Nucleotide Synthesis: De Novo and Salvage Pathways

Nucleotide Synthesis Overview

• Nucleotides are synthesized as ribonucleoside monophosphates (NMPs)
• Starting substrate for purine nucleotide synthesis is 5-phosphoribosyl-1-pyrophosphate (PRPP)
• PRPP is synthesized from ribose-5 phosphate and requires ATP, Mg2+, and is activated by free Pi
• End-product inhibition is a recurring theme in nucleotide synthesis

De Novo Pathway: Purine Synthesis

• Uses 5-phosphoribosyl 1-pyrophosphate (PRPP) as the starting "scaffold"
• The purine ring is built by additions of carbons and nitrogens from various sources including aspartate, glycine, glutamine, CO2, and N10-formyltetrahydrofolate
• The end product is inosine monophosphate (IMP)

Clinical Correlates

• Antibiotics that target folic acid metabolism:
  • Sulfonamides/sulfa drugs competitively inhibit bacterial synthesis of folic acid and slow down purine synthesis
  • Trimethoprim inhibits the reduction of dihydrofolate to tetrahydrofolate, which slows down purine synthesis
• Folic acid analogs:
  • Methotrexate is an analog of dihydrofolate

Nucleotide Functions

• Nucleotides have three components: nitrogen-base derivatives, pentose, and phosphates
• Functions:
  • Monomeric units of DNA and RNA
  • Provide free energy for chemical reactions (ATP and GTP)
  • Components of coenzymes (Flavin adenine dinucleotide, Nicotinamide adenine dinucleotide, coenzyme A)

De Novo Pyrimidine Synthesis

• Purpose: Synthesis of a pyrimidine ring from scratch through the additions of carbons and nitrogens from aspartate, glutamine, and CO2
• Then it is added to ribose-5-phosphate (from PRPP)
• The end product is UMP, an important intermediate is orotic acid
• UMP is modified to generate UTP and CTP for RNA synthesis and dCTP and dTTP for DNA synthesis
• Key steps:
  • The rate-limiting step is the formation of carbamoyl phosphate by carbamoyl phosphate synthetase II (CPSII)
  • Another important step is the generation of (d)TMP from dUMP by thymidylate synthase, which is the rate-limiting step for DNA synthesis and requires tetrahydrofolate
• Hormonal control: N/A
• Allosteric/local control: CPSII is activated by PRPP and inhibited by UTP
• Deficiencies/diseases: Orotic aciduria with high orotic acid in urine, failure to thrive, and anemia

Test your knowledge on the biosynthesis of nucleotides, including the de novo and salvage pathways, and the role of the liver in this process.