Orotidine decarboxylase synthesis

Michalski J, Dabkowski W (2003) State of the Art. Chemical Synthesis of Biophosphates and Their Analogues via P Derivatives. 232 93-144 Miller BG (2004) Insight into the Catalytic Mechanism of Orotidine 5 -Phosphate Decarboxylase fi"om Crystallography and Mutagenesis. 238 43-62 Mikolajezyk M, Balczewski P (2003) Phosphonate Chemistry and Reagents in the Synthesis of Biologically Active and Natural Products. 223 161-214 Mikolajezyk M, see Drabowicz J (2000) 208 143-176... 

Low activities of orotidine phosphate decarboxylase and (usually) orotate phosphoribosyltransferase are associated with a genetic disease in children that is characterized by abnormal growth, megaloblastic anemia, and the excretion of large amounts of orotate. When affected children are fed a pyrimidine nucleoside, usually uridine, the anemia decreases and the excretion of orotate diminishes. A likely explanation for the improvement is that the ingested uridine is phosphorylated to UMP, which is then converted to other pyrimidine nucleotides so that nucleic acid and protein synthesis can resume. In addition, the increased intracellular concentrations of pyrimidine nucleotides inhibit carbamoyl phosphate synthase, the first enzyme in the. naibwav of aro-tate synthesis. 

De novo pathway of uridine-5 -monophosphate (UMP) synthesis. Enzymes (1) carbamoyl phosphate synthetase II (2) aspartate transcarbamoylase (3) dihydroorotase (4) dihydroorotate dehydrogenase (5) orotate phosphoribosyltransferase (6) orotidine-5 -monophosphate decarboxylase (orotidylate decarboxyla.se). 

Deficiency of folate or vitamin Bn can cause hematological changes similar to hereditary orotic aciduria. Folate is directly involved in thymidylic acid synthesis and indirectly involved in vitamin Bn synthesis. Orotic aciduria without the characteristic hematological abnormalities occurs in disorders of the urea cycle that lead to accumulation of carbamoyl phosphate in mitochondria (e.g., ornithine transcarbamoylase deficiency see Chapter 17). The carbamoyl phosphate exits from the mitochondria and augments cytosolic pyrimidine biosynthesis. Treatment with allopurinol or 6-azauridine also produces orotic aciduria as a result of inhibition of orotidine-5 phosphate decarboxylase by their metabolic products. 

The first three reactions are catalyzed by a trifunctional protein which contains carbamoyl-phosphate synthetase II, aspartate carbamoyltransferase and dihydro-orotase. This set of reactions begins with the synthesis of carbamoyl phosphate followed by its condensation with aspartic acid. The third step involves the closure of the ring through the removal of water by the action of dihydro-orotase to yield dihydro-orotate. The fourth enzyme, dihydro-orotate oxidase, oxidizes dihydro-orotate to orotate and is a mitochondrial flavoprotein enzyme located on the outer surface of the inner membrane and utilizes NAD" " as the electron acceptor. The synthesis of UMP from orotate is catalyzed by a bifunctional protein which comprises orotate PRTase and orotidine 5 -phosphate (OMP) decarboxylase. The former phosphoribosylates orotate to give OMP the latter decarboxylates OMP to UMP, the immediate precursor for the other pyrimidine nucleotides. It is interesting to note that whereas five molecules of ATP (including the ATP used in the synthesis of PRPP) are used in the de novo synthesis of IMP, no net ATP is used in the de novo synthesis of UMP. In de novo pyrimidine synthesis, two ATP molecules are used to synthesize carbamoyl phosphate and one ATP is needed to synthesize the PRPP used by orotate PRTase but 3 ATPs... 

Orotidyhc acid decarboxylase (orotidine 5 -phosphate carboxy-lyase, EC 4.1.1.23) catalyses the only irreversible step in the pyrimidine synthesis de novo. The enzyme is competitively inhibited by UMP and CMP [114-116] and some anomalous pyrimidine nucleoside 5 -monophosphates. The activity of orotidylic acid decarboxylase in excess of that of orotate phosphoribosyltransferase accounts for the absence of orotidine 5 -phosphate in the pool of low molecular weight compounds in animal cells. 

The final steps of pyrimidine biosynthesis novo which are catalyzed by two sequential enzymes, orotate phosphoribosyltransfer-ase (OPRT) and orotidylic decarboxylase (ODC), involve the PP-ribose P dependent conversion of orotic acid to orotidine-5 -monophosphate (OMP) followed by decarboxylation at the 7 position to form uridine 5 -monophosphate (UMP) (Fig. 1). UMP is then utilized further in the synthesis of nucleic acids and co-enzymes. Defects at this site in this metabolic pathway are important for they can result in "pyrimidine starvation" from depletion of the intracellular pool of pyrimidine nucleotides. In man the rare genetic disease, orotic aciduria, involves a deficiency of both OPRT and ODC (Type 1) (Smith, Sullivan and Huguley, 1961) or, less commonly, only ODC (Type II) (Fox, 0 Sullivan and Firken, 1969). 

Orotidine 5 -phosphate pyrophosphoiylase has been found in liver and yeast. The enzyme is specific for orotic acid and catalyzes the reversible reaction (16A). The equilibrium constant is approximately 0.1 but the reaction is pulled in the direction of ribotide synthesis by the irreversible decarboxylation of orotidine 5 -phosphate by orotidine 5 -phosphate decarboxylase [Eq. (16B)] (Sa, 48). 

Recently this transformation was applied for synthesis of orotidine-5 -monophosphate decarboxylase inhibitors [207]. Unexpected results was obtained during the methylation... 

Decarboxylation reactions are common in Nature and they are involved in many pathways, including decarboxylation of keto acids, amino acid conversions, and carbohydrate synthesis. Many decarboxylases use cofactors such as metal ions, pyridoxal 5 -phosphate, biotin, and flavin, but a small subset, for example, orotidine 5 -phosphate decarboxylase (ODCase) and methyhnalonyl CoA decarboxylase do not utilize any cofactor. ODCase catalyzes the decarboxylation of orotic acid (shown in Figure 8), and it generates one of the largest rate enhancements known to be produced by any enzyme (rate of the reaction is enhanced by a factor of Several... 

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