Nucleotides ribonucleotide synthesis

Overview of Nucleotide Metabolism Synthesis of Purine Ribonucleotides de Novo... 

RNA (ribonucleic acid) A large linear molecule made up of nucleotides, ribonucleotides which contain the bases uracil, guanine, cytosine and adenine. RNA occurs in several forms, transfer RNA (tRNA), ribosomal RNA (rRNA) and messenger RNA (mRNA). All are concerned with protein synthesis. All cellular RNAs are synthesized by transcription of chromosomal DNA which acts as the template. 

See also Figure 4.3, RNA, De Novo Biosynthesis of Purine Nucleotides, Ribonucleotide Reductase and Deoxyribonucleotide Biosynthesis, Salvage Routes to Deoxyribonucleotide Synthesis... 

The most conspicuous use of iron in biological systems is in our blood, where the erythrocytes are filled with the oxygen-binding protein hemoglobin. The red color of blood is due to the iron atom bound to the heme group in hemoglobin. Similar heme-bound iron atoms are present in a number of proteins involved in electron-transfer reactions, notably cytochromes. A chemically more sophisticated use of iron is found in an enzyme, ribo nucleotide reductase, that catalyzes the conversion of ribonucleotides to deoxyribonucleotides, an important step in the synthesis of the building blocks of DNA. 

Mercaptopurine (6-MP) is an oral purine analog that is converted to a ribonucleotide to inhibit purine synthesis. Mercaptopurine is converted into thiopurine nucleotides, which are catabolized by thiopurine S-methyltransferase (TPMT), which is subject to genetic polymorphisms and may cause severe myelosuppression. TPMT status may be assessed prior to therapy to reduce drug-induced morbidity and the costs of hospitalizations for neutropenic events. Mercaptopurine is poorly absorbed, with a time to peak concentration of 1 to 2 hours after an oral dose. The half-life is 21 minutes in pediatric patients and 47 minutes in adults. Mercaptopurine is used in the treatment of acute lymphocytic leukemia and chronic myelogenous leukemia. Significant side effects include myelosuppression, mild nausea, skin rash, and cholestasis. When allopurinol is used in combination with 6-MP, the dose of 6-MP must be reduced by 66% to 75% of the usual dose because allopurinol blocks the metabolism of 6-MP. 

Ribonucleotide reductase is required for the formation of the deoxyribonucleotides for DNA synthesis. Figure 1-18-2 shows its role in dTMP synthesis, and Figure 1-18-3 shows all four nucleotide substrates ... 

Figure 20.5 Nucleotides that are required for RNA or DNA synthesis. Note that the ribonucleotide diphosphates are the precursors for the formation of deoxyribonucleotides. It is the triphosphates that are required for polymerisation to form either RNA or DNA (see text).

The two classes of nucleotide that must be synthesised are the pyrimidine and purine ribonucleotides for RNA synthesis and the deoxyribonucleotides for DNA synthesis. For the original sources of the nitrogen atoms in the bases of the pyrimidine and purine nucleotides, see Figure 20.7. The pathway for the synthesis of the pyrimidine nucleotides is... 

These enzymes use DNA as a template and the ribonucleotide substrates must be present in the nucleus, i.e. ATP, GTP, CTP and UTP. Similarly, for the synthesis of DNA, the deoxyribonucleotides dATP, dGTP, dCTP and dTTP must be present in the nucleus. In addition, since the ribonucleoside diphosphates are required for synthesis of deoxyribonucleotides, these diphosphates must also be present. The concentrations of these various nucleotides have not been measured in the nucleus but it may be assumed that the concentrations of the ribonucleotides will be similar in the nucleus to those in the cytosol. 

During the past 15 years data from experiments with different types of animal tissues and micro-organisms, using intact cells, crude extracts or purified enzymes, have firmly established the general occurrence of nucleotide reductases and have stressed their importance for DNA synthesis in essentially all types of rapidly growing cells [54]. It has been proposed that ribonucleotide diphosphates lose a hydroxide ion from C-2 to form a carbonium ion which is then stero-specifically reduced by a hydride ion derived from thioredoxin [54]. Adenosine diphosphate and guanosine diphosphate (as well as uridine and cytidine diphosphates) are reduced in this manner. 

The effect of 6-mercaptopurine on the incorporation of a number of C-labelled compounds into soluble purine nucleotides and into RNA and DNA has been studied in leukemia L1210, Ehrlich ascites carcinoma, and solid sarcoma 180. At a level of 6-mercaptopurine that markedly inhibited the incorporation of formate and glycine, the utilization of adenine or 2-aminoadenine was not affected. There was no inhibition of the incorporation of 5(or 4)-aminoimidazole-4(5)-carboxamide (AIC) into adenine derivatives and no marked or consistent inhibition of its incorporation into guanine derivatives. The conversion of AIC to purines in ascites cells was not inhibited at levels of 6-mercaptopurine 8-20 times those that produced 50 per cent or greater inhibition of de novo synthesis [292]. Furthermore, AIC reverses the inhibition of growth of S180 cells (AH/5) in culture by 6-mercaptopurine [293]. These results suggest that in all these systems, in vitro and in vivo, the principal site at which 6-mercaptopurine inhibits nucleic acid biosynthesis is prior to the formation of AIC, and that the interconversion of purine ribonucleotides (see below) is not the primary site of action [292]. Presumably, this early step is the conversion of PRPP to 5-phosphoribosylamine inhibited allosterically by 6-mercaptopurine ribonucleotide (feedback inhibition is not observed in cells that cannot convert 6-mercaptopurine to its ribonucleotide [244]. 

The synthesis of purine nucleotides (1) starts from IMP. The base it contains, hypoxanthine, is converted in two steps each into adenine or guanine. The nucleoside monophosphates AMP and CMP that are formed are then phos-phorylated by nucleoside phosphate kinases to yield the diphosphates ADP and GDP, and these are finally phosphorylated into the triphosphates ATP and CTP. The nucleoside triphosphates serve as components for RNA, or function as coenzymes (see p. 106). Conversion of the ribonucleotides into deoxyribo-nucleotides occurs at the level of the diphosphates and is catalyzed by nucleoside diphosphate reductase (B). 

The answer is D. Impaired immune function in severe combined immunodeficiency (SCID) is the direct result of blocked DNA synthesis due to inadequate supplies of de-oxyribonucleotides in B and T cells. This effect arises by dATP-induced allosteric inhibition of ribonucleotide reductase, which catalyzes reduction of the 2 -hydroxyl groups on ADP and GDP to form dADP and dCDP. The ultimate cause of many cases of SCID is adenosine deaminase deficiency, which leads to accumulation of dATP and consequent inhibition of ribonucleotide reductase. Although the other enzymes mentioned are also involved in purine nucleotide metabolism, their deficiencies do not lead to SCID. 

Pemetrexed is chemically similar to folic acid. It inhibits three enzymes used in purine and pyrimidine synthesis - thymidylate synthetase, dihydrofolate reductase, and glycinamide ribonucleotide formyl transferase. By inhibiting the formation of precursor purine and pyrimidine nucleotides, pemetrexed prevents the formation of DNA and RNA. In 2004 it was approved for treatment of malignant pleural mesothelioma and as a second-line agent for the treatment of non-small cell lung cancer. Adverse effects include gastrointestinal complaints, bone marrow suppression, alopecia, allergic and neurotoxic reactions. 

Transcription It involves copying of DNA base sequences into a mRNA. A small portion of DNA double helix unwinds and are of the two DNA strands act as the template for the synthesis of /wRNA. Ribonucleotides assemble along the uncoiled template in accordance with the base pairing principle. For example, U in the RNA being formed appears opposite A of DNA C opposite G A opposite T and G opposite C of DNA. Then occurs the bond formation between the various nucleotides thus assembled. 

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... 

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).]... 

The nucleotides described thus far in this chapter all contain ribose (ribonucleotides). The nucleotides required for DNA synthesis, however, are 2 -deoxyribonucleotides, which are produced from ribonucleoside diphosphates by the enzyme ribonucleotide reductase. 

Ribonucleotides are reduced to the 2 -deoxyribo-nucleotides (Eq. 16-21) that are needed for DNA synthesis by enzymes that act on either the di- or triphosphates of the purine and pyrimidine nucleosides348-351 (Chapter 25). These ribonucleotide reductases utilize either thioredoxin or glutaredoxin (Box 15-C) as the immediate hydrogen donors (Eq. 16-22). The pair of closely spaced -SH groups in the reduced thioredoxin or glutaredoxin are converted into a disulfide bridge at the same time that the 2 -OH of the ribonucleotide di- (or tri-) phosphate is converted to H20. While some organisms employ a vitamin B12-... 

IMP Is Converted into AMP and GMP Synthesis of Pyrimidine Ribonucleotides de Novo UMP Is a Precursor of Other Pyrimidine Nucleotides... 

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