Study Notes on Nucleotide Biosynthesis

NUCLEOTIDE BIOSYNTHESIS

Introduction to Nucleotide Functions

  • Nucleotides perform a wide variety of functions including:

    • Building blocks for nucleic acids (DNA and RNA).

    • Universal energy carriers (e.g., ATP, GTP).

    • Activators (e.g., UDP-glucose).

    • Components of signal transduction pathways (e.g., cAMP, cGMP).

Composition of Nucleotides

  • Nucleotides contain:

    • Sugar: Ribose or deoxyribose.

    • Phosphate Groups: One to three phosphate groups.

    • Nitrogen Base: Either purine or pyrimidine heterocyclic nitrogen base.

Focus Areas of Study

  • We will focus on:

    • The nucleotide bases.

    • The carbon scaffold is provided by glycine and aspartate.

    • Aspartate and glutamine will provide the nitrogen.

    • Specific areas include:

    • De novo synthesis of pyrimidine bases.

    • De novo synthesis of purines bases.

    • Synthesis of deoxyribonucleotides.

    • Regulation of nucleotide synthesis.

De Novo versus Salvage Pathways

  • Salvage Pathway:

    • Activated ribose (PRPP) + base → Nucleotide.

  • De Novo Pathway:

    • Activated ribose (PRPP) + amino acids + ATP + CO2 + … → Nucleotide.

NUCLEOTIDE STRUCTURE

Basic Structure of Nucleotides

  • Nucleotides are composed of:

    • A nitrogenous base (N).

    • A pentose monosaccharide (carbohydrate).

    • One, two, or three phosphate groups (P).

  • The nitrogen-containing bases belong to two families of compounds: the purines and the pyrimidines.

Purine and Pyrimidine Structures

  • DNA and RNA include:

    • Purines: Adenine (A) and Guanine (G).

    • Pyrimidines: Cytosine (C) is found in both DNA and RNA.

    • DNA features thymine (T): A, G, C, T.

    • RNA features uracil (U): A, G, C, U.

Modified Bases

  • Certain types of DNA and RNA, such as viral DNA and tRNA, may have modified bases.

  • Modifications including methylation and acetylation may help enzymes identify specific sequences or protect them from degradation by nucleases.

Carbon Numbering System

  • The prime (') notation distinguishes the carbon atoms in the sugar from those in the nitrogenous base.

  • Carbon atoms in the pentose sugar are numbered 1' to 5', starting from the carbon attached to the nitrogenous base and moving clockwise.

Nucleosides and Nucleotides

  • The addition of a pentose sugar to a base produces a nucleoside.

    • 2-deoxyribose: This sugar results from the removal of oxygen from carbon 2 of aldopentose ribose.

  • If the sugar is ribose, it generates a ribonucleoside; if it is 2-deoxyribose, it is a deoxyribonucleoside.

Nomenclature

  • (RNA) Nucleosides of A, G, C, and U are named:

    • Adenosine (A), Guanosine (G), Cytidine (C), Uridine (U).

  • (DNA) Deoxyribonucleosides of A, G, C, and T:

    • Prefix “deoxy-” indicates deoxyribonucleosides, e.g., deoxyadenosine.

Conversion of Nucleosides to Nucleotides

  • A nucleoside becomes a nucleotide when its C5' is bonded to one or more phosphate groups.

  • The first phosphate group is attached by an ester linkage to the 5'-OH of the pentose, forming a nucleoside 5'-phosphate or a 5'-nucleotide.

High-Energy Bonds

  • The second and third phosphate groups are each connected to the nucleotide by a “high-energy” bond, commonly seen in adenine monophosphate (AMP) and adenosine diphosphate (ADP) or triphosphate (ATP).

SYNTHESIS OF PURINE NUCLEOTIDES

Step 1: PRPP Synthesis

  • Activator: PRPP (5-phosphoribosyl 1-pyrophosphate).

  • Catalyzed by: PRPP synthetase, requires ATP and Mg2+.

  • Inhibitors: Purine ribonucleotides.

Step 2: Synthesis of 5'-phosphoribosylamine

  • The amide group from glutamine replaces the pyrophosphate group on carbon 1 of PRPP.

Step 3: Inosine Monophosphate Synthesis

  • Inosine monophosphate (IMP) is created from the aforementioned compounds.

    • Requires ATP and involves two steps using N10-formyl-tetrahydrofolate as a coenzyme.

Step 4: Chemical Inhibition of Purine Synthesis

  • Sulfa Drugs: Structural analogs of para-aminobenzoic acid that inhibit bacterial purine synthesis by targeting tetrahydrofolate availability.

  • Methotrexate: Folic acid analog that inhibits dihydrofolate reductase, crucial in the conversion of dihydrofolate to tetrahydrofolate. This leads to reduced availability for purine synthesis, slowing down DNA replication, especially in cancer cells.

Step 5: Conversion of IMP to AMP and GMP

  • This conversion is a two-step energy-requiring pathway.

    • AMP Requires: GTP as an energy source.

    • GMP Requires: ATP as an energy source.

    • Both pathways are inhibited by their respective end products.

Step 6: Formation of Diphosphates and Triphosphates

  • Nucleoside diphosphates are synthesized from the corresponding nucleoside monophosphates via base-specific nucleoside monophosphate kinases.

  • Adenylate kinase functions in liver and muscle cells to maintain balance among AMP, ADP, and ATP.

  • Interconversion occurs for nucleoside diphosphates and triphosphates via nucleoside diphosphate kinase.

Step 7: Salvage Pathway for Purines

  • Purines from cellular turnover or dietary sources can be salvaged into nucleoside triphosphates through specific kinases.

  • Pyrimidines from dietary or endogenous sources are effectively salvaged and are converted to nucleotides.

CLINICAL SIGNIFICANCE

Lesch-Nyhan Syndrome

  • Inheritance: X-linked recessive disorder.

  • Cause: Hypoxanthine-guanine phosphoribosyltransferase deficiency.

  • Symptoms: High PRPP, low IMP/GMP levels, leading to excessive uric acid production; results in neurological features like self-mutilation and involuntary movements.

Orotic Aciduria

  • Characteristics: Elevated levels of orotic acid in urine signal a rare genetic disorder due to enzyme deficiency affecting the pyrimidine synthesis pathway.

  • Symptoms: Megaloblastic anemia, potential developmental delays due to inability to convert orotic acid to UMP.

Treatment of Orotic Aciduria

  • Administration of uridine monophosphate (UMP) or uridine triacetate provides a source of pyrimidines, bypassing the enzymatic deficiency.

  • Importantly, thymidine cannot convert to other pyrimidines, and purine bases alone do not substitute for pyrimidine deficiencies.

Case Study: Orotic Aciduria Treatment

  • A 1-year-old female patient presents with lethargy, weakness, and elevated urine orotic acid levels. The most effective treatment is administration of UMP or uridine triacetate.

SYNTHESIS OF DEOXYRIBONUCLEOTIDES

Requirement for DNA Synthesis

  • DNA synthesis necessitates 2'-deoxyribonucleotides, derived from ribonucleoside diphosphates via ribonucleotide reductase during the S-phase.

Regulation of Deoxyribonucleotide Synthesis

  • Ribonucleotide reductase maintains an appropriate balance of deoxyribonucleotides through complex regulation involving:

    • A catalytic site and multiple allosteric sites that modulate enzyme activity.

  • Consists of two non-identical subunits, R1 and R2, that convert nucleoside diphosphates (ADP, GDP, CDP, UDP) into their deoxy forms (dADP, dGDP, dCDP, dUDP).

Hydroxyurea as an Inhibitor

  • Mechanism: Hydroxyurea inhibits DNA synthesis by disrupting the necessary free radical for ribonucleotide reductase activity.

  • Uses: Commonly used to treat cancers such as chronic myelogenous leukemia and sickle cell disease, aiding in fetal hemoglobin production, though its effects are not directly due to ribonucleotide reductase inhibition.