Human Biochemistry, Volume 2 - Murray R. 1993

Structure, Function, and Replication of Information Macromolecules
Metabolism of Purine and Pyrimidine Nucleotides
Pyrimidines

Pyrimidine Biosynthesis

The pyrimidine ring Structure is simpler, and its biosynthetic pathway is shorter than that of Purines. At the same time, both pathways share A number of common precursors. PRPP, glutamine, СО₂, and aspartate are required for the synthesis of all pyrimidine and purine NUCLEOTIDES. The synthesis of thymidine nucleotides, as well as all purine nucleotides, requires the presence of tetrahydrofolate derivatives. One significant difference can be noted between the biosynthetic pathways of purine and pyrimidine nucleotides. In the former case, synthesis begins with a ribose phosphate molecule as an integral part of the future nucleotide precursor molecule, whereas in the latter case, the pyrimidine base is synthesized first, and the ribose phosphate residue is attached only at the final stages.

The Synthesis of the pyrimidine ring (Fig. 35.15) begins with The formation of carbamoyl phosphate from glutamine, ATP, and СО₂ in a cytosolic reaction catalyzed by carbamoyl phosphate synthetase (reaction 1). Note that the carbamoyl phosphate synthetase responsible for the Cytology/cytology/16.html">Early stages of urea synthesis is localized in the Cell/35.html">Mitochondria.

The first step unique to pyrimidine biosynthesis—the formation of carbamoylaspartate via the Condensation of carbamoyl phosphate and aspartate—is catalyzed by aspartate transcarbamoylase (reaction 2). Next, in a reaction catalyzed by dihydroorotase, Н₂О is eliminated, and a ring structure is formed (reaction 3).

The next step involves dehydrogenation by dihydroorotase dehydrogenase using NAD as a cofactor, yielding orotic acid (reaction 4).

In reaction 5, a ribose phosphate residue is added to orotic acid to yield orotidylate (orotidine monophosphate, OMP). This process is carried out by orotate phosphoribosyltransferase, an enzyme analogous to hypoxanthine-guanine phosphoribosyltransferase and adenine phosphoribosyltransferase, which are involved in the phosphoribosylation of purine rings.

The first true pyrimidine ribonucleotide, uridylate (uridine monophosphate, UMP), is formed by the decarboxylation of orotidylate (reaction 6). Thus, phosphoribosylation of the heterocycle occurs only at the penultimate stage of UMP formation.

Dihydroorotase dehydrogenase is a mitochondrial enzyme. All Other Enzymes involved in de novo pyrimidine synthesis are localized in the Cytosol.

The phosphorylation of pyrimidine nucleoside monophosphates to their corresponding di- and triphosphates proceeds analogously to that described for purine nucleoside monophosphates (reactions 7–12). UTP is aminated to CTP in a reaction involving glutamine and ATP (reaction 9). The Mechanism of Reduction of pyrimidine nucleoside diphosphates to the corresponding 2'-deoxynucleoside diphosphates (reaction 10) is also analogous to that described for purine nucleoside diphosphates (Figs. 35.6 and 35.13).

The formation of thymidylate (thymidine monophosphate, TMP) (reaction 12) is the only reaction in the pyrimidine nucleotide biosynthetic pathway that requires a tetrahydrofolate derivative as a single-carbon donor. 2'-Deoxy-UMP is methylated by thymidylate synthase, which uses N⁵, N¹⁰-methylenetetrahydrofolate as the methyl group donor. During the reaction, the methylene group of N⁵, N¹⁰-methylenetetrahydrofolate is reduced to a methyl group and attached to the C-5 atom of dUMP. This process is accompanied by The oxidation of the tetrahydrofolate carrier to dihydrofolate. It can be considered that the methylation of dUMP to form TMP results in the complete reduction of the hydroxymethyl group of Serine (transferred to tetrahydrofolate during the formation of N⁵, N¹⁰-methylenetetrahydrofolate) to a methyl group, with the simultaneous oxidation of tetrahydrofolate to dihydrofolate. For the folate carrier to continue functioning, dihydrofolate must be reduced back to tetrahydrofolate. This reaction is catalyzed by Dihydrofolate Reductase. For this reason, dividing Cells, which are forced to synthesize TMP with the concomitant production of dihydrofolate, are particularly sensitive to dihydrofolate reductase inhibitors. One such inhibitor, methotrexate (amethopterin), is widely used as an antitumor drug.

Class="center">

Fig. 35.15. Pathway of Pyrimidine Nucleotide Biosynthesis.

Pyrimidine Nucleotide Salvage Pathways

Mammalian cells lack efficient means of utilizing free pyrimidine bases for nucleotide synthesis. At the same time, they are capable of utilizing the pyrimidine ribonucleosides uridine and cytidine, as well as the 2'-deoxyribonucleosides thymidine and deoxycytidine, by converting them into their corresponding nucleotides (Fig. 35.16). 2'-Deoxycytidine is phosphorylated by deoxycytidine kinase, an enzyme that can also phosphorylate deoxyguanosine and deoxyadenosine.

The enzyme orotate phosphoribosyltransferase, which is required for de novo pyrimidine synthesis, is responsible for the phosphoribosylation of orotic acid to form OMP, although orotic acid is not strictly a true pyrimidine base. Orotate phosphoribosyltransferase cannot use normal pyrimidine bases as substrates, but it is able to phosphoribosylate allopurinol (4-hydroxypyrazolopyrimidine) into a nucleotide derivative in which the ribosyl phosphate is attached to the N-1 atom of the pyrimidine ring of this drug molecule. The antitumor drug 5-fluorouracil is also phosphoribosylated by this enzyme.

Pyrimidine Catabolism

Pyrimidine catabolism, which occurs primarily in the Liver, yields highly soluble end products (Fig. 35.17). This distinguishes them from the End products of purine catabolism (uric acid and its sodium salt have poor solubility). The release of СО₂, derived from the ureido carbon (C-2) of the pyrimidine ring, represents the major catabolic pathway for uracil, cytosine, and thymine. The main end products of the catabolism of these bases are ß-Alanine and ß-aminoisobutyrate.

Thymine acts as a precursor of ß-aminoisobutyrate in humans and common laboratory animals. The excretion of ß-aminoisobutyrate increases in leukemia and following X-irradiation, which undoubtedly reflects accelerated cell death and DNA destruction. The excretion of abnormally large amounts of ß-aminoisobutyrate may also be observed in otherwise healthy individuals. This trait is inherited as recessive and, consequently, manifests only in homozygotes for the corresponding allele. Approximately 25% of the individuals examined (of Japanese and Chinese descent) were found to have elevated levels of ß-aminoisobutyrate excretion. Relatively little is known about The Mechanism of ß-aminoisobutyrate degradation in The Human Body. An enzyme catalyzing the reversible Transamination of this compound has been discovered in pig Kidney. ß-Aminoisobutyrate is converted to methylmalonyl semialdehyde, then to propionate, which is subsequently converted to succinate.

Fig. 35.16. Reactions of pyrimidine nucleoside monophosphate formation from the corresponding pyrimidine nucleosides, catalyzed by nucleoside kinase.

The Initial Stages of pyrimidine nucleotide degradation, which include the Cleavage of the carbohydrate-phosphate moiety at the N-glycosidic bond, closely resemble the reverse of the final Stages of the biosynthetic pathway. For pseudouridine, which is formed in situ As a result of internal rearrangement, no mechanism of Hydrolysis or phosphorolysis to uracil exists. Consequently, this unusual nucleotide is excreted unchanged in the urine of healthy individuals.

Fig. 35.17. Pyrimidine catabolism.

Regulation of Pyrimidine Biosynthesis

The biosynthetic pathway of pyrimidine nucleotides is regulated by two distinct mechanisms. The activity of the first two enzymes is controlled by allosteric effectors. In addition, the first three and the last two enzymes are subject to coordinate repression and derepression. Carbamoyl phosphate synthetase is inhibited by UTP and purine nucleotides, but is activated by PRPP (Fig. 35.18). Aspartate transcarbamoylase is particularly sensitive to the inhibitory effect of CTP. The allosteric properties of microbial aspartate transcarbamoylase have been the subject of intensive and now classical studies on the mechanisms of allosteric enzyme regulation.

The rate of pyrimidine biosynthesis correlates with that of Purine Biosynthesis, indicating a coordinated control over the synthesis of both types of nucleotides. PRPP synthetase, The enzyme catalyzing the Formation of the common precursor for both biosynthetic pathways, is feedback-inhibited by both purine and pyrimidine nucleotides. Carbamoyl phosphate synthetase is similarly subject to feedback inhibition by both nucleotide types, whereas PRPP activates this enzyme. Thus, Cross-Regulation is implemented at multiple stages of purine and pyrimidine nucleotide biosynthesis.

Fig. 35.18. Regulation of the pyrimidine nucleotide biosynthetic pathway. Solid lines indicate The pathway of chemical conversions. Dashed lines denote positive (⊕) and negative (⊝) feedback regulation. Abbreviations are defined in Fig. 35.15.