Nucleosides Nucleotides and Roles of Nitrogenous Bases

Introduction

Nucleosides and nucleotides are the fundamental building blocks of nucleic acids (DNA and RNA) and participate in nearly every biochemical process. A nucleoside consists of a nitrogenous base covalently linked to a five‑carbon sugar (β-D-ribose in RNA or β-D-2-deoxyribose in DNA) via a β‑N‑glycosidic bond at the anomeric carbon (1′). A nucleotide is a nucleoside with one or more phosphate groups esterified, most commonly at the 5′ hydroxyl (though 3′-phosphate nucleotides also exist, e.g., in RNA degradation).

Additional data:

• The glycosidic bond has a high activation energy for hydrolysis (~30–40 kcal/mol), making it kinetically stable at physiological pH.

• The conformation of the glycosidic bond is typically anti for B-DNA and syn for certain purine nucleotides in Z-DNA or tRNA.

Structure of Nitrogenous Bases

Purines: Adenine (A) and Guanine (G) — bicyclic aromatic heterocycles (a six‑membered pyrimidine ring fused to a five‑membered imidazole ring). Numbering: purine ring atoms are 1–9.

Pyrimidines: Cytosine (C), Thymine (T, DNA only), Uracil (U, RNA only) — single six‑membered aromatic ring.

Key chemical features:

• Hydrogen‑bond donors/acceptors determine Watson–Crick base pairing:

  • A–T (or A–U): 2 hydrogen bonds.

  • G–C: 3 hydrogen bonds (greater thermal stability).

• Tautomerism: Bases can exist in rare enol or imino forms (e.g., keto-enol tautomerization of thymine), leading to mispairing and spontaneous mutations. The common form is keto (for T/U) and amino (for A/C).

• pKa values of bases (approximate):

  • Adenine: pKa (N1) = 3.5, N7 = 9.8

  • Guanine: pKa (N7) = 3.3, N1 = 9.2

  • Cytosine: pKa (N3) = 4.5

  • Thymine: pKa (N3) = 9.9

  • Uracil: pKa (N3) = 9.5
    These values become relevant in catalytic RNA (ribozymes) and pH-dependent base modifications.

Nucleoside and Nucleotide Chemistry

Glycosidic Bond:

• Pyrimidines attach to sugar at N1; purines attach at N9.

• The β‑configuration means the base is on the same side of the sugar ring as the 3′ hydroxyl (opposite to the 5′ CH₂OH group in the standard Haworth projection).

• Stability: Purine glycosidic bonds are slightly more acid-labile than pyrimidine bonds (depurination occurs at ~10,000 sites per cell per day in mammals; repaired by base excision repair).

Phosphorylation:

• Sequential addition of phosphate groups yields:

  • NMP (nucleoside monophosphate, e.g., AMP)

  • NDP (nucleoside diphosphate, e.g., ADP)

  • NTP (nucleoside triphosphate, e.g., ATP, GTP, CTP, UTP, and dNTPs for DNA)

• High-energy bonds: The α-β and β-γ phosphoanhydride bonds have ΔG°′ of hydrolysis ≈ −30.5 kJ/mol each (ATP → ADP + Pi). Pyrophosphate (PPi) hydrolysis provides additional driving force in polymerization.

• NTPs are substrates for DNA/RNA polymerases and also serve as allosteric regulators.

Additional chemical data:

• Cyclic nucleotides: 3′,5′-cAMP and 3′,5′-cGMP have a phosphate bridging the 3′ and 5′ hydroxyls – key second messengers.

• Modified sugar nucleotides: UDP-glucose, GDP-mannose – used in glycosylation reactions.

• Thio- and seleno-analogs: e.g., α-thio-ATP – resistant to some nucleases, used in molecular biology.

Expanded Biological Roles of Bases and Nucleotides

1. Information Storage and Transfer

• Sequence of bases encodes genetic information; complementarity enables replication, transcription, and translation.

• Degeneracy of codons: 64 codons encode 20 amino acids + stop signals.

• Epigenetic modifications: 5-methylcytosine (5mC) in DNA regulates gene expression; 5-hydroxymethylcytosine in neurons. In RNA, N6-methyladenosine (m⁶A) controls RNA stability and translation.

2. Energy Currency

• ATP is the universal energy carrier. GTP powers protein synthesis and signal transduction.

• Energy charge ([ATP] + 0.5[ADP]) / ([ATP]+[ADP]+[AMP]) regulates anabolic and catabolic pathways. Healthy cells maintain 0.85–0.95.

• Creatine phosphate and phosphoarginine serve as rapid ATP reserves in muscle.

3. Signalling and Regulation

• cAMP: activates PKA; synthesized from ATP by adenylyl cyclase; degraded by phosphodiesterase (PDE – drug target for sildenafil).

• cGMP: activates PKG; involved in vasodilation and vision.

• ATP and ADP act as extracellular purinergic signals (P2 receptors) in inflammation, pain, and platelet aggregation.

• Adenosine (a nucleoside) is a tissue-protective signal (A1/A2A receptors); caffeine antagonizes adenosine receptors.

4. Structural and Catalytic Roles

• Cofactors: NAD⁺/NADH, FAD/FADH₂, CoA (all contain nucleotides).

• Ribozymes: catalytic RNA (e.g., RNase P, self-splicing introns) require specific nucleotide conformations.

• tRNA and rRNA: modified nucleotides (pseudouridine, dihydrouridine, thiouridine) stabilize tertiary structure.

Examples, Clinical Relevance, and Pathologies

Thymidine vs Uridine:

• Thymine’s methyl group at C5 protects DNA from uracil-DNA glycosylase (prevents removal of T from T-A pairs) and stabilizes the double helix via hydrophobic interactions.

• Uracil in RNA allows enzymatic discrimination of DNA vs RNA damage; cytosine deamination produces uracil (mutagenic if unrepaired).

Mutagenesis and Repair:

• Deamination: Cytosine → Uracil (mutation if replicated); 5-methylcytosine → Thymine (frequent C→T transition at CpG islands).

• Alkylation: O⁶-methylguanine pairs with thymine instead of cytosine; repaired by MGMT.

• Oxidation: 8-oxoguanine miscodes A → C; repaired by OGG1 glycosylase.

• Base analog drugs:

  • 5-fluorouracil (5-FU) – inhibits thymidylate synthase; incorporated into RNA.

  • Acyclovir – guanosine analog; chain terminator for herpesvirus DNA polymerase.

  • Azidothymidine (AZT) – thymidine analog; inhibits HIV reverse transcriptase.

Genetic diseases:

• Lesch-Nyhan syndrome: HGPRT deficiency → unable to salvage purines → excess uric acid → self-mutilation, gout, neurological dysfunction.

• Severe combined immunodeficiency (SCID): Adenosine deaminase (ADA) deficiency → accumulation of dATP → toxic to lymphocytes.

• Mitochondrial DNA depletion syndromes: mutations in thymidine kinase 2 (TK2) or deoxyguanosine kinase (DGUOK).

Synthetic and Pharmacological Nucleotides

• Remdesivir: adenosine analog with 1′-cyano modification; terminates RNA synthesis in RNA viruses (SARS-CoV-2, Ebola).

• Fludarabine: purine analog used in CLL – resistant to deamination.

• Cytarabine (Ara-C): cytosine arabinoside; leukemia chemotherapy.

Key Takeaways (Expanded)

Feature	Nucleoside	Nucleotide
Composition	Base + sugar	Base + sugar + phosphate(s)
Examples	Adenosine, Cytidine, Uridine	ATP, dGTP, cAMP
Phosphates	0	1–3 (or cyclic)

• Purines (A, G) are bicyclic; pyrimidines (C, T, U) are monocyclic with distinct H-bonding patterns.

• NTPs power polymerases, energy transfer (ATP/GTP), and signaling (cAMP/cGMP).

• Clinical relevance includes base analog antivirals/chemotherapeutics, repair disorders, and metabolic diseases.

• Modified nucleotides (m⁶A, 5mC) are central to epigenetics and RNA regulation.