Pyrimidine de novo synthesis

Utilizes pre-formed purine or pyrimidine De novo synthesis pathway... 

Gero, A. M., Brown, G. V., and O Sullivan, W. J. (1984). Pyrimidine de novo synthesis during the life cycle of the intraerythrocytic stage of Plasmodium falciparum, f. Parasitol. 70,536-541. 

FIG. 6.12 Pyrimidine de novo synthesis pathway. Enzymes are as follows (1) carbamoyl-phosphate synthetase II (2) asparate carbamoyl-transferase (3) dihydro-orotase (4) dihydro-orotate oxidase (5) orotate phosphoribosyltransferase (6) orotidine-5 -phosphate decarboxylase (7) nucleoside monophosphate kinase (8) nucleotide diphospho kinase (9) CTP synthetase. 

Methotrexate is a potent inhibitor of DHFR, the enzyme responsible for the reduction of folic acid to dihydro- and tetrahydrotolic acid, precursors to one-carbon donation in purine and pyrimidine de novo synthesis. Methotrexate is... 

Ruckemann K, Fairbanks LD, Carrey EA, et al. Leflunomide inhibits pyrimidine de novo synthesis in mitogen-stimulated T-lymphocytes from healthy humans. J Biol Chem 1998 273 21682-1691. 

The mode of action has been a subject for research for a number of years. While it was originally thought that maleic hydrazide replaced uracil in the RNA sequence, it has been deterrnined that the molecule may be a pyrimidine or purine analogue and therefore base-pair formation is possible with uracil and thymine and there exists the probabiHty of base-pair formation with adenine however, if maleic hydrazide occurs in an in vivo system as the diketo species, then there remains the possibiHty of base-pairing with guanine (50). Whatever the mechanism, it is apparent that the inhibitory effects are the result of a shutdown of the de novo synthesis of protein. 

In many cells, the capacity for de novo synthesis to supply purines and pyrimidines is insufficient, and the salvage pathway is essential for adequate nucleotide synthesis. In patients with Lesch-Nyhan disease, an enzyme for purine salvage (hypoxanthine guanine phosphoribosyl pyrophosphate transferase, HPRT) is absent. People with this genetic deficiency have CNS deterioration, mental retardation, and spastic cerebral palsy associated with compulsive self-mutilation, Cells in the basal ganglia of the brain (fine motor control) normally have very high HPRT activity. These patients also all have hyperuricemia because purines cannot be salvaged. 

Figure 20.8 Summary of pathways for de novo synthesis of purine and pyrimidine nucleotides. C represents transfer of a single carbon atom (a one-carbon transfer). Details are provided in Appendix 20.1. IMP - inosine monophosphate. For thymi-dylate synthesis, see Figure 20.12a.

De novo synthesis of purines and pyrimidines yields the monophosphates IMP and UMP, respectively (see p. 188). All other nucleotides and deoxynucleotides are synthesized from these two precursors. An overview of the pathways involved is presented here further details are given on p. 417. Nucleotide synthesis by recycling of bases (the salvage pathway) is discussed on p. 186. 

A. Salvage pathways allow synthesis of nucleotides from free purines or pyrimidines that arise from nucleic acid degradation or dietary sources, which is more economical for the cell than de novo synthesis. 

The answer is E. Methotrexate is an analog of folic acid that binds with very high affinity to the substrate-binding site of dihydrofolate reductase, the enzyme that catalyzes conversion of DHF to THE, which is used in various forms by enzymes of both the purine and pyrimidine de novo synthetic pathways. Thus, synthesis of dTMP from dUMP catalyzed by thymidylate synthetase and several steps in purine synthesis catalyzed by formyltransferase are indirectly blocked by the action of methotrexate because both those enzymes require THE coenzymes. 

Fusion with the cells compensates for this deficiency. When fused and unfused cells are incubated in the presence of the folic acid antagonist aminopterin, the de novo synthesis of purines and pyrimidines for DNA is blocked. Cells deficient in HGPRT die, whereas hybrid cells are able to bypass aminopterin blockage by metabolism of hypoxanthine and thymidine added to the medium. In the generation of mouse hybridomas, an number of myelomas deficient in HGPRT are available, all originating from MOPC 21, a spontaneous myeloma from the BALB/c mouse strain. 

Purine and pyrimidine nucleotides are essential for a vast number of biological processes such as the synthesis of RNA, DNA, phospholipids, glycogen, and the si-alylation and glycosylation of proteins. Both purines and pyrimidines can be synthesized de novo in mammalian cells through multistep processes. In addition to the de novo synthesis, purine nucleotides can also be synthesized via the salvage of... 

FIGURE 22-36 De novo synthesis of pyrimidine nucleotides biosynthesis of UTP and CTP via orotidylate. The pyrimidine is constructed from carbamoyl phosphate and aspartate. The ribose 5-phosphate is then added to the completed pyrimidine ring by orotate phosphori-bosyltransferase. The first step in this pathway (not shown here see Fig. 18-11a) is the synthesis of carbamoyl phosphate from C02 and NH), catalyzed in eukaryotes by carbamoyl phosphate synthetase II. 

Free purine and pyrimidine bases are constantly released in cells during the metabolic degradation of nucleotides. Free purines are in large part salvaged and reused to make nucleotides, in a pathway much simpler than the de novo synthesis of purine nucleotides described earlier. One of the primary salvage pathways consists of a single reaction catalyzed by adenosine phosphoribosyltransferase, in which free adenine reacts with PRPP to yield the corresponding adenine nucleotide ... 

The de novo synthesis of pyrimidines and purines, particularly purines, is energetically expensive and hence most (80%) of the purines and pyrimidines obtained from the degradation of nucleic acids. 

The fourth step in the de novo synthesis of pyrimidine nucleotides—the conversion of dihydroorotic acid to orotic acid—is catalyzed by dihydroorotic acid dehydrogenase. The enzyme, located on the cytosolic side of the inner membrane of mitochondria, is a target for antitumor agents. 

Dihydroorotase catalyzes the intramolecular cyclization of 7V-carbamyl-L-aspartic acid to L-dihydroorotic acid. In mammals, the activity is present in a trifunctional enzyme that catalyzes the first three steps in the de novo synthesis of pyrimidine nucleotides. 

In vivo, Leflunomide is rapidly converted into its pharmacologically active metabolite A77 1726 (Herrmann et al., 2000). Although the precise mode of action of Leflunomide in vivo remains elusive, A77 1726 has been shown in vitro to inhibit reversibly dihydro-orotate dehydrogenase (DHODH), which catalyzes a rate-limiting step in the de novo synthesis of pyrimidines (Cherwinski et al., 1995 Williamson et al., 1996). The inhibition of DHODH activity by A77 1726 might explain part of its mechanism of action in suppressing inflammation. 

In de Novo Synthesis, the Pyrimidine Ring Is Assembled from Biearbonate, Aspartate, and Glutamine... 

In de novo synthesis of pyrimidines, the ring is synthesized first and then it is attached to ribose to form a pyrimidine nucleotide (Figure 25.2). Pyrimidine rings are assembled from bicarbonate, aspartic acid, and ammonia. Although ammonia can be used directly, it is usually produced from the hydrolysis of the side chain of glutamine. 

Scheme 12-9 De novo synthesis of pyrimidine nucleotides (simplified).

Figure 7-20. De novo synthesis of purines and pyrimidines. Ribonucleotide reductase (R.R.) catalyzes the reduction of the ribose moiety in ADP, GDP, and CDP to deoxyribose. The source of each of the atoms is indicated in the boxes at the bottom of the figure. In hereditary orotic aciduria, the enzymes converting orotate to UMP are defective ( ).

Nucleotides are synthesized by two types of metabolic pathways de novo synthesis and salvage pathways. The former refers to synthesis of purines and pyrimidines from precursor molecules the latter refers to the conversion of preformed purines and pyrimidines—derived from dietary sources, the surrounding medium, or nucleotide catabolism—to nucleotides, usually by addition of ribose-5-phosphate to the base. De novo synthesis of purines is based on the metabolism of one-carbon compounds. 

Phosphoribosyl-l-pyrophosphate (PRPP) is a key intermediate in nucleotide biosynthesis. It is required for de novo synthesis of purine and pyrimidine nucleotides and the salvage pathways, in which purines are converted to their respective nucleotides via transfer of ribose 1-phosphate group from PRPP to the base that is. 

Folic acid or the folate coenzyme [6] is a nutritional factor both for the parasites and the hosts. It exists in two forms, viz. dihydro- and tetrahydrofolic acids [4,5] which act as cofactors involved in the transfer of one carbon units like methyl, hydroxymethyl and formyl. The transfer of a one carbon unit is associated with de novo synthesis of purines, pyrimidines and amino acids. Mammals can not synthesize folate and, therefore, depend on preformed dietary folates, which are converted into dihydrofolate by folate reductase. Contrary to this, a number of protozoal parasites like plasmodia, trypanosomes and leishmania can not utilize exogenous folate. Consequently, they carry out a de novo biosynthesis of their necessary folate coenzymes [12]. The synthesis of various folates follows a sequence of reactions starting from 2-amino-4-hydroxy-6-hydroxymethyldihydropteridine (1), which is described in Chart 4 [13,14]. 

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