Purines/purine nucleotides synthesis

Figure 34-7 summarizes the roles of the intermediates and enzymes of pyrimidine nucleotide biosynthesis. The catalyst for the initial reaction is cytosolic carbamoyl phosphate synthase II, a different enzyme from the mitochondrial carbamoyl phosphate synthase I of urea synthesis (Figure 29-9). Compartmentation thus provides two independent pools of carbamoyl phosphate. PRPP, an early participant in purine nucleotide synthesis (Figure 34-2), is a much later participant in pyrimidine biosynthesis.

Zalkin H, Dixon JE De novo purine nucleotide synthesis. Prog Nucleic Acid Res Mol Biol 1992 42 259. 

Figure 20.10 The positions in the pathway for de novo purine nucleotide synthesis where GLUCOSE provides the ribose molecule and GLUTAMINE provides nitrogen atoms. The pathway begins with glucose which provides ribose 5-phosphate, via the pentose phosphate pathway (Chapter 6). Glutamine provides its amide nitrogen in two reactions formation of 5-phosphoribosylamine and formation of guanosine monophosphate (GMP) from xantho-sine 5-phosphate (XMP).

Methotrexate Inhibits DHFR inhibits TS inhibits de novo purine nucleotide synthesis Breast cancer, head and neck cancer, osteogenic sarcoma, primary central nervous system lymphoma, non-Hodgkin s lymphoma, bladder cancer, chorioca rcinoma Mucositis, diarrhea, myelosuppression with neutropenia and thrombocytopenia... 

Pemetrexed Inhibits TS, DHFR, and purine nucleotide synthesis Mesothelioma, non-small cell lung cancer Myelosuppression, skin... 

Mercaptopurine Inhibits de novo purine nucleotide synthesis incorporation of triphosphate into RNA incorporation of triphosphate into DNA AML Myelosuppression, immunosuppression, and hepatotoxicity... 

De Novo Purine Nucleotide Synthesis Begins with PRPP... 

Three major feedback mechanisms cooperate in regulating the overall rate of de novo purine nucleotide synthesis and the relative rates of formation of the two end products, adenylate and guanylate (Fig. 22-35). The first mechanism is exerted on the first reaction that is unique to purine synthesis—transfer of an amino group to PRPP to form 5-phosphoribosylamine. This reaction is catalyzed by the allosteric enzyme glutamine-PRPP amidotransferase, which is inhibited by the end products IMP, AMP, and GMP. AMP and GMP act synergisti-cally in this concerted inhibition. Thus, whenever either AMP or GMP accumulates to excess, the first step in its biosynthesis from PRPP is partially inhibited. 

The common pyrimidine ribonucleotides are cytidine 5 -monophosphate (CMP cytidylate) and uridine 5 -monophosphate (UMP uridylate), which contain the pyrimidines cytosine and uracil. De novo pyrimidine nucleotide biosynthesis (Fig. 22-36) proceeds in a somewhat different manner from purine nucleotide synthesis the six-membered pyrimidine ring is made first and then attached to ribose 5-phosphate. Required in this process is carbamoyl phosphate, also an intermediate in the urea cycle (see Fig. 18-10). However, as we noted... 

Biosynthesis of UMP. The parts of the intermediates derived from aspartate are shown in red. Bold type indicates atoms derived from carbamoyl phosphate. In contrast to purine nucleotide synthesis, where ring formation starts on the sugar, in pyrimidine biosynthesis the pyrimidine ring is completed before being attached to the ribose. 

The absence of de novo purine nucleotide synthesis in protozoal parasites as well as in the trematode Schistosoma mansoni is reflected in the relative importance of purine phosphoribosyl transferases in many parasite species. 

Ebbole, D. J., and Zalkin, H. (1987). Cloning and characterization of a 12-gene cluster from Bacillus subtilis encoding nine enzymes for de novo purine nucleotide synthesis. J. Biol. Chem., 262, 8274-8287. 

Methotrexate is a potent inhibitor of dihydrofolate reductase, with an affinity 1,000-fold greater than that of dUiydrofolate. Chemotherapy consists of alternating periods of administration of methotrexate and folate (normally as 5-formyl-tetrahydrofolate, leucovorin) to replete the normal tissues and avoid induction of folate deficiency- so-called leucovorin rescue. As well as depleting tissue pools of tetrahydrofolate, methotrexate leads to the accumulation of relatively large amounts of 10-formyl-dihydrofolate, which is apotentinhibitor of both thymidylate synthetase and glycinamide ribotide transformylase, an intermediate step in purine nucleotide synthesis. It is likely that this, rather than simple depletion of tetrahydrofolate, is the basis of the cytotoxic action of methotrexate (Barametal., 1988). 

Figure 25.6. de Novo Pathway for Purine Nucleotide Synthesis. The origins of the atoms in the purine ring are indicated. 

Mycophenolate mofetil is the 2-moiphohnoethyl ester of mycophenolic acid (MPA). It is a prodrug that is rapidly hydrolyzed to the active form, mycophenolic acid. Mycophenolic acid is a selective, uncompetitive and reversible inhibitor of inosine monophosphate dehydrogenase (IMPDH). IMPDH is an important enzyme in the de novo pathway of purine nucleotide synthesis. This pathway is very important in B and T lymphocytes for proliferation. Other cells can use salvage pathways. Therefore MPA inhibits lymphocyte proliferation and functions. The mofetil ester is first converted to MPA which then is metabolized to an inactive glucuronide (Alhson and Eugui, 2000). MPA has a half-hfe of about 16 hours (Fulton and Markham, 1996). 

Salvage pathways of purine nucleotide synthesis. The preformed purines can be converted to mononucleotides in a single step, using PRPP. 

The names of nucleotides containing both deoxyribose and thymine do not have the prefix deoxy. Inosine-5 -monophosphate (IMP) is an intermediate in purine nucleotide synthesis. The base component of IMP is hypoxanthine. 

The relative importance of the de novo and salvage pathways is unclear. However, the severe symptoms of hereditary HGPRT deficiency indicate that the purine salvage pathway is vitally important. In addition, investigations of purine nucleotide synthesis inhibitors for treating cancer indicate that both pathways must be inhibited for significant tumor growth suppression. 

Aspartate has many fates, too. For example, its nitrogen is used in the biosynthesis of arginine and urea. Similar reactions are involved in purine nucleotide synthesis. The entire aspartate molecule is used in pyrimidine nucleotide biosynthesis. In plants and bacteria, aspartate is a precursor to three other amino acids (i.e., methionine,threonine, and isoleucine) via its conversion to homoserine (see here). Homoserine then leads in separate pathways to methionine and threonine. Threonine, in turn, can be converted to isoleucine. In bacteria, aspartic / -semialdehyde is a precursor to lysine. 

De novo purine nucleotide synthesis proceeds by the synthesis of the purine base upon the ribose sugar moiety. 

Figure 22.5 Transformylation reactions in purine nucleotide synthesis. 

Azaserine is an antibiotic that is a potent inhibitor of purine nucleotide synthesis. Azaserine is similar in structure to glutamine and is an irreversible inhibitor of glutamine amidotransferases, which catalyze the ATP-dependent transfer of the amido nitrogen of glutamine to an acceptor. Four such reactions occur in nucleotide synthesis. 

Boonlayangoor, P., Albach, R. A. and Booden, T. (1980) Purine nucleotide synthesis in Entamoeba histolytica a preliminary study. Arch. Invest. Med. 11 (Mex, 1 Suppl.) 83-88. 

L8. Lowy, B. A., Cook, J, K., and London, I. M., Enzymatic deficiencies of purine nucleotide synthesis in the human eiythrocyte. J. Biol. Chem. 237, 1622... 

Historically, the first route of purine nucleotide synthesis to be studied in detail was that involving the phosphorylation of adenosine by ATP. More recently, evidence has been presented for the existence in animal tissues of two other purine nucleoside kinases. The general reaction is... 

Fig. 10-3. Possible relationship between purine catabolism and purine nucleotide synthesis.

Hartman, S.C., Buchanan, J.M. Nucleic acids, purines, pyrimidines (nucleotide synthesis). Annu. Rev. Biochem. 28, 365-410 (1959)... 

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