Comprehensive Notes on Pyrimidine Metabolism

Metabolic Overview and Learning Objectives

• Course and Topic Codes: C2K23 Year-02 Final version vol.2; TOPIC R-B-002, R-B-004, R-B-006.

• Learning Objectives:

  • Discuss the synthesis and degradation of pyrimidine (De-Novo and salvage pathways).

  • Discuss the regulation of pyrimidine biosynthesis and degradation.

  • Interpret Orotic aciduria in relevance to nucleotides and the urea cycle.

  • Differentiate between Carbamoyl Phosphate Synthetase I ($CPS-I$) and II ($CPS-II$).

  • Understand pyrimidine metabolism disorders.

General Structure and Sources of Pyrimidines

• Pyrimidine Structures:

  • Cytosine: Found in both DNA and RNA.

  • Thymine: Found in DNA only; characterized by a methyl group ($CH_3$).

  • Uracil: Found in RNA only. (CTU)

• Synthesis Overview:

  • Unlike purines, the pyrimidine ring is synthesized before being attached to the ribose 5-phosphate donated by $PRPP$ (5-Phosphoribosyl-1-pyrophosphate).

• Sources of Atoms in the Pyrimidine Ring:

  • Glutamine: Provides the amide nitrogen ($N_3$).

  • $CO_2$: Contributes the $C_2$ atom.

  • Aspartate: Contributes the rest of the ring ($N_1$, $C_4$, $C_5$, and $C_6$).

De Novo Pyrimidine Biosynthesis Pathway

Step 1: Formation of Carbamoyl Phosphate

• Reaction:   $2 ATP + CO_2 + \text{Glutamine} + H_2O \rightarrow \text{Carbamoyl phosphate} + 2 ADP + P_i + \text{Glutamate}$

• Enzyme: Carbamoyl Phosphate Synthetase II ($CPS-II$).

• Location: Cytosol.

• Regulation in Mammals: This is the regulated step. It is inhibited by $UTP$ (end product) and activated by $PRPP$.

Step 2: Synthesis of Carbamoyl Aspartate

• Reaction: Carbamoyl phosphate combines with Aspartate to form Carbamoyl aspartate.

• Enzyme: Aspartate transcarbamoylase ($ATC$).

• Bacterial Regulation: In prokaryotic cells, this is the regulated step, inhibited by $CTP$.

Step 3: Ring Closure to Dihydroorotate

• Reaction: Carbamoyl aspartate undergoes ring closure (losing $H_2O$) to form Dihydroorotate.

• Enzyme: Dihydroorotase.

Step 4: Oxidation to Orotate

• Reaction:   $\text{Dihydroorotate} + NAD^+ \rightarrow \text{Orotate} + NADH + H^+$

• Enzyme: Dihydroorotate dehydrogenase.

• Cofactor: Flavin mononucleotide ($FMN$).

• Key Detail: This enzyme is a flavoprotein associated with the inner mitochondrial membrane. All other enzymes in this pathway are cytosolic.

Step 5: Formation of OMP

• Reaction: Orotate reacts with $PRPP$ to form Orotidine 5'-monophosphate ($OMP$), releasing pyrophosphate ($PP_i$).

• Enzyme: Orotate phosphoribosyltransferase.

• Significance: This is the point where the ribose 5-phosphate is attached to the base.

Step 6: Decarboxylation to UMP

• Reaction:   $\text{OMP} \rightarrow \text{UMP} + CO_2$

• Enzyme: Orotidylate decarboxylase (also known as OMP decarboxylase).

• Product: Uridine 5'-monophosphate ($UMP$), the parent pyrimidine mononucleotide.

Multifunctional Polypeptides in Pyrimidine Synthesis

• Pyrimidine biosynthesis employs multifunctional enzymes to facilitate "channeling" of intermediates, increasing efficiency:

• CAD: A single polypeptide catalyzing the first three reactions. It contains three domains:

  • Carbamoyl Phosphate Synthetase II

  • Aspartate transcarbamoylase

  • Dihydroorotase

• UMP Synthase: A bifunctional enzyme catalyzing the final two reactions:

  • Orotate phosphoribosyltransferase

  • Orotidylate decarboxylase

Comparison of Carbamoyl Phosphate Synthetase I and II

• CPS-I:

  • Pathway: Urea Synthesis.

  • Location: Mitochondria.

  • Nitrogen Source: Ammonia ($NH_3$).

  • Regulator: Activated by N-Acetyl Glutamate.

• CPS-II:

  • Pathway: Pyrimidine Synthesis.

  • Location: Cytosol.

  • Nitrogen Source: Glutamine (Amide group).

  • Regulator: Activated by $PRPP$; Inhibited by $UTP$.

• Compartmentation: These separate pools of carbamoyl phosphate ensure that the two metabolic processes do not interfere with one another.

Orotic Aciduria and Related Disorders

Hereditary Orotic Aciduria

• Cause: Deficiency in the bifunctional enzyme UMP Synthase (low activity of one or both domains).

• Subtypes:

  • Type I: Deficiency of both Orotate phosphoribosyltransferase and Orotidylate decarboxylase.

  • Type II: Deficiency specifically of Orotidylate decarboxylase.

• Symptoms:

  • Excessive excretion of orotic acid in urine.

  • Poor growth and failure to thrive.

  • Megaloblastic Anemia (unresponsive to vitamin $B_{12}$ or folate).

  • Mental and physical retardation.

  • Inhibition of RNA and DNA synthesis red.

• Treatment: Administration of Uridine. Uridine bypasses the metabolic block, is salvaged to $UMP$, and provides feedback inhibition for $CPS-II$, reducing orotic acid production.

Secondary Orotic Aciduria (Urea Cycle Defects)

• Mechanism: Defects in Ornithine Transcarbamylase (OTC) cause an accumulation of carbamoyl phosphate in the mitochondria. The excess leaks into the cytosol, overstimulating the de-novo pyrimidine pathway.

• Outcome: Increased excretion of orotic acid, uracil, and uridine. Mild symptoms that worsen with high nitrogen (protein) intake.

Orotic Aciduria in Reye's Syndrome

• Definition: A serious condition involving swelling of the liver and brain, often following viral infections (flu, chickenpox) in children treated with aspirin.

• Mechanism: Severe mitochondrial damage prevents the utilization of carbamoyl phosphate in the urea cycle, diverting it to cytosolic pyrimidine synthesis, leading to orotic aciduria.

Drug-Induced Orotic Aciduria

• Allopurinol: Competes with orotic acid for $PRPP$ at the OPRT site. The resulting nucleotide inhibits orotidylate decarboxylase.

• 6-azauridine: Competitively inhibits orotidylate decarboxylase.

Nucleotide Conversions and Thymidylate Synthesis

• Formation of CTP:

  • $UMP \rightarrow UDP \rightarrow UTP$.

  • $UTP + \text{Glutamine} \rightarrow CTP + \text{Glutamate}$ (catalyzed by CTP synthetase).

• Formation of dUMP:

  • $UDP \rightarrow dUDP$ (via ribonucleotide reductase).

  • $dUDP \rightarrow dUTP \rightarrow dUMP$ (via $dUTPase$). $dUTPase$ prevents the erroneous incorporation of uracil into DNA by keeping $dUTP$ levels low.

• Synthesis of dTMP (Thymidylate):

  • Enzyme: Thymidylate synthase.

  • Mechanism: Converts $dUMP$ to $dTMP$ using $N^5, N^{10}\text{-Methylene tetrahydrofolate}$ as a methyl donor.

  • Unique Feature: Tetrahydrofolate ($THF$) is oxidized to dihydrofolate ($DHF$) because it donates both a carbon unit and two hydrogen atoms. $DHF$ must then be reduced back to $THF$ by Dihydrofolate Reductase.

Pharmacological Inhibitors

• 5-Fluorouracil (5-FU): An antitumor agent. It is converted to $5-FdUMP$, which binds permanently to thymidylate synthase. It is described as a suicide inhibitor.

• Methotrexate: Inhibits Dihydrofolate Reductase. This depletes the pool of $THF$ required for both purine synthesis and the conversion of $dUMP$ to $dTMP$, thereby slowing DNA replication and cell growth in cancers.

• Acyclovir: A purine analog used for Herpes Simplex Virus; inhibits viral DNA polymerase.

• AZT (3'-azido-3'-deoxythymidine): A pyrimidine analog used for HIV; inhibits viral DNA polymerase (Reverse Transcriptase). Tbh

Pyrimidine Catabolism and Salvage

Degradation Pathway

• Unlike purines, the pyrimidine ring is opened and degraded into highly water-soluble products.

• End Products:

  • $CO_2$ and $NH_3$.

  • $\beta\text{-alanine}$: Derived from Cytosine and Uracil degradation.

  • $\beta\text{-aminoisobutyrate}$: Derived from Thymine degradation.

• Clinical Significance of $\beta\text{-aminoisobutyrate}$:

  • Increased excretion is seen in leukemia or severe X-ray exposure due to high DNA turnover.

  • Chinese and Japanese populations exhibit a genetic tendency to excrete this metabolite routinely.

• Enzyme Note: Hepatic $\beta\text{-ureidopropionase}$ is used for the final steps of these pathways.

Other Catabolic Conditions

• Combined uraciluria-thyminuria: Leads to serious neurological complications.

• $\beta\text{-hydroxybutyric aciduria}$: Due to deficiency in dihydropyrimidine dehydrogenase. Patients with this deficiency risk severe toxicity if given 5-fluorouracil.

• Pseudouridine ($\text{ψ}$): Derived from RNA degradation. Humans lack enzymes to hydrolyze or phosphorolyze it, so it is excreted unchanged in the urine.

Salvage Pathway

• Mammalian cells salvage few free pyrimidines due to the high solubility of the bases.

• Reactions: Pyrimidine nucleosides (uridine, cytidine, thymidine, deoxycytidine) are phosphorylated to their respective nucleotides.

• Clinical Use: This pathway is the basis for using uridine to treat hereditary orotic aciduria.

Comparison: Purine vs. Pyrimidine Metabolism