Nucleotide Metabolism

Roles of Nucleotides

Precursors for DNA and RNA, energy carriers (ATP, GTP), signal transduction (cAMP, G-proteins), and biosynthesis (UTP for polysaccharides, CTP for phospholipids).

Energy Contribution of Nucleotides

ATP drives cellular reactions; GTP is essential for protein synthesis.

Nucleotides in Signal Transduction

cAMP activates protein kinase A, while G-proteins bind GTP to relay hormone signals.

Main Pathways of Nucleotide Metabolism

Includes purine metabolism, pyrimidine metabolism, and deoxyribonucleotide metabolism.

Types of Nucleotide Synthesis

De novo (from simple molecules) and salvage pathways (from existing nucleotide pieces).

Impact of Pathway Inhibition

Blocking de novo synthesis has little effect if salvage pathways function; blocking salvage can be lethal for organisms like Plasmodium falciparum.

First Purine Nucleotide

Inosine monophosphate (IMP) is the first formed nucleotide.

Energy Requirements for IMP Formation

Requires 5 ATP molecules and occurs in the cytosol.

Regulated Step in Purine Synthesis

Formation of phosphoribosylamine from PRPP by amidotransferase.

Sources of Purine Atoms

Glycine (C4, C5, C7), glutamine (N3, N9), N10-formyl-THF (C2, C8), CO₂ (C6), and aspartate (N1).

PRPP Activation in Purine Synthesis

PRPP synthetase catalyzes ribose-5-phosphate conversion to PRPP.

Conversion of IMP to AMP & GMP

Uses aspartate (AMP pathway) and glutamine (GMP pathway).

Purine Synthesis Regulation

IMP, AMP, and GMP inhibit amidotransferase; PRPP activates it.

Initial Pyrimidine Nucleotide

Orotidine monophosphate (OMP), later converted to UMP.

Formation of Carbamoyl Phosphate

Requires ATP and glutamine in eukaryotes; catalyzed by CPS II.

Pyrimidine Ring Construction

Formed before attaching to ribose-phosphate (contrast to purine synthesis).

OMP Decarboxylation Deficiency

Leads to orotic acidemia; treated with uridine supplementation.

Conversion of UMP to UTP & CTP

Involves phosphorylation and amination using glutamine.

Regulation of Pyrimidine Synthesis

UTP inhibits CPS II, PRPP activates it.

Nucleoside Monophosphate Kinases

Convert NMPs to NDPs using ATP.

Nucleoside Diphosphate Kinases

Convert NDPs to NTPs using ATP.

Role of ATP Synthase

Regenerates ATP from ADP using a proton gradient.

Three Types of Salvage Enzymes

Nucleoside kinases, nucleoside phosphorylases, phosphoribosyl transferases.

Example: Thymidine Kinase

Converts thymidine to TMP using ATP.

Lesch-Nyhan Syndrome

Caused by HGPRT deficiency, leading to excess urate.

Breakdown of Nucleic Acids

DNA/RNA → nucleotides via nucleases, then to nucleosides via phosphodiesterases.

Conversion of Nucleotides to Nucleosides

5’-Nucleotidases remove phosphate groups.

Breakdown of Purines to Urate

Hypoxanthine and guanine are converted to urate via xanthine oxidase.

Gout and Purine Catabolism

High urate levels cause joint pain; treated with allopurinol.

Pyrimidine Catabolism Products

Pyrimidines are broken down into beta-ureidopropionic acid.

Enzyme for dNTP Formation

Ribonucleotide reductase reduces ribonucleotides to deoxyribonucleotides.

Electron Donors in Reduction

Thioredoxin or glutaredoxin.

Synthesis of dTMP from dUMP

Catalyzed by thymidylate synthase, using N5,N10-methylene tetrahydrofolate.

Regeneration of THF

Dihydrofolate must be reduced back to tetrahydrofolate by dihydrofolate reductase.

Thymidylate Synthase Inhibition

Fluorouracil (5-FU) irreversibly inhibits the enzyme.

Two Allosteric Sites in RNR

Activity site (ATP activates, dATP inhibits); specificity site controls substrate preference.

Effect of dTTP on dCTP Synthesis

High dTTP inhibits reduction of CDP, stopping DNA synthesis.

Thioredoxin & Glutaredoxin in RNR Regulation

Maintain the reduced state of ribonucleotide reductase.

Methotrexate Mechanism

Inhibits dihydrofolate reductase, blocking THF regeneration.

Fluorouracil (5-FU) as a Suicide Inhibitor

Converted to FdUMP, which irreversibly inhibits thymidylate synthase.

Trimethopterin and Sulfa Drugs

Block folate synthesis in microbes, used as antibiotics.

Impact of Chemotherapy on Normal Cells

Affects rapidly dividing cells, leading to anemia, hair loss, and nausea.

Differences in Purine vs. Pyrimidine Synthesis

Purines built onto ribose, pyrimidines built separately before attachment.

Xanthine Oxidase and Allopurinol

Allopurinol inhibits xanthine oxidase, reducing urate production.

PRPP as a Central Metabolite

Used in purine and pyrimidine synthesis, NAD biosynthesis.