Ara 
Sac Soy Vit
Ara Sac Soy Vit
 |
Pathway was created on Thu Jul 19, 2007 | |
 | General background: Purine nucleotides are crucial compounds that are central to primary metabolism but are also connected to many facets of secondary plant metabolism. They are involved in many vital cellular processes which are essential for plant growth and development  . Despite its fundamental importance purine metabolism, especially the regulatory aspect of it is not well understood but starts to come more into scientist's focus as the tools in molecular biology become more and more sophisticated. De novo purine nucleotide biosynthesis is of special importance in meristematic cells as it provides the building blocks for DNA and RNA in those dividing and elongating tissues . Further phosphotransfer (see |FRAME:DENOVOPURINE2-PWY|) results in the formation of ATP and GTP which serve as the universal energy source for a huge number of chemical reactions. In addition, purine nucleotides are precursors in the synthesis of a number of indispensable coenzymes such as in |FRAME:PYRIDNUCSYN-PWY| , |FRAME:RIBOSYN2-PWY| and |FRAME:PWY-3742| |CITS: [Basset05]|. Besides the energy-consuming de novo biosynthesis of purine nucleotides, purine components can be recycled through the energy-conservative |FRAME:PWY-5043| which provides substrates for the formation of cytokinins (e.g. |FRAME:PWY-2681|) or purine alkaloids such as theobromine and caffeine . Moreover, it has been demonstrated in legumes that purine metabolism (see |FRAME:PWY-5044|) is employed to assimilate nitrogen in form of ureides (compare |FRAME:URSIN-PWY|) that are derivatives of inosine 5'-monophosphate (IMP), an branch intermediate in the de novo biosynthesis |CITS: [REINBOTHE62]| |CITS: [SCHUBERT90]| although the metabolic reason behind remains obscure . Interestingly, the characteristics of the enzymes involved in the de novo purine nucleotide biosynthesis in plants resemble the pathway in prokaryotes and differ from the pathway observed in other eukaryotes. Plant and prokaryote enzymes are mostly mono-functional whereas the enzymes in other eukaryotes are multi-functional (e.g. ). Except for the PRPP amidotransferase and the adenylosuccinate lyase (and probably AIR carboxylase) all other enzymes in the Arabidopsis de novo purine nucleotide biosynthesis are encoded by a single gene About the pathway: The de novo biosynthesis of purine nucleotides that produces adenosine 5'-monophosphate (AMP) and guanosine 5'-monophosphate (GMP) via inosine 5'-monophosphate (IMP) comprises 14 enzymatic steps. All of the genes involved in the formation of IMP and AMP contain putative plastid transit sequences indicating a plastid location of the pathway which has experimentally been proven for the entry enzyme of the pathway, PRPP amidotransferase (ATase) . The enzymatic conversion of IMP to GMP is suspected to occur in the cytosol but experimental evidence for that proposal is not yet provided. However, the discovery of a plastidic adenine nucleotide uniporter in Solanum tuberosum exporting AMP, ADP, and ATP to the cytosol supports that hypothesis. There are 10 enzymatic steps leading to inosine 5'-monophosphate (IMP), the first complete purine molecule. The first enzyme, PRPP amidotransferase catalyzes the formation of phosphorybosylamine (PRA) from PRPP and glutamine encoded by a small gene family of three members . PRA is then converted to phosphoribosyl glycineamide (GAR) by the glycinamide ribonucleotide synthetase (GAR synthetase) . In the next step GAR is further converted to phosphoribosyl N-formylglycineamide mediated by glycinamide ribonucleotide transformylase (GAR transformylase) using 10-formyltetrahydrofolate . The formation of 5-aminoimidazole ribonucleotide (AIR) through the intermediate 5-phosphoribosyl-N-formylglycineamidine (FGAM - enzyme not yet characterized in Arabidopsis) is carried out by the AIR synthetase (phosphoribosylformylglycinamidine cyclo-ligase) . The synthesis proceeds further via carboxylation to yield 4-carboxy aminoimidazole ribonucleotide (CAIR) which is then converted to N-succinyl-5-aminoimidazole-4-carboxamide ribonucleotide (SAICAR) by adding asperatate which requires the consumption of ATP ). The remaining steps towards IMP, the formation of 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR) and 5-formaminoimidazole-4-carboxamide ribonucleotide (FAICAR) which undergoes dehydration and ring closure to form IMP have not been furnished with Arabidopsis enzymes yet. At this point the pathway branches, one branch leading to the purine nucleotide AMP (two enzymatic steps) and the other one to GMP (two enzymatic steps). Adenylosuccinate synthetase (AdSS), the enzyme catalyzing the first of the two-step conversion of IMP to AMP, i.e. forming the intermediate adenylo-succinate |CITS: [ 8819867]| has been identified and characterized . For the remaining step to AMP the enzyme has not been identified yet. In the supposedly cytosolic steps that form GMP one clone encoding IMP dehydrogenase has been obtained but the enzyme remains to be functionally characterized. There is a growing body of evidence that the GMP synthesis can be fed by cytosolic IMP. AMP and derivatives are exported to the cytosol which are then re-converted to IMP via AMP deaminase . -  aracycPart of the pathway occurs in cytosol and rest of the pathway occurs in plastids. References: - siva | |
 | Parts of this pathway occur in: cytosol plastid nucleus |
| Type | |
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 Enzymatic reaction | |
| Translation | |
| Transcription | |
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| Composition-OR | |
| Catalysis |
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metabolite [32] | |
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polypeptide [17] | |
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protein complex [11] | |
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RNA [15] | |
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gene [15] |
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