If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
Locations of Mapped Genes:
|Superclasses:||Biosynthesis → Nucleosides and Nucleotides Biosynthesis → 2'-Deoxyribonucleotides Biosynthesis → Pyrimidine Deoxyribonucleotides De Novo Biosynthesis|
|Biosynthesis → Nucleosides and Nucleotides Biosynthesis → Pyrimidine Nucleotide Biosynthesis → Pyrimidine Nucleotides De Novo Biosynthesis → Pyrimidine Deoxyribonucleotides De Novo Biosynthesis|
Pyrimidine and purine nucleoside triphosphates are the activated precursors of DNA and RNA. Their diphosphates form activated derivatives of other molecules, such as UDP-D-glucose, for use in biosynthesis. The pathway for de novo biosynthesis of pyrimidine deoxyribonucleotides is derived from the pathway for superpathway of pyrimidine ribonucleotides de novo biosynthesis as shown above in the pathway links. In addition to de novo biosynthesis, salvage pathways re-utilize exogenous free bases and nucleosides and some of the resulting pyrimidine nucleotides can enter the de novo biosynthesis pathways (see salvage pathways of pyrimidine deoxyribonucleotides and salvage pathways of pyrimidine ribonucleotides). These essential, evolutionarily conserved biosynthetic and salvage pathways are found in both prokaryotes and eukaryotes.
The pyrimidine nucleoside triphosphate products of these pathways are incorporated into DNA (dCTP and dTTP) and RNA (CTP and UTP). The de novo biosynthetic pathways are necessary in order to supply the cell with nucleoside triphosphates for stable RNA and DNA synthesis when precursors are limiting. These biosynthetic pathways consume relatively large amounts of high energy phosphate and reducing power. In Escherichia coli they are regulated both by allosteric enzymes and at the gene expression level in order to conserve resources under different growth conditions.
The pyrimidine nucleotide de novo biosynthetic pathways derive in part from the central metabolic precursors oxaloacetate and D-ribose 5-phosphate. Oxaloacetate from the TCA cycle I (prokaryotic) gives rise to the aspartate family of amino acids and L-aspartate is a precursor of both pyrimidine ribonucleotides and nicotinamide coenzymes (see L-aspartate biosynthesis). The overall pathway begins with the biosynthesis of carbamoyl-phosphate from bicarbonate, phosphate from ATP and nitrogen from glutamine. This compound then condenses with L-aspartate to form N-carbamoyl-L-aspartate as shown in pathways superpathway of pyrimidine ribonucleotides de novo biosynthesis and UMP biosynthesis. The overall pathway branches at carbamoyl-phosphate, which is an intermediate in L-arginine biosynthesis (see L-arginine biosynthesis I (via L-ornithine)). The activated intermediate 5-phospho-α-D-ribose 1-diphosphate is derived from D-ribose 5-phosphate and is a precursor in the de novo and salvage pathways for pyrimidine and purine nucleotides, as well as in biosynthetic pathways for nucleotide coenzymes, L-histidine and L-tryptophan (see superpathway of histidine, purine, and pyrimidine biosynthesis, PRPP biosynthesis I and PRPP biosynthesis II).
About This Pathway
The pathway shown here illustrates the point at which the pyrimidine ribonucleotides CDP, CTP, UDP and UTP are converted to their corresponding deoxyribonucleotides and the DNA-specific thymine deoxyribonucleoside triphosphate dTTP is formed. Deoxyribonucleotides are synthesized by reduction of ribonucleotides, which is regulated by the rate-limiting enzyme ribonucleotide reductase that carries out reduction at the C2' position. This enzyme has a central role in DNA replication and repair. In E. coli this ribo- to deoxyribo- conversion is catalyzed by aerobic ribonucleoside diphosphate reductase, or anaerobic ribonucleoside triphosphate reductase. Electrons for these reductions are derived from NADPH and are transferred via a reduced thioredoxin, a reduced glutaredoxin, or a reduced flavodoxin.
The ribonucleotide reductase product of genes nrdA and nrdB functions aerobically and under microaerophilic conditions. A normally cryptic enzyme encoded by genes nrdE and nrdF cannot support aerobic, or microaerophilic growth in a nrdAB deletion mutant. However, it supports aerobic growth in such a mutant when genes nrdHIEF are overexpressed on a plasmid [Gon06]. It has been speculated that this additional aerobic enzyme may enhance ribonucleotide reductase activity under conditions of oxidative stress. The anaerobic enzyme is encoded by gene nrdD which functions in a multienzyme complex with the products of genes nrdG and fpr. Pathways for the reduction of these reductase enzymes have also been studied [Gon06].
The major pathway for the aerobic, de novo formation of the intermediate dUMP, the immediate precursor of thymine-containing nucleotides, is via phosphorylation of dCDP to dCTP and deamination of dCTP to dUTP which is hydrolyzed to dUMP. This pathway supplies 70-80% of total dUMP [Johansson07]. Most of the remainder is presumed to be formed from dUDP, as shown. In E. coli dcd, deoA mutants, an alternate de novo deoxycytidine pathway to dUMP is used that relies on salvage enzymes [Weiss07]. Formation of the DNA-specific end product dTTP requires reductive methylation of dUMP to dTMP by 5,10-methylenetetrahydropteroyl mono-L-glutamate catalyzed by thymidylate synthase. Two phosphorylations follow to form dTTP. The first step is catalyzed by the highly specific dTMP kinase, while the second step is catalyzed by the product of gene ndk which also catalyzes two preceding reactions.
Review: Neuhard J. and R.A. Kelln, Biosynthesis and Conversions of Pyrimidines, Chapter 35 in [Neidhardt96]
Martha Arnaud on Tue Jan 21, 2003:
The pathways "de novo biosynthesis of pyrimidine deoxyribonucleotides" and "salvage pathways of pyrimidine deoxyribonucleotides" supersede the pathway formerly called "deoxypyrimidine nucleotide/nucleoside metabolism.
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