# Lecture 23: Nucleotide Metabolism
## Biochemistry
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## Learning Objectives
By the end of this lecture, students will be able to:
1. Describe the de novo synthesis of purine and pyrimidine nucleotides, including key enzymes and regulatory steps
2. Explain the salvage pathways for purine and pyrimidine bases and their clinical significance
3. Describe the conversion of ribonucleotides to deoxyribonucleotides by ribonucleotide reductase
4. Outline the synthesis of thymidylate (dTMP) and its dependence on folate metabolism
5. Describe the degradation pathways for purines and pyrimidines
6. Discuss clinical disorders of nucleotide metabolism including gout, Lesch-Nyhan syndrome, and targets for cancer chemotherapy
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## Lecture Content
### I. Overview of Nucleotide Structure and Function
Nucleotides consist of three components: a nitrogenous base, a sugar (ribose or deoxyribose), and one or more phosphate groups. The bases fall into two structural classes. **Purines** -- adenine (A) and guanine (G) -- have a two-ring structure, while **pyrimidines** -- cytosine (C), uracil (U, found in RNA), and thymine (T, found in DNA) -- have a single-ring structure. Nucleotides serve diverse functions beyond their role as DNA and RNA building blocks: they act as energy carriers (ATP, GTP), signaling molecules (cAMP, cGMP), components of coenzymes (NAD+, FAD, CoA), and activated intermediates in biosynthetic reactions (UDP-glucose, CDP-diacylglycerol).
### II. De Novo Purine Synthesis
De novo purine synthesis occurs in the **cytoplasm**, primarily in the liver. A distinctive feature of purine biosynthesis is that the purine ring is built step by step directly on the sugar phosphate backbone -- the ring is not assembled independently and then attached. The starting material is ribose-5-phosphate from the pentose phosphate pathway.
The pathway begins when **PRPP synthetase** converts ribose-5-phosphate and ATP to PRPP (5-phosphoribosyl-1-pyrophosphate), a critically important activated sugar used in both purine and pyrimidine salvage and de novo synthesis. Next, **glutamine-PRPP amidotransferase** catalyzes the **committed step** of purine synthesis, converting PRPP and glutamine to phosphoribosylamine, glutamate, and PPi. This enzyme is feedback-inhibited by IMP, AMP, and GMP (end-product inhibition) and activated by PRPP.
A series of nine additional reactions then build the purine ring, using glycine, two molecules of N10-formyl-THF, glutamine, CO2, and aspartate as sources of individual atoms. The atoms of the purine ring derive from specific donors: N1 from aspartate, C2 and C8 from N10-formyl-THF (one-carbon units from folate), N3 and N9 from glutamine amide nitrogen, C4, C5, and N7 from glycine (the entire molecule), and C6 from CO2. The first complete purine nucleotide produced is **IMP** (inosine monophosphate, containing the base hypoxanthine).
IMP then sits at a branch point. Conversion of **IMP to AMP** requires GTP and aspartate and proceeds through adenylosuccinate synthetase and adenylosuccinate lyase. Conversion of **IMP to GMP** requires ATP and glutamine and proceeds through IMP dehydrogenase (using NAD+) and GMP synthetase. An elegant cross-regulation ensures balanced purine pools: AMP synthesis requires GTP, while GMP synthesis requires ATP.
Regulation of purine synthesis occurs at multiple points. **PRPP synthetase** is inhibited by purine nucleotides (ADP, GDP). **Glutamine-PRPP amidotransferase**, the committed step enzyme, is feedback-inhibited by IMP, AMP, and GMP and activated by PRPP. At the branch point, AMP inhibits its own formation and GMP inhibits its own formation.
A diagram of de novo purine synthesis. Panel A: The purine ring structure with atoms numbered and color-coded by their metabolic source: N1 (aspartate, red), C2 and C8 (N10-formyl-THF, green), N3 and N9 (glutamine, blue), C4, C5, N7 (glycine, orange), C6 (CO2, purple). Panel B: A simplified pathway from ribose-5-phosphate to PRPP (PRPP synthetase) to phosphoribosylamine (glutamine-PRPP amidotransferase, committed step) through 10 steps to IMP. From IMP, the pathway branches: one arm to AMP (requiring GTP and aspartate) and one arm to GMP (requiring ATP and glutamine). Feedback inhibition arrows are shown: AMP and GMP inhibit glutamine-PRPP amidotransferase; AMP inhibits adenylosuccinate synthetase; GMP inhibits IMP dehydrogenase. Cross-regulation (GTP needed for AMP; ATP needed for GMP) is highlighted.
### III. De Novo Pyrimidine Synthesis
A key difference between pyrimidine and purine synthesis is that the **pyrimidine ring is built first** and then attached to ribose-5-phosphate, rather than being assembled on the sugar. The first three enzymatic activities of the pathway in mammals reside on a single trifunctional enzyme called **CAD**.
**Carbamoyl phosphate synthetase II (CPS-II)** catalyzes the **rate-limiting step** of pyrimidine synthesis in the cytoplasm, combining glutamine, bicarbonate, and 2 ATP to produce carbamoyl phosphate, glutamate, 2 ADP, and Pi. CPS-II is the cytoplasmic isoform and uses glutamine as its nitrogen donor, distinguishing it from CPS-I, the mitochondrial isoform that uses NH4+ and participates in the urea cycle. CPS-II is inhibited by UTP and activated by ATP and PRPP.
**Aspartate transcarbamylase (ATCase)** then combines carbamoyl phosphate with aspartate to form carbamoyl aspartate. **Dihydroorotase** catalyzes ring closure to produce dihydroorotate. **Dihydroorotate dehydrogenase (DHODH)** oxidizes dihydroorotate to orotate in the only mitochondrial step of the pathway, located on the outer surface of the inner mitochondrial membrane and using FMN and ubiquinone as electron acceptors. This enzyme is the target of **leflunomide**, an immunosuppressant used in rheumatoid arthritis.
**Orotate phosphoribosyltransferase** attaches orotate to PRPP, yielding OMP (orotidine-5'-monophosphate), and **OMP decarboxylase** converts OMP to **UMP** (uridine monophosphate). Steps 5 and 6 are catalyzed by a single bifunctional enzyme called **UMP synthase**. UMP is then phosphorylated to UDP and UTP by kinases using ATP, and **CTP synthetase** converts UTP to **CTP** using glutamine and ATP.
### IV. Salvage Pathways
Salvage pathways recycle free bases and nucleosides arising from nucleic acid turnover and dietary sources, providing a more energy-efficient alternative to de novo synthesis.
For **purine salvage**, two phosphoribosyltransferases are important. **HGPRT (hypoxanthine-guanine phosphoribosyltransferase)** converts hypoxanthine plus PRPP to IMP, and guanine plus PRPP to GMP. **APRT (adenine phosphoribosyltransferase)** converts adenine plus PRPP to AMP. **Pyrimidine salvage** is less clinically significant and involves uridine kinase and thymidine kinase, which recycle nucleosides.
### V. Deoxyribonucleotide Synthesis
**Ribonucleotide reductase (RNR)** converts ribonucleoside diphosphates (NDPs) to deoxyribonucleoside diphosphates (dNDPs) by reducing the 2' position of ribose. Its substrates are ADP, GDP, CDP, and UDP. The enzyme requires **thioredoxin** in its reduced form as the electron donor, and thioredoxin is regenerated by **thioredoxin reductase** using NADPH. RNR employs a free radical mechanism involving a tyrosyl radical.
Regulation of RNR is critical for maintaining balanced dNTP pools. An **activity site** determines overall enzyme activity: ATP activates while dATP inhibits. A **specificity site** determines which substrate is reduced, based on which dNTP is bound as an effector. This sophisticated allosteric control ensures that all four deoxyribonucleotides are produced in the proportions needed for DNA replication.
### VI. Thymidylate (dTMP) Synthesis
**Thymidylate synthase** converts dUMP to **dTMP** using N5,N10-methylene-THF, which donates both the one-carbon unit and the reducing equivalents. In this reaction, THF is oxidized to DHF. **DHF must be regenerated** by dihydrofolate reductase (DHFR), which reduces DHF back to THF using NADPH. This cycle is critical for DNA synthesis because thymidylate is needed exclusively for DNA, not RNA.
#### Drug Targets in Thymidylate Synthesis:
Several important anticancer and antimicrobial drugs target this pathway. **Methotrexate** inhibits DHFR, depleting THF and blocking both dTMP and purine synthesis. **Trimethoprim** inhibits bacterial DHFR selectively, acting as an antibiotic. **5-Fluorouracil (5-FU)** is converted to 5-FdUMP, which irreversibly inhibits thymidylate synthase. **Hydroxyurea** inhibits ribonucleotide reductase and is used in cancer treatment and sickle cell disease management.
A diagram of thymidylate synthesis and its pharmacological targets. The central cycle shows: dUMP is converted to dTMP by thymidylate synthase, with N5,N10-methylene-THF donating a one-carbon unit and being oxidized to DHF. DHF is reduced back to THF by dihydrofolate reductase (DHFR) using NADPH. THF receives a one-carbon unit from serine (via serine hydroxymethyltransferase) to regenerate N5,N10-methylene-THF. Drug inhibition sites are marked with red X symbols: methotrexate and trimethoprim blocking DHFR, 5-FU (as 5-FdUMP) blocking thymidylate synthase, and hydroxyurea blocking ribonucleotide reductase (shown converting UDP to dUDP upstream). The downstream effect of blocking this cycle (impaired DNA synthesis leading to cell death) is indicated.
### VII. Purine Degradation
Purine nucleotides are degraded through a stepwise pathway to uric acid. AMP is dephosphorylated to adenosine by 5'-nucleotidase, then deaminated to inosine by adenosine deaminase (ADA), and cleaved to hypoxanthine by purine nucleoside phosphorylase (PNP). GMP is dephosphorylated to guanosine, cleaved to guanine by PNP, and deaminated to xanthine by guanine deaminase. **Xanthine oxidase** then oxidizes hypoxanthine to xanthine and xanthine to **uric acid**, using O2 and producing H2O2.
Uric acid is the final product of purine catabolism in humans and is excreted by the kidneys. Notably, humans lack uricase, the enzyme that converts uric acid to the more soluble allantoin found in most other mammals.
### VIII. Pyrimidine Degradation
In contrast to purine degradation, pyrimidine bases are degraded to highly soluble products that pose no clinical problems. CMP and UMP are degraded through cytosine and uracil to **beta-alanine**, NH4+, and CO2. dTMP is degraded through thymine to **beta-aminoisobutyrate**, NH4+, and CO2. These products are water-soluble and easily excreted.
### IX. Clinical Correlations
#### A. Gout
Gout is caused by **hyperuricemia** (elevated serum uric acid). When uric acid exceeds its solubility limit, it crystallizes as monosodium urate in joints (especially the first metatarsophalangeal joint, a presentation called podagra), soft tissues, and kidneys. Causes include overproduction of purines, underexcretion of uric acid, high-purine diets, and cell lysis (as in tumor lysis syndrome). Treatment options include **allopurinol** and **febuxostat** (xanthine oxidase inhibitors that reduce uric acid production), **colchicine** (an anti-inflammatory that inhibits neutrophil microtubules, used for acute flares), **probenecid** (a uricosuric agent that increases renal uric acid excretion), and **rasburicase** (recombinant uricase that converts uric acid to the soluble allantoin, used in tumor lysis syndrome).
#### B. Lesch-Nyhan Syndrome
This X-linked recessive disorder results from **HGPRT deficiency**. Without the ability to salvage hypoxanthine or guanine, PRPP accumulates, driving increased de novo purine synthesis and massive uric acid overproduction. The syndrome presents with severe hyperuricemia and gout, intellectual disability, **self-mutilating behavior** (biting lips and fingers), choreoathetosis, and spasticity. Partial HGPRT deficiency causes Kelley-Seegmiller syndrome, which features gout without neurological symptoms.
#### C. Adenosine Deaminase (ADA) Deficiency
ADA deficiency leads to accumulation of deoxyadenosine and dATP. Because dATP allosterically inhibits ribonucleotide reductase, DNA synthesis is impaired. Lymphocytes are particularly vulnerable, resulting in **severe combined immunodeficiency (SCID)**. ADA deficiency was one of the first diseases treated with gene therapy.
#### D. Orotic Aciduria
Deficiency of **UMP synthase** (orotate phosphoribosyltransferase and/or OMP decarboxylase) causes accumulation of orotic acid in the urine. The condition presents as megaloblastic anemia that does **not** respond to folate or B12. Treatment consists of uridine supplementation, which bypasses the enzymatic block. It is important to distinguish orotic aciduria from OTC deficiency, which also elevates urinary orotic acid but is accompanied by hyperammonemia.
A degradation pathway diagram for purines. Starting from AMP and GMP at the top, the pathway shows stepwise conversion through nucleosides and free bases. AMP is dephosphorylated to adenosine, then deaminated (ADA) to inosine, which is cleaved to hypoxanthine. GMP is dephosphorylated to guanosine, cleaved to guanine, then deaminated to xanthine. Hypoxanthine is oxidized to xanthine (xanthine oxidase), and xanthine is oxidized to uric acid (xanthine oxidase). Uric acid is shown as the end product in humans, with a note that most mammals convert it further to allantoin (uricase, absent in humans). Clinical annotations: ADA deficiency causing SCID is marked at the adenosine deaminase step; HGPRT deficiency (Lesch-Nyhan) is marked at the salvage step; allopurinol inhibition is marked at xanthine oxidase. Gout crystals in a joint are depicted in an inset.
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