Dihydroorotic acid synthesis

Anodic oxidation of dihydroorotic acid, its isomer, and the 1-benzyl analog of dihydroorotic acid in H20-NaOH media is an efficient synthesis of uracils (51-91%) under mild conditions 416... 

The fourth step in the de novo synthesis of pyrimidine nucleotides—the conversion of dihydroorotic acid to orotic acid—is catalyzed by dihydroorotic acid dehydrogenase. The enzyme, located on the cytosolic side of the inner membrane of mitochondria, is a target for antitumor agents. 

Dihydroorotase catalyzes the intramolecular cyclization of 7V-carbamyl-L-aspartic acid to L-dihydroorotic acid. In mammals, the activity is present in a trifunctional enzyme that catalyzes the first three steps in the de novo synthesis of pyrimidine nucleotides. 

Leflunomide (Arava) is an anti-proliferative drug that inhibits dihydroorotate dehydrogenase, an enzyme essential for the synthesis of pyrimidines such as uracil, via aromatisation of dihydroorotic acid. It also has some immunomodulatory effects and is useful for intractable cases of rheumatoid arthritis, despite being liable to cause a number of serious side-effects. It is actually a prodrug, being converted into the active compound, a nitrile formed by cleavage of the isoxazole ring (cf. 25.5.1). 

Leflunomide This drug inhibits dihydroorotic acid dehydrogenase, an enzyme involved in ribonucleotide synthesis. Leflunomide arrests lymphocytes in the Gj phase of the cell cycle. Leflunomide is used in rheumatoid arthritis. The drug causes alopecia, rash, and diarrhea. 

The suggestion of a control of pyrimidine synthesis stems from the observation that pyrimidine-requiring mutants of E. coli in pyrimidine-free medium accumulated carbamylaspartic acid and, to a lesser extent, dihy-droorotic acid and orotic acid this accumulation was prevented by the addition of uracil and cytosine to the medium (443). It was shown further employing enzyme preparations that cytidine and particularly cytidine 5 -phosphate were effective inhibitors of carbamylaspartic acid synthesis, suggesting that the inhibition of this enzyme by a pyrimidine nucleotide was the mechanism for the feedback control of pyrimidine biosynthesis in bacteria. The decreased formation of dihydroorotic acid and orotic acid were probably secondary events reflecting the earlier metabolic block. 

Steric effects and FMO control have been combined in an elegant way to achieve regiospecific synthesis of pyrazole inhibitors of dihydroorotate dehydrogenase <2006SL901>. When the size of the propargylic acid ester 86 is increased from ethyl to diphenylmethyl, pyrazole 87 is formed from compound 85 regiospecifically (Scheme 3 Table 4) <2006H(68)1007>. 

The second step in pyrimidine synthesis is the formation of car-bamoylaspartate, catalyzed by aspartate transcarbamoylase. The pyrimidine ring is then closed hydrolytically by dihydroorotase. Thi resulting dihydroorotate is oxidized to produce orotic acid (onotate, Figure 22.21). The enzyme that produces orotate, dihydroorotate dehydrogenase, is located inside the mitochondria. All other reactions in pyrimidine biosynthesis are cytosolic. [Note The first three enzymes in this pathway (CPS II, aspartate transcarbamoylase, and dihydroorotase) are all domains of the same polypeptide chain. (See k p. 19 for a discussion of domains.) This is an example of a multifunctional or multicatalytic polypeptide that facilitates the ordered synthesis of an important compound.]... 

The first step in de novo pyrimidine biosynthesis is the synthesis of carbamoyl phosphate from bicarbonate and ammonia in a multistep process, requiring the cleavage of two molecules of ATP. This reaction is catalyzed by carbamoyl phosphate synthetase (CPS), and the bicarbonate is phosphorylated by ATP to form carboxyphosphate and ADP (adenine dinucleotide phosphate). Ammonia then reacts with carboxyphosphate to form carbamic acid. The latter is phosphorylated by another molecule of ATP with the mediation of CPS to form carbamoyl phosphate, which reacts with aspartate by aspartate transcarbamoy-lase to form A-carbamoylaspartate. The latter cyclizes to form dihydroorotate, which is then oxidized by NAD-1- to generate orotate. Reaction of orotate with 5-phosphoribosyl-l-pyrophosphate (PRPP), catalyzed by pyrimidine PT, forms the pyrimidine nucleotide orotidylate. This reaction is driven by the hydrolysis of pyrophosphate. Decarboxylatin of orotidylate, catalyzed by orotidylate decarboxylase, forms uridylate (uridine-5 -monophosphate, UMP), a major pyrimidine nucleotide that is a precursor of RNA (Figure 6.53). 

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