Nucleotide Metabolism and DNA Structure – Comprehensive Study Notes

PART 1: Nucleotide Metabolism

Overview & Importance

  • Nucleotides are fundamental building blocks for DNA and RNA, essential for genetic information storage, retrieval, and transmission.
  • They also serve as:
    • Energy carriers (e.g., ATP, GTP)
    • Signaling molecules (e.g., cAMP, cGMP)
    • Components of coenzymes (e.g., NAD, FAD, CoA)
    • Allosteric effectors regulating metabolic enzymes.
  • Disruption of nucleotide metabolism can lead to diseases (e.g., gout, immune deficiencies), and is a target for several chemotherapeutic drugs.

Nucleotide Structure

  • Bases: Nitrogen-containing rings.
    • Purines: Two rings (Adenine [A], Guanine [G]).
    • Pyrimidines: One ring (Cytosine [C], Thymine [T] in DNA, Uracil [U] in RNA).
  • Nucleosides: A base covalently linked to a pentose sugar (ribose for RNA, deoxyribose for DNA).
  • Nucleotides: A nucleoside with one or more phosphate groups attached to the sugar.
    • Example: Adenosine Triphosphate (ATP) has adenosine (base + ribose) and three phosphates.
  • Conversion: Ribonucleotides (RNA building blocks) are converted to deoxyribonucleotides (DNA building blocks) by ribonucleotide reductase, except for dTMP, which has a distinct synthesis pathway.

Pathways for Nucleotide Synthesis
All nucleotide synthesis pathways require PRPP (5-phosphoribosyl-1-pyrophosphate), which provides the ribose and phosphate backbone.

  1. De Novo Synthesis: Builds nucleotides from simpler precursors (amino acids, CO2\text{CO}_2, PRPP).
  2. Salvage Pathways: Recycles pre-existing free bases from nucleic acid degradation to form new nucleotides, which is energy-efficient.
    • These pathways utilize phosphoribosyltransferases (PRTs) to transfer PRPP onto a base. Key enzymes include:
      • HGPRT (Hypoxanthine-Guanine Phosphoribosyltransferase): Catalyzes:
        Hypoxanthine+PRPP→IMP+PPi\text{Hypoxanthine} + \text{PRPP} \rightarrow \text{IMP} + \text{PP}_i
        Guanine+PRPP→GMP+PPi\text{Guanine} + \text{PRPP} \rightarrow \text{GMP} + \text{PP}_i
      • APRT (Adenine Phosphoribosyltransferase): Catalyzes:
        Adenine+PRPP→AMP+PPi\text{Adenine} + \text{PRPP} \rightarrow \text{AMP} + \text{PP}_i

Purine Metabolism

  • De Novo Synthesis: Starts with PRPP.
    • The first purine nucleotide formed is IMP (Inosine Monophosphate), which then serves as a branch point for synthesizing AMP and GMP.
    • Amido PRT (Amido Phosphoribosyltransferase) is a key regulated step; it's activated by PRPP and negatively inhibited by purine nucleotides (feedback inhibition).
  • Degradation: Purines are broken down into hypoxanthine, then xanthine, and finally uric acid by the enzyme xanthine oxidase. Humans excrete uric acid as the end product due to the absence of uricase.

Uric Acid and Gout

  • Uric Acid Elimination: Approximately 65% is eliminated by the kidneys and 35% by the GI tract.
  • Reabsorption: About 90% of filtered uric acid is reabsorbed in the kidneys, primarily by the URAT1 transporter. Genetic variations in URAT1 can affect gout risk.
  • Hyperuricemia: Elevated plasma uric acid levels ( > 7.0 mg/dL in men, > 6.0 mg/dL in women) can lead to the formation of urate crystals, which deposit in joints and tissues, causing gout.
  • Gout Management:
    • Acute attacks: NSAIDs, colchicine, glucocorticoids.
    • Chronic management (hyperuricemia/tophi): Allopurinol (xanthine oxidase inhibitor), probenecid, sulfinpyrazone (increase uric acid excretion).

Pyrimidine Metabolism

  • De Novo Synthesis: Begins with carbamoyl phosphate (formed by CPSII from glutamine, CO2\text{CO}_2, ATP).
    • The first pyrimidine base formed is orotate, which then combines with PRPP via orotate phosphoribosyltransferase (OPRT) to form OMP (Orotate Monophosphate).
    • OMP is decarboxylated to form UMP (Uridine Monophosphate), which is then phosphorylated to UDP and UTP. CTP (Cytidine Triphosphate) is formed from UTP by CTP synthetase.
  • dTMP Synthesis: dTMP (deoxythymidine monophosphate) is synthesized from dUMP (deoxyuridine monophosphate) by thymidylate synthase.
    • This reaction requires tetrahydrofolate (THF) as a one-carbon donor (methylene group), which is oxidized to dihydrofolate (DHF).
    • Dihydrofolate reductase (DHFR) regenerates THF from DHF, a critical step for continuous dTMP and DNA synthesis.
    • Therapeutic relevance: Inhibitors of thymidylate synthase (e.g., 5-fluorouracil) or DHFR (e.g., methotrexate) are important anti-cancer drugs, as they block DNA synthesis (specifically dTMP production) in rapidly dividing cells.
  • Degradation: Pyrimidine bases (uracil, thymidine) are broken down into beta-alanine and beta-aminoisobutyrate, which feed into central metabolic pathways.
PART 2: DNA Structure

DNA vs. RNA

  • DNA (Deoxyribonucleic Acid): Primary molecule for storing genetic information (the genome). Replicated and transmitted through cell division.
  • RNA (Ribonucleic Acid): Involved in expressing genetic information (e.g., mRNA, tRNA, rRNA).
  • Key Differences:
    • Sugar: DNA has deoxyribose, RNA has ribose.
    • Bases: DNA uses A, T, C, G. RNA uses A, U, C, G (Uracil replaces Thymine).
    • Structure: DNA is typically a double helix; RNA is usually single-stranded (but can fold into complex 3D structures).

DNA Architecture - The Double Helix

  • DNA exists as a double helix composed of two antiparallel polynucleotide strands.
    • One strand runs 5' to 3', and the complementary strand runs 3' to 5'.
  • The two strands are held together by hydrogen bonds between complementary bases:
    • Adenine (A) always pairs with Thymine (T) via two hydrogen bonds.
    • Guanine (G) always pairs with Cytosine (C) via three hydrogen bonds.
  • This base pairing rule (A-T, G-C) ensures the strands are complementary, not identical.
    • Example: If a DNA strand is 5'-GCATCA-3', its complementary strand is 3'-CGTAGT-5'.

DNA Packaging: Chromosomes and Chromatin

  • Chromosomes: Structures within the nucleus of eukaryotic cells that contain genetic material.
    • Humans have 23 pairs (22 autosomes, 1 pair of sex chromosomes X/Y).
    • Prokaryotes typically have a single circular chromosome.
  • Chromatin: The complex of DNA wrapped around associated proteins, primarily histones.
    • Allows DNA to be compact enough to fit into the cell nucleus.
    • Regulates gene expression by controlling DNA accessibility.
    • Nucleosome: The basic unit of chromatin, consisting of DNA wrapped around a core of eight histone proteins (histone octamer).
  • Chromatin States:
    • Heterochromatin: Highly condensed, transcriptionally less active.
    • Euchromatin: Less condensed, transcriptionally more active.

Chromosomes, Replication, and Structural Elements

  • For proper replication and segregation, chromosomes require specific structural elements:
    • Origins of Replication: Specific DNA sequences where replication initiation complexes assemble, marking the start points of DNA replication.
    • Centromeres: Constricted regions that serve as attachment sites for spindle fibers during cell division, ensuring accurate segregation of duplicated chromosomes (sister chromatids).
    • Telomeres: Protective caps at the ends of linear eukaryotic chromosomes, preventing degradation and ensuring complete replication of the lagging strand.

Cell Division: Mitosis vs. Meiosis

  • Mitosis:
    • Produces two genetically identical somatic cells from a single parent cell.
    • Involved in growth, repair, and asexual reproduction.
    • Sister chromatids separate in anaphase.
  • Meiosis:
    • Produces four genetically distinct gametes (sperm or egg cells) with half the number of chromosomes.
    • Involves two rounds of division (Meiosis I and Meiosis II).
    • Homologous chromosomes pair and separate in Meiosis I; sister chromatids separate in Meiosis II.
  • Ploidy: Human somatic cells are diploid (2n2n chromosomes), while gametes are haploid (nn chromosomes).
  • Aneuploidy: Abnormal number of chromosomes (e.g., Down syndrome, trisomy 21).

Extrachromosomal DNA

  • Not all genetic material is in the cell nucleus; mitochondria in human cells contain their own circular DNA.

Clinical Connections

  • Understanding DNA structure is crucial for genetics, heredity, and disease mechanisms (e.g., mutations).
  • Therapeutic strategies often target nucleotide metabolism or DNA synthesis, such as in cancer chemotherapy (e.g., antifolates blocking dTMP synthesis).
  • Genetic variations, like URAT1 polymorphisms, can influence disease risk and drug responses, highlighting the importance of personalized medicine.