Aspartate pyrimidine ring

ACTase catalyzes the transfer of a carbamoyl residue from carbamoyl phosphate to the amino group of L-aspartate. The N-carbamoyl L-aspartate formed in this way already contains all of the atoms of the later pyrimidine ring (see p. 188). The ACTase of the bacterium Escherichia coli is inhibited by cytidine triphosphate (CTP), an end product of the anabolic metabolism of pyrimidines, and is activated by the precursor ATP. 

The pyrimidine ring is made up of three components the nitrogen atom N-1 and carbons C-4 to C-6 are derived from aspartate, carbon C-2 comes from HCOa", and the second nitrogen (N-3) is taken from the amide group of glutamine. 

Condensation of carbamoyl phosphate with aspartate brings together all the atoms needed to make the main pyrimidine ring (Figure 10-4). 

FIGURE 22-36 De novo synthesis of pyrimidine nucleotides biosynthesis of UTP and CTP via orotidylate. The pyrimidine is constructed from carbamoyl phosphate and aspartate. The ribose 5-phosphate is then added to the completed pyrimidine ring by orotate phosphori-bosyltransferase. The first step in this pathway (not shown here see Fig. 18-11a) is the synthesis of carbamoyl phosphate from C02 and NH), catalyzed in eukaryotes by carbamoyl phosphate synthetase II. 

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 sources of the atoms in the pyrimidine ring are glutamine, CO2, and aspartic acid. 

Biosynthesis of UMP. The parts of the intermediates derived from aspartate are shown in red. Bold type indicates atoms derived from carbamoyl phosphate. In contrast to purine nucleotide synthesis, where ring formation starts on the sugar, in pyrimidine biosynthesis the pyrimidine ring is completed before being attached to the ribose. 

Unlike in purine biosynthesis, the pyrimidine ring is synthesized before it is conjugated to PRPP. The first reaction is the conjugation of carbamoyl phosphate and aspartate to make N-carbamoylaspartate. The carbamoyl phosphate synthetase used in pyrimidine biosynthesis is located in the cytoplasm, in contrast to the carbamoyl phosphate used in urea synthesis, which is made in the mitochondrion. The enzyme that carries out the reaction is aspartate transcarbamoylase, an enzyme that is closely regulated. 

Figure 10.9 De now pyrimidine nucleotide biosynthesis pathway. Note the numbering of the pyrimidine ring in UMP atoms 2 and 3 come from carbamoyl phosphate and atoms 1, 4, 5, and 6 from aspartate.

The atoms of the pyrimidine ring are derived from carbamoyl phosphate and aspartate, as shown in Fig. 15-14. The de novo biosynthesis of pyrimidine nucleotides is shown in Fig. 15-15. The first completely formed pyrimidine ring is that of dihydroorotate. Only after oxidation to orotate is the ribose attached to produce orotidylate. The compound 5-phosphoribosyl 1-pyrophosphate (P-Rib-PP) provides the ribose phosphate. L-Glutamine is used as a substrate donating nitrogen atoms at reactions 1 and 9, catalyzed by carbamoyl phosphate synthetase II and CTP synthetase, respectively a second... 

In de Novo Synthesis, the Pyrimidine Ring Is Assembled from Biearbonate, Aspartate, and Glutamine... 

In de novo synthesis of pyrimidines, the ring is synthesized first and then it is attached to ribose to form a pyrimidine nucleotide (Figure 25.2). Pyrimidine rings are assembled from bicarbonate, aspartic acid, and ammonia. Although ammonia can be used directly, it is usually produced from the hydrolysis of the side chain of glutamine. 

Figure 25.2. de Novo Pathway for Pyrimidine Nucleotide Synthesis. The C-2 and N-3 atoms in the pyrimidine ring come from carbamoyl phosphate, whereas the other atoms of the ring come from aspartate. 

The pyrimidine ring is assembled first and then linked to ribose phosphate to form a pyrimidine nucleotide. PRPP is the donor of the ribose phosphate moiety. The synthesis of the pyrimidine ring starts with the formation of carbamoylaspartate from carbamoyl phosphate and aspartate, a reaction catalyzed by aspartate transcarbamoylase. Dehydration, cyclization, and oxidation yield orotate, which reacts with PRPP to give orotidylate. Decarboxylation of this pyrimidine nucleotide yields UMP. CTP is then formed by the amination of UTP. 

FIGURE 9.7 Pyrimidine biosynthetic pathway. The pathway of pyrimidine biosynthesis involves sbc steps and results in the production of uridine 58-monophosphate. Folate is not used in this pathway The pathway commences with the transfer of the amide nitrogen of glutamine to bicarbonate to produce caibamy I phosphate. This molecule then reacts with aspartate to form the beginnings of the siK-membered pyrimidine ring. 

The answer is a. (Murray, pp 375-401. Scriver, pp 2513-2570. Sack, pp 121-138. Wilson, pp 287-320.) During purine ring biosynthesis, the amino acid glycine is completely incorporated to provide C4, C5, and N7. Glutamine contributes N3 and N9, aspartate provides Nl, and derivatives of tetrahydrofolate furnish C2 and C8. Carbon dioxide is the source of C6. In pyrimidine ring synthesis, C2 and N3 are derived from carbamoyl phosphate, while Nl, C4, C5, and C6 come from aspartate. 

Pyrimidine bases are first synthesized as the free base and then converted to a nucleotide. Aspartate and carbamoyl phosphate form all components of the pyrimidine ring. Ribose 5-phosphate, which is converted to phosphoribosyl pyrophosphate (PRPP), is required to donate the sugar phosphate to form a nucleotide. The first pyrimidine nucleotide produced is orotate monophosphate (OMP). The OMP is converted to uridine monophosphate (UMP), which will become the precursor for both cytidine triphosphate (CTP) and deoxythymidine monophosphate (dTMP) production. 

The overall scheme of pyrimidine nucleotide biosynthesis differs from that of purine nucleotides in that the pyrimidine ring is assembled before it is attached to ribose-5-phosphate. The carbon and nitrogen atoms of the pyrimidine ring come from carbamoyl phosphate and aspartate. The production of carbamoyl phosphate for pyrimidine biosynthesis takes place in the cytosol, and the nitrogen donor is glutamine. (We already saw a reaction for the production of carbamoyl phosphate when we discussed the urea cycle in Section 23.6. That reaction differs from this one because it takes place in mitochondria and the nitrogen donor is NH/). 

Succinate can be converted to oxaloacetate by the citric acid cycle. The oxaloacetate can then be transaminated to yield asparate, a key precursor of pyrimidines. The carbons of aspartate then will label positions 4, 5, and 6 in the pyrimidine rings ... 

The precursors of the pyrimidine ring are bicarbonate, glutamine and aspartate (Fig. 58.1). The regulatory enzyme is carbamoylphos-... 

Pyrimidine biosynthesis, de novo pyrimidine biosynthesis total synthesis of the pyrimidine ring of uracil, thymine, cytosine and their derivatives from carbamoyl phosphate and aspartate in all living cells. The pyrimidine ring of thiamin (vitamin Bj) has a different biosynthetic origin (see below). 

The pyrimidine ring moiety of thiamine has recently been shown to be derived in part from an intermediate in the pathway of purine biosynthesis de novo, although many details regarding this synthesis still remain unclear. The proposed scheme is shown in Fig. 3-3. Aspartate may provide the other part of this ring. 

Historically, carbamyl aspartate was recognized as a likely intermediate in pyrimidine biosynthesis because (a) this compound is an assembly of two elementary precursors of the pyrimidine ring, (b) carbamyl aspartate would satisfy the nutritional requirement of L. bvlgaricus 09 for orotate, and (c) labeled carbamyl aspartate was incorporated into ribonucleic acid pyrimidines in L. bulgaricus and, as well, served as an orotate precursor in liver slice trapping experiments such as those mentioned above. 

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