Salvage synthesis of nucleotides

Important enzymes in the salvage synthesis of nucleotides (Figure 22.2) are as follows ... 

Non- oxidative branch Pentose-5 -Phosphates Ribose-5-P 2 deoxy ribose-5-P 5 -phosphoribosyl-1 -pyrophosphate (PRPP) i) Structural components of nucleotides a. Basal structural component of RNA b. Basal structural component of DNA c. Precursor of both de novo and salvage synthesis of nucleotides ii) Intermediate products of purine metabolism and act as precursor molecules of cofactors, e g., riboflavin, flavin mononucleotide (FMN), flavin adenine di nucleotide (FAD) iii) Precursor of the amino acid. Histidine. 

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. 

Two types of pathways lead to nucleotides the de novo pathways and the salvage pathways. De novo synthesis of nucleotides begins with their metabolic precursors amino acids, ribose 5-phosphate, C02, and NH3. Salvage pathways recycle the free bases and nucleosides released from nucleic acid breakdown. Both types of... 

The equilibria in these phosphoribosyltransferase reactions favor nucleotide synthesis, and since the inorganic pyrophosphate released is rapidly hydrolyzed by inorganic pyrophosphatase, the coupling of these reactions makes the synthesis of nucleotide irreversible. However, the efficiency of salvage is heavily dependent on the intracellular concentration of PRPP. 

In the purine salvage pathway, purine bases obtained from the normal turnover of cellular nucleic acids or (to a lesser extent) from the diet are reconverted into nucleotides. Because the de novo synthesis of nucleotides is metabolically expensive (i.e., relatively large amounts of phosphoryl bond energy are used), many cells have mechanisms to retrieve purine bases. Hypoxanthine-guaninephos-phoribosyltransferase (HGPRT) catalyzes nucleotide synthesis using PRPP and either hypoxanthine or guanine. The hydrolysis of pyrophosphate makes these reactions irreversible. 

Phospho-ot-D-ribosyl-l-pyrophosphate (PRPP) is an intermediate in both the de novo synthesis of nucleotides (Figure 22.1) and the salvage synthesis (reutilization) of nucleotides (Figure 22.2). 

In this chapter, the authors complete their treatment of the biosyntheses of the major classes of macromolecular precursors by describing the synthesis of the purine and pyrimidine nucleotides. Besides being the precursors of RNA and DNA, these compounds serve a number of other important roles that are reviewed in the opening paragraph of the chapter. Nucleotide nomenclature is reviewed in the introduction to the chapter, as is an outline for the synthesis of nucleotides through de novo and salvage pathways. 

FIGURE 8 De novo and salvage pathways of nucleotide synthesis. 

The de novo synthesis of inosinic acid The salvage pathways Purine nucleotide interconversions Other enzymes... 

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. 

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... 

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 ... 

PRPP is an "activated pentose" that participates in the synthesis of purines and pyrimidines, and in the salvage of purine bases (see p. 294). Synthesis of PRPP from ATP and ribose 5-phosphate is catalyzed by PRPP synthetase (ribose phosphate pyrophosphokinase, Figure 22.6). This enzyme is activated by inorganic phosphate (Pi) and inhibited by purine nucleotides (end-product inhibition). [Note The sugar moiety of PRPP is ribose, and therefore ribonucleotides are the end products of de novo purine synthesis. When deoxy-ribonucleotides are required for DNA synthesis, the ribose sugar moiety is reduced (see p. 295).]... 

An alternative pathway for synthesis of quinoli-nate from aspartate and a triose phosphate exists in bacteria and in plants and provides the major route of nicotinic acid synthesis in nature. In E. coli the reaction is catalyzed by two enzymes, one an FAD-containing L-aspartate oxidase which oxidizes aspartate to a-iminoaspartate.228 The latter condenses with dihydroxyacetone-P to form quinolinate (Eq. 25-13).229 There are at least two other pathways for synthesis of quinolinic acid as well as five or more salvage pathways for resynthesis of degraded pyridine nucleotide coenzymes.224/230/231... 

In addition to the pathways for synthesis de novo, mammalian cells and microorganisms can readily form mononucleotides from purine bases and their nucleosides and to a lesser extent from pyrimidine bases and their nucleosides. In this way bases and nucleosides formed by constant breakdown of mRNA and other nucleic acids can be reconverted (or salvaged ) to useful nucleotides, and the energy expended by the cell in synthesizing the bases is retained. 

Azathioprine [a zah THIO preen] has been the cornerstone of immunosuppressive therapy over the last several decades. It has a nitroimidazoloyl side chain attached to the sulfur of 6-mercap-topurine, which is removed by non-enzymatic reduction in the body by glutathione to yield 6-mercaptopurine (6-MP). The latter is then converted to the corresponding nucleotide, thioinosinic acid (TIMP), by the salvage pathway enzyme, hypoxanthine-gua-nine phosphoribosyl transferase. The immunosuppressant effects of azathioprine are due to this fraudulent nucleotide. (See pp. 380-381 for a discussion of 6-MP s mechanism of action, resistance, pharmacokinetics, and adverse effects.) Because of their rapid proliferation in the immune response, and their dependence on de novo synthesis of purines required for cell division, lymphocytes are predominantly affected by the cytotoxic effects of azathioprine. The drug has little effect on suppressing a secondary immune response. 

While the PRTases salvage nucleobases within cells, nucleosides such as adenosine and uridine are present in the blood at much higher concentrations ( 1 pM) than the equivalent nucleobases, adenine and uracil. Indeed, the brain synthesizes pyrimidine nucleotides (UTP and CTP) via salvage synthesis from uridine produced by the liver and released into the circulation. Human cells may contain at least three types of nonspecific nucleoside transporters, and nucleosides are internalized more rapidly than nucleobases. 

B lymphocytes will be eliminated during continuous culture because these cells have a short life span in culture. Commercially available myeloma cells for hybridoma production have mutations in one of the enzymes of the salvage pathway of purine nucleotide biosynthesis. Hybridoma cells are cultured in medium that forces the cells to utilize the salvage pathway for nucleotide synthesis. The mutated myeloma cells or hybridization products of two myeloma cells will die in this selection medium since they are incapable of nucleotide synthesis under these propagation conditions. However, myeloma cells that have fused to the B lymphocytes derived from the spleen of the immunized animal will have an intact salvage pathway and will survive in the selection medium. Thus, only the B lymphocytes-myeloma hybridomas will survive prolonged culture in the selection medium. 

Purines are degraded to urate in human beings. Gout, a disease that affects joints and leads to arthritis, is associated with the excessive accumulation of urate. The Lesch-Nyhan syndrome, a genetic disease characterized by self-mutilation, mental deficiency, and gout, is caused by the absence of hypoxanthine-guanine phosphoribosyltransferase. This enzyme is essential for the synthesis of purine nucleotides by the salvage pathway. 

PRPP is the activated intermediate in the synthesis of phosphoribosylamine in the de novo pathway of purine formation of purine nucleotides from free bases by the salvage pathway of orotidylate in the formation of pyrimidines of nicotinate ribonucleotide of phosphoribosyl ATP in the pathway leading to histidine and of phosphoribosylanthranilate in the pathway leading to tryptophan. 

The ability to acquire choline and amino acids from the diet and to salvage the purine nucleotides makes the role of N, NlO-methylene-THF in dTMP synthesis the most metabolically significant function for this vitamin. 

Most of the free purines derived from the breakdown of DNA, RNA, and nucleotides in the diet are catabolized to xanthine and then to uric acid in the gut mucosa. The AMP and GMP biosynthesized in the body can also be bmken down to free purines, such as adenine, guanine, and hypoxanthine. These purines, in contrast to those derived frcim the diet, are largely reused for the synthesis of ATP and GTP- They are first converted back to AMP or GMP in a pathway of reutiliza-lion called the purine salvage pathway. For example, adenine phosphoribosyl-transferase (PRPP) catalyzes the conversion of adenine to AMP. Here, PRPP serves as the source of the phosphoribose group. Pyrophosphate is a product of the reaction. 

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. 

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