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Carbamylaspartic acid

Since the orotic acid required by Lactobacillus could be replaced partially by carbamylaspartic acid, the latter compound was investigated further as a possible pyrimidine precursor labeled carbamylaspartic acid was as effective as orotic acid in labeling the nucleic acid pyrimidines of this microorganism (339). If ureidosuccinic acid were formed from oxalacetate or aspartate, it would be possible to outline the formation of the pyrimi-... [Pg.432]

Even though orotidylic acid or orotidine was implicated in pyrimidine formation, the precise role of orotic acid per se remained to be evaluated. On the other hand, it was posable that orotic acid was a normal intermediate that condensed with a ribose compound to yield orotidine or orotidylic acid during the biosynthetic process. In support of this thesis, it was found that 5-phosphoribosyl-l-pyrophosphate was utilized for nucleotide formation from orotic acid (83). On the other hand, it was equally posable that an aliphatic compound, such as aminofumaric acid (335) or carbamylaspartic acid (339), could have coupled with a ribose compound and formed orotidine or orotidylic acid directly without the existence or participation of orotic acid per se. In this latter instance, orotic acid would not be conadered a true intermediate in pyrimidine biosynthesis but merely an accidental cleavage product of hi ly labile orotidine or orotidylic acid. At this time research in the area of purine biosynthesis indicated that a series of acyclic intermediates attached to ribose 6-phosphate were biosynthetic intermediates and that free purines per se were not (Section II, B.). [Pg.434]

The formation of carbamylaspartic acid from carbamyl phosphate and aspartic acid (Fig. 20) has been demonstrated in pigeon liver, several tissues of the rat, E. colt, and yeast (363S67). It appears that only one enz3rme was involved in thb interesting reaction 368) and it has been named ureidosuccinic (carbamylaspartic) acid synthetase 355). The reaction was essentially irreversible, even though slight reversal was shown with labeled substrates 357). [Pg.435]

By ring closure carbamylaspartic acid was transformed into dihydro-orotic acid (Fig. 21). The reaction was freely reversible and the enzyme responsible for this conversion was named dihydroorotase. The enzyme has been found in several bacteria grown in the presence of orotic acid 369,370), in rat liver 371), and in extracts of E. coli 372). The compound originally prepared as dihydroorotic acid 373) was not identical with the enzymic product 374)- However, the authenticity of the enzymic product was proven by several synthetic methods 371, 375). [Pg.435]

The formation of the first nucleotide in the pyrimidine sequence, orotidylic acid (orotidine 5 -phosphate), was accomplished by the reaction of 5-phosphoribosyl-l-pyrophosphate (PRPP) with orotic acid 83) (Fig. 22). Other pyrimidines and carbamylaspartic acid did not react with PRPP in the presence of the enzyme, which has been named orotidine 5 -phosphate pyrophosphorylase. Several purine analogs, e.g., 6-uracilsulfonic acid, 6-uracil methyl sulfone, which inhibited the growth of several organisms (S78, 379), probably inhibited the formation of orotidylic acid. [Pg.435]

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. [Pg.443]

The location of the metabolic block appears to be advantageous to cellular economy, sparing energy, and metabolites. The formation of carbamylaspartic acid was essentially an irreversible reaction and inhibition of the sequence at a subsequent step would be considered inefficient since carbamylaspartic acid would be produced whether it were needed or not. It is of further interest that the steps preceding the blocking point, the formation of aspartic acid and carbamyl phosphate, were freely reversible reactions. [Pg.444]

The Biosynthesis of the Pyrimidine Ring begins with aspartic acid and carbamyl phosphate. The latter is an energy-rich compound which reacts with the former to give carbamylaspartic acid. Ring closure consumes ATP and is in principle an acid amide formation (peptide synthesis). The intermediate dihydro-orotic acid is dehydrogenated to orotic acid, probably by action of a flavoprotein. Orotic acid is the key precursor of pyrimidine nucleotides. It reacts with phosphoribosyl pyrophosphate. The removal of pyrophosphate yields the nucleotide of orotic acid, whose enzymic decarboxylation produces uridine 5 -phosphate. Phosphorylation with ATP yields uridine pyrophosphate and, finally, uridine triphosphate. Beside the above pathway, there is the further possibility of converting free uracil and ribose 1-phosphate to the nucleoside and from there with ATP to the nucleotide. [Pg.119]

The first step of the biosynthesis of pyrimidine nucleotides is the irreversible carbamylation of L-aspartate by carbamyl-phosphate to form carbamylaspartate (catalyzed by the enzyme aspartate transcarbamylase). Next, carbamylaspartate is converted, by ring closure, to dihydro-orotic acid which, in turn, is reduced to orotic acid, catalyzed by the enzyme orotic acid dehydrogenase (OAD). Orotic acid (6-carboxyuracil) reacts with 5 -phosphoribosyl--1-pyrophosphate (PRPP) to form orotidine monophosphate (OMP). [Pg.153]


See other pages where Carbamylaspartic acid is mentioned: [Pg.39]    [Pg.257]    [Pg.432]    [Pg.434]    [Pg.303]    [Pg.39]    [Pg.257]    [Pg.432]    [Pg.434]    [Pg.303]    [Pg.432]   
See also in sourсe #XX -- [ Pg.39 ]

See also in sourсe #XX -- [ Pg.257 ]




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Carbamylaspartate

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