4. 

The first step of the biosynthesis of pyrimidine nucleotides is the irreversible carbamylation of L-aspartate by carbamyl-phosphate to form carbamylaspartate (catalyzed by the enzyme aspartate transcarbamylase). Next, carbamylaspartate is converted, by ring closure, to dihydro-orotic acid which, in turn, is reduced to orotic acid, catalyzed by the enzyme orotic acid dehydrogenase (OAD). Orotic acid (6-carboxyuracil) reacts with 5 -phosphoribosyl--1-pyrophosphate (PRPP) to form orotidine monophosphate (OMP). 

In reversal experiments using the human B-cell line (RPMI 8422), pyrazofurin inhibited cell growth at a concentration of 10 M. This PF effect was completely reversed with the addition of 10 M to 10 M of uridine or cytidine but not by orotic acid or orotidine. This is additional confirmation of the assumption that the site of inhibition in the biosynthesis of pyrimidine nucleotides is at the site of orotidylic carboxylase ... 

In Figure 8 (top, left) the structure of orotidine, the nucleoside of the above nucleotide, is shown a stability ccmstant of its Cd complex has been measured [93], Orotidinate 5 -monophosphate (OMP is involved in the biosynthesis of pyrimidine-nucleotides OMP is decarboxylated to UMP (cf. Figures 1 and 8), which is then further transformed, e.g., to UTP or CTP (Figure 10). OMP itself exists predominately in the syn conformation, which is depictured for H(Or) in Figure 8 (top, left) (i.e., the (C2)0 group being above the ribose ring). 

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