Orotidine 5 -monophosphate decarboxylase acidities

The process of substitution undertaken on carboxylic acids and the derivatives of carboxylic acids (anhydrides, acid halides, esters, amides, and nitriles) generally involves a series of replacement processes. Thus, individually, substitution may involve replacement of (a) the proton attached to oxygen of the -OH group (i.e., ionization of the acid) (b) the hydroxyl (-OH) portion of the carboxylic acid (or derivative) (e.g., esterification) (c) the carbonyl oxygen and the hydroxyl (-OH) (e.g., orthoester formation, vide infra) (d) the entire carboxylic acid functionality (e.g., the Hunsdiecker reaction, already discussed Scheme 9.101) and the decarboxylation of orotic acid (as orotidine monophosphate) to uracil (as uridine monophosphate)—catalyzed by the enzyme orotidine monophosphate decarboxylase (Scheme 9.115) or (e) the protons (if any) on the carbon to which the carboxylic acid functional group is attached (e.g., the Dieckman cycUzation, already discussed earlier, c Equation 9.91). Indeed, processes already discussed (i.e., reduction and oxidation) have also accomplished some of these ends. Some additional substitutions for the carboxylic acid group itself are presented in Table 9.6, while other substitutions for derivatives of carboxylic acids are shown in Tables 9.7-9.10 and discussed subsequently. 

The thermal decarboxylation of 1,3-dimethylorotic acid 56 was studied as a model for uncatalysed decarboxylation of orotidine 5 -monophosphate to form uridine 5 -monophosphate.97 This reaction catalysed by the enzyme oritidine 5 -monophosphate decarboxylase is the essential step in nucleic acid biosynthesis.98 Samples of 1,3-dimethylorotic acid were heated at 190°C to conversion over 90% and unreacted substrate was recovered after conversion into the methyl ester. 

Orotidine S -monophosphate decarboxylase (ODCase) is a key enzyme in the biosynthesis of nucleic acids, effecting the decarboxylation of orotidine 5 -monophosphate (OMP, 1) to form uridine S -monophosphate (UMP, 2, Scheme l).1,2 The conversion of OMP to UMP is biomechanistically intriguing, because the decarboxylation appears to result, uniquely, in a carbanion (3, mechanism i, Scheme 2) that cannot delocalize into a it orbital.3,4 The uncatalyzed reaction in solution is therefore extremely unfavorable, with a AG of... 

T.W. Traut and B.R. Temple. 2000. The chemistry of the reaction determines the invariant amino acids during the evolution and divergence of orotidine 5 -monophosphate decarboxylase T. Biol. Chem. 275 28675-28681. (PubMed)... 

Using radioactive bicarbonate (l4C-NaHCC>3), we found it to be incorporated by infected red cells and free parasites into pyrimidines suggesting that P. lophurae synthesized cytosine, uracil and thymine de novo (Walsh and Sherman, 1968b). Further, evidence was found for the pyrimidine-pathway enzymes orotidine-5 -monophosphate pyropho-sphorylase (OMPDC also named orotidine-5 -decarboxylase) and thymi-dylate synthase (TS) in parasites but not in the red cells. The presence of TS made it possible for us to interpret the action of folic acid analogues in the same way as other microbes inhibition of DHFR would lead to a... 

The first known drug affecting the orotate pathway was 6-azauridine [245,246], This analogue is phosphorylated to 6-azauridine 5 -monophosphate which acts as a competitive inhibitor of orotidylic acid decarboxylase [247], Therapeutic use of 6-azauridine [248,249] is occasionally complicated by a pronounced crystalluria. Owing to the block of orotidylic acid decarboxylase, large amounts of orotidine and orotic acid are excreted in urine. After the infusion of 6-azauridine the excretion of orotic acid precedes orotidine and the former disappears more rapidly from the urine. Psoriatic patients on azaribine (triacetylated form of 6-azauridine given orally) excreted 0.2-1.3 g of orotic acid and orotidine per day [250]. 

This is in agreement with our results obtained in human cultured lymphoblasts. Oxipurinol inhibits orotidyl decarboxylase(ODC), Purines might inhibit the conversion of orotic acid to orotidine monophosphate by lowering the intracellular PRPP-concentration (Crandall et al. 1978). 

In hereditary orotic aciduria, orotic acid is excreted in the urine because the enzymes that convert it to uridine monophosphate, orotate phosphoribosyl transferase and orotidine 5 -phosphate decarboxylase, are defective (see Figure 7-20). Pyrimidines cannot be synthesized, and therefore, normal growth does not occur. Oral administration of uridine bypasses the metabolic block and provides the body with a source of pyrimidines. 

Reaction of aspartic acid (14) with carbamoyl phosphoric acid (17) in the presence of the allosteric enzyme aspartate carbamoyltransferase (aspartate transcar-bamoylase) gives N-carbamoyl aspartic acid (18), which is cyclised to L-dihy-droorotic acid (19) by dihydroorotase. Oxidation of L-dihydroorotic acid by flavoprotein, orotate reductase gives orotic acid (20), which reacts with 5-phosphori-bosy 1-1-pyrophosphate (PRPP) in the presence of orotate phosphoribosyl transferase to form orotidine 5 -monophosphate (OMP, 21). Decarboxylation of OMP by orotid-ine 5 -phosphate decarboxylase yields uridine 5 -monophosphate (UMP, 22), which acts as precursor for the cytidine nucleotides (CTP) (Chart 6). 

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). 

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