Structure, Nomenclature, and Function of Nucleosides and Nucleotides

Course and Curriculum Overview

• Academic Institution: Faculty of Medicine, BADR University in Cairo (BUC).
• Instructional Department: Molecular Cell Biology and Genetics.
• Module Identifier: CBG 1103.
• Instructional Faculty: Prof. Dr Dina Sabry and Dr/Shimaa Mohsen.
• Module Purpose and Scope:
  • Equips medical students with the foundational principles of molecular biology and genetics.
  • Details molecular mechanisms responsible for governing the stability, integrity, and structural maintenance of the genetic material.
  • Utilizes clinical and pathological examples to illustrate the cellular, biochemical, and genetic basis of human diseases.
  • Applies fundamental scientific principles to solve clinical and medical problems.
• Intended Learning Outcomes (ILOs):
  • Identify the structural components, chemistry, and systematic nomenclature of nucleotides and nucleosides.
  • Enumerate nucleotides of biological significance and articulate their precise physiological and metabolic functions.
  • Describe higher-order DNA architecture and elucidate the chemical mechanics of phosphodiester bond formation.

Nucleic Acid Chemistry and Structural Architecture

• Definition and General Characteristics:
  • Nucleic acids, specifically deoxyribonucleic acid ($\text{DNA}$) and ribonucleic acid ($\text{RNA}$), represent biopolymers constructed from repeating linear arrays of monomeric building blocks termed nucleotides.
  • Nucleotides function both in polymerized macromolecular configurations ($\text{DNA}$ and $\text{RNA}$ chains) and as free, monomeric molecules executing crucial biochemical roles throughout the cell.

• Fundamental Components of a Nucleotide:
  • Every complete monomeric nucleotide consists of three covalently bonded constituents:
  • A nitrogenous base (purine or pyrimidine heterocyclic ring).
  • A five-carbon pentose sugar (ribose or $2'\text{-deoxyribose}$).
  • One or more phosphate groups (derived from phosphoric acid, $\text{H}_3\text{PO}_4$).

The Pentose Sugar Component

• Classification and Structural Stereochemistry:
  • Pentose sugars found in nucleic acids exist as five-membered furanose rings classified into two forms:
  • Ribose ($\text{D-ribofuranose}$): Present exclusively in ribonucleic acid ($\text{RNA}$); contains a hydroxyl group ($-\text{OH}$) bonded to carbon $2'$ ($\text{C}2'$).
  • Deoxyribose ($2'\text{-deoxy-D-ribofuranose}$): Present exclusively in deoxyribonucleic acid ($\text{DNA}$); structurally characterized by the absence of an oxygen atom at carbon $2'$ ($\text{C}2'$), possessing two hydrogen atoms ($-\text{H}$) bonded to this position instead of a hydroxyl moiety.

The Phosphate Group

• Molecular Configuration and Linkage:
  • Derived from inorganic phosphoric acid ($\text{H}_3\text{PO}_4$).
  • Covalently links to the carbon $5'$ ($\text{C}5'$) position of the pentose sugar via an ester bond (phosphoester linkage).
  • In extended nucleic acid polynucleotides, phosphate groups bridge adjacent sugars by forming a phosphodiester bond connecting the $3'\text{-hydroxyl}$ of one pentose to the $5'\text{-hydroxyl}$ of the subsequent pentose.

Nitrogenous Bases

• Nitrogenous Base Classification:
  • Nitrogen-containing heterocyclic aromatic ring systems classified into two distinct structural families:
  • Purines: Planar, bicyclic molecules consisting of a six-membered pyrimidine ring fused to a five-membered imidazole ring.
  • Pyrimidines: Planar, monocyclic six-membered heterocyclic rings containing two nitrogen atoms.

• Major Purine Bases:
  • Adenine ($\text{A}$): $6\text{-aminopurine}$; possesses an exocyclic amino group ($-\text{NH}_2$) at carbon 6 ($\text{C}6$).
  • Guanine ($\text{G}$): $2\text{-amino-6-oxypurine}$; possesses an exocyclic amino group at carbon 2 ($\text{C}2$) and a carbonyl oxygen at carbon 6 ($\text{C}6$).
• Major Pyrimidine Bases:
  • Cytosine ($\text{C}$): $4\text{-amino-2-oxypyrimidine}$; found in both $\text{DNA}$ and $\text{RNA}$; contains an amino group at $\text{C}4$ and a carbonyl group at $\text{C}2$.
  • Thymine ($\text{T}$): $2,4\text{-dioxy-5-methylpyrimidine}$; found exclusively in $\text{DNA}$; contains carbonyl groups at $\text{C}2$ and $\text{C}4$ and a unique methyl group ($-\text{CH}_3$) at carbon 5 ($\text{C}5$).
  • Uracil ($\text{U}$): $2,4\text{-dioxypyrimidine}$; found exclusively in $\text{RNA}$; contains carbonyl groups at $\text{C}2$ and $\text{C}4$ without methylation at $\text{C}5$.

• Minor Purine Bases and Metabolic Intermediates:
  • Metabolic intermediates formed during purine degradation and biosynthesis, typically absent from mature primary nucleic acid chains:
  • Hypoxanthine: $6\text{-oxypurine}$; intermediate formed during the catabolism of adenosine.
  • Xanthine: $2,6\text{-dioxypurine}$; formed by the oxidation of hypoxanthine or the deamination of guanine.
  • Uric Acid: $2,6,8\text{-trioxypurine}$; end product of purine catabolism in humans.
  • Caffeine: $1,3,7\text{-trimethylxanthine}$; pharmacologically active methylated purine derivative.

Assembly and Chemistry of Nucleosides and Nucleotides

• Nucleoside Synthesis and Chemical Bonding:
  • Formed by covalently coupling a nitrogenous base directly to carbon $1'$ ($\text{C}1'$) of a pentose sugar via an $\beta\text{-N-glycosidic}$ bond.
  • Regiospecific attachment points:
  • Purines: Atom nitrogen 9 ($\text{N}9$) covalently attaches to $\text{C}1'$ of the sugar.
  • Pyrimidines: Atom nitrogen 1 ($\text{N}1$) covalently attaches to $\text{C}1'$ of the sugar.
  • Chemical Equation: $\text{Nitrogenous base} + \text{Pentose sugar} \rightarrow \text{Nucleoside}$.
• Nucleotide Synthesis and High-Energy Bonding:
  • Formed when one, two, or three phosphate groups esterify to the hydroxyl group on carbon $5'$ ($\text{C}5'$) of the pentose sugar of a nucleoside.
  • Chemical Equation: $\text{Nucleoside} + \text{Phosphate group(s)} \rightarrow \text{Nucleotide}$.
  • Energetics:
  • Nucleoside monophosphates contain a single low-energy ester linkage ($\text{C}5'-\text{O}-\text{P}$).
  • Nucleoside diphosphates ($\text{NDP}$) and nucleoside triphosphates ($\text{NTP}$) possess additional phosphoanhydride bonds between successive phosphate groups.
  • Phosphoanhydride bonds are thermodynamically unstable and high-energy; cleavage releases substantial free energy ($\Delta G$) to drive endergonic cellular reactions.

Nomenclature of Ribonucleosides and Ribonucleotides

• Naming Standards for Ribose-Containing Compounds:
  • Bases, corresponding nucleosides, monophosphorylated acids, and higher-energy phosphorylated forms follow standardized nomenclature:
  • Adenine:
    • Ribonucleoside: Adenosine.
    • Monophosphate Nucleotide: Adenylic acid or Adenosine monophosphate ($\text{AMP}$).
    • Diphosphate Nucleotide: Adenosine diphosphate ($\text{ADP}$).
    • Triphosphate Nucleotide: Adenosine triphosphate ($\text{ATP}$).
  • Guanine:
    • Ribonucleoside: Guanosine.
    • Monophosphate Nucleotide: Guanylic acid or Guanosine monophosphate ($\text{GMP}$).
    • Diphosphate Nucleotide: Guanosine diphosphate ($\text{GDP}$).
    • Triphosphate Nucleotide: Guanosine triphosphate ($\text{GTP}$).
  • Cytosine:
    • Ribonucleoside: Cytidine.
    • Monophosphate Nucleotide: Cytidylic acid or Cytidine monophosphate ($\text{CMP}$).
    • Diphosphate Nucleotide: Cytidine diphosphate ($\text{CDP}$).
    • Triphosphate Nucleotide: Cytidine triphosphate ($\text{CTP}$).
  • Uracil:
    • Ribonucleoside: Uridine.
    • Monophosphate Nucleotide: Uridylic acid or Uridine monophosphate ($\text{UMP}$).
    • Diphosphate Nucleotide: Uridine diphosphate ($\text{UDP}$).
    • Triphosphate Nucleotide: Uridine triphosphate ($\text{UTP}$).
  • Hypoxanthine:
    • Ribonucleoside: Inosine.
    • Monophosphate Nucleotide: Inosine monophosphate ($\text{IMP}$).
  • Xanthine:
    • Ribonucleoside: Xanthosine.
    • Monophosphate Nucleotide: Xanthosine monophosphate ($\text{XMP}$).

Nomenclature of Deoxyribonucleosides and Deoxyribonucleotides

• Naming Standards for $2'\text{-Deoxyribose}$-Containing Compounds:
  • Utilizes the prefix "deoxy-" (abbreviated with lowercase "d") to denote the missing $2'\text{-oxygen}$ atom:
  • Adenine:
    • Deoxyribonucleoside: Deoxyadenosine.
    • Monophosphate Nucleotide: Deoxyadenylic acid or Deoxyadenosine monophosphate ($\text{dAMP}$).
    • Diphosphate Nucleotide: Deoxyadenosine diphosphate ($\text{dADP}$).
    • Triphosphate Nucleotide: Deoxyadenosine triphosphate ($\text{dATP}$).
  • Guanine:
    • Deoxyribonucleoside: Deoxyguanosine.
    • Monophosphate Nucleotide: Deoxyguanylic acid or Deoxyguanosine monophosphate ($\text{dGMP}$).
    • Diphosphate Nucleotide: Deoxyguanosine diphosphate ($\text{dGDP}$).
    • Triphosphate Nucleotide: Deoxyguanosine triphosphate ($\text{dGTP}$).
  • Cytosine:
    • Deoxyribonucleoside: Deoxycytidine.
    • Monophosphate Nucleotide: Deoxycytidylic acid or Deoxycytidine monophosphate ($\text{dCMP}$).
    • Diphosphate Nucleotide: Deoxycytidine diphosphate ($\text{dCDP}$).
    • Triphosphate Nucleotide: Deoxycytidine triphosphate ($\text{dCTP}$).
  • Thymine:
    • Deoxyribonucleoside: Deoxythymidine (often referred to simply as thymidine).
    • Monophosphate Nucleotide: Deoxythymidylic acid or Deoxythymidine monophosphate ($\text{dTMP}$).
    • Diphosphate Nucleotide: Deoxythymidine diphosphate ($\text{dTDP}$).
    • Triphosphate Nucleotide: Deoxythymidine triphosphate ($\text{dTTP}$).

Specialized Nucleotides of Biological Significance

• S-Adenosylmethionine ($\text{SAM}$):
  • Structure: Formed by the enzymatic condensation of adenosine triphosphate ($\text{ATP}$) and the amino acid methionine, creating a positively charged sulfonium center ($\text{S}^+$).
  • Biological Function: Primary methyl group ($-\text{CH}_3$) donor in cellular transmethylation reactions, transferring methyl groups to $\text{DNA}$, $\text{RNA}$, proteins, and membrane phospholipids.

• $3'\text{-Phosphoadenosine-5'-phosphosulfate}$ ($\text{PAPS}$):
  • Structure: Adenosine derivative containing a phosphate esterified at carbon $3'$ and a mixed anhydride bond between a $5'\text{-phosphate}$ and a sulfate group ($-\text{SO}_3^-$).
  • Biological Function: Active, high-energy sulfate donor utilized by sulfotransferases for the biosynthesis of sulfated glycosaminoglycans, sulfolipids, and the phase II detoxification of xenobiotics.

• Cyclic Adenosine Monophosphate ($\text{cAMP}$):
  • Structure: Mononucleotide containing an intramolecular diester bond formed between the phosphate group and both the carbon $3'$ ($\text{C}3'$) and carbon $5'$ ($\text{C}5'$) hydroxyl positions of the same ribose ring.
  • Biological Function: Universal intracellular second messenger mediating the physiological responses of peptide hormones, catecholamines, and neurotransmitters.

Physiological Functions of Nucleotides

• Polymeric Precursors:
  • Function as the essential, direct building blocks for the enzymatic synthesis and repair of genetic polymers, $\text{DNA}$ and $\text{RNA}$.
• Intracellular Energy Reservoirs:
  • Nucleoside triphosphates serve as cellular energy currency.
  • $\text{ATP}$ provides energy for cellular work, mechanical contraction, active membrane transport, and enzymatic catalysis.
  • $\text{GTP}$ provides specific energy input during ribosomal protein translation, microtubule assembly, and intracellular vesicle trafficking.
• Second Messengers in Signal Transduction:
  • Cyclic nucleotides ($\text{cAMP}$ and $\text{cGMP}$) transduce extracellular hormonal cues into defined intracellular cascades:
  • Primary hormone binds to a specific target G-protein-coupled receptor on the plasma membrane.
  • Receptor activates the transmembrane effector enzyme adenylate cyclase (adenyl cyclase).
  • Adenylate cyclase cyclizes intracellular $\text{ATP}$ into $\text{cAMP}$.
  • Elevated $\text{cAMP}$ allosterically binds to inactive protein kinase A ($\text{PKA}$), transforming it into active protein kinase.
  • Active kinase phosphorylates downstream intracellular target proteins, triggering a specific cellular response.

• Essential Coenzyme Components:
  • Nucleotide components form the structural scaffolding of redox and acyl-transfer coenzymes:
  • Nicotinamide Adenine Dinucleotide ($\text{NAD}^+$ / $\text{NADH}$): Dinucleotide serving as a mobile hydrogen and electron carrier ($2\text{H}^+ + 2\text{e}^-$) in catabolic oxidation-reduction reactions, such as the reversible interconversion of lactate and pyruvate catalyzed by lactate dehydrogenase:       $\text{Lactate} + \text{NAD}^+ \rightleftharpoons \text{Pyruvate} + \text{NADH} + \text{H}^+$
  • Flavin Adenine Dinucleotide ($\text{FAD}$ / $\text{FADH}_2$): Hydrogen carrier participating in intermediate metabolism and oxidative phosphorylation.
  • Coenzyme A ($\text{CoA-SH}$): Nucleotide-derived carrier providing a reactive thiol group to bind and transfer acyl and acetyl groups.

• Metabolic Activation of Intermediates:
  • Nucleotides covalently bond to biosynthetic intermediates to form high-energy activated complexes:
  • $\text{UDP-glucose}$: Activated sugar nucleotide required as the glucosyl donor for the biosynthesis of glycogen.
  • $\text{CDP-choline}$ and $\text{CDP-ethanolamine}$: Activated cytidine intermediates essential for the de novo synthesis of membrane phospholipids.
• Chemical Group Donors:
  • Specialized nucleotide conjugates serve as activated donor molecules:
  • $\text{PAPS}$ provides activated sulfate ($-\text{SO}_3^-$) for sulfolipid synthesis.
  • $\text{SAM}$ provides activated methyl groups ($-\text{CH}_3$) for transmethylation reactions.

Synthetic Nucleotide Analogues and Clinical Pharmacology

• Concept and Chemical Design:
  • Synthetic nucleotide analogues are clinically manufactured molecules produced by modifying either the heterocyclic aromatic base or the pentose sugar moiety.
  • Function as antimetabolites by competing with natural substrates, inhibiting specific biosynthetic enzymes, or incorporating into nascent nucleic acids to induce chain termination or lethal mutations.
• Anti-Gout Therapy — Allopurinol:
  • Mechanism of Action: Structural analogue of hypoxanthine; acts via substrate competition to inhibit the enzyme xanthine oxidase ($\text{XO}$).
  • Normal Pathway: Xanthine oxidase converts hypoxanthine into xanthine, and subsequently converts xanthine into uric acid.
  • Therapeutic Effect: Allopurinol blocks xanthine oxidase, preventing the accumulation and tissue deposition of insoluble uric acid crystals, thereby effectively treating gout and hyperuricemia.

• Antineoplastic Therapy — 5-Fluorouracil ($5\text{-FU}$):
  • Mechanism of Action: Halogenated synthetic analogue of uracil containing a fluorine atom at carbon 5 ($\text{C}5$).
  • Enzymatic Blockade: Inhibits the enzyme thymidylate synthase, preventing the methylation of uracil derivatives into thymine.
  • Cascading Effect: Decreased thymine synthesis ($\downarrow \text{thymine synthesis}$) causes depleted $\text{dTTP}$ pools, which inhibits $\text{DNA}$ replication ($\downarrow \text{DNA synthesis}$), arresting the proliferation and growth of rapidly dividing cancer cells ($\downarrow \text{growth of cancer cells}$).

Diagnostic Exercises and Self-Assessment Review

• Exercise 1: Structural Identification of Nucleosides

• Structural Component Analysis:
  • Purine Ring: Left molecule (adenosine) shows a bicyclic purine (adenine) ring containing fused six- and five-membered rings.
  • Pyrimidine Ring: Right molecule (deoxythymidine) shows a monocyclic six-membered pyrimidine (thymine) ring containing a $5\text{-methyl}$ substitution.
  • $\beta\text{-N-Glycosidic}$ Bond: The covalent bond linking atom $\text{N}9$ of adenine to $\text{C}1'$ of ribose (left), and atom $\text{N}1$ of thymine to $\text{C}1'$ of deoxyribose (right).
  • Pentose Sugar (Ribose): Left molecule possesses hydroxyl groups on both $\text{C}2'$ and $\text{C}3'$, identifying it as ribose.
  • Deoxypentose Sugar ($2'\text{-Deoxyribose}$): Right molecule displays a hydrogen atom without oxygen at $\text{C}2'$ and a hydroxyl group at $\text{C}3'$, identifying it as $2'\text{-deoxyribose}$.
  • Exercise 2: Signal Transduction Pathway Identification

• Product Identification: The molecule designated at ?????? is cyclic adenosine monophosphate ($\text{cAMP}$).
• Chemical Nature: A cyclic ribonucleotide monophosphate characterized by an intramolecular $3',5'\text{-phosphodiester}$ linkage.
• Physiological Function: Functions as an intracellular second messenger that allosterically binds to and activates inactive protein kinase A ($\text{PKA}$), triggering phosphorylation cascades that execute the target cellular response.
  • Exercise 3: Coenzyme Identification in Dehydrogenase Reactions

• Reactant Identification: The oxidized compound entering the forward reaction at the dashed line is $\text{NAD}^+$, and the reduced product released is $\text{NADH} + \text{H}^+$.
• Chemical Nature: A pyridine dinucleotide coenzyme consisting of two mononucleotides (nicotinamide mononucleotide and adenosine monophosphate) linked via a phosphoanhydride bridge.
• Physiological Function: Operates as an electron and hydride ion ($\text{H}^-$) carrier, accepting two electrons and one proton ($2\text{e}^- + 2\text{H}^+$) during the oxidation of lactate to pyruvate, and releasing them during reversible reduction.