Pyrimidine Nucleotide Synthesis Flashcards
What is the precursor for denovo synthesis of pyrimidines?
PRPP
How does pyrimidine synthesis differ from purine synthesis?
Pyrimidine base is synthesized prior to attachment to ribose sugar
Where do the nitrogens in pyrimidine synthesis come from?
Glutamine and aspartate
Into what molecule is the amine from glutamine incorporated?
Carbamoyl phosphate
How is orotate formed?
Carbamoyl phosphate is covalently linked to aspartate, then the molecule cyclizes
What is produced when orotate reacts with PRPP?
OMP (pyrimidine nucleotide)
What enzyme generates carbamoyl phosphate? What are the reactants?
ATP, HCO3-, and glutamine are reacted by carbamoyl phosphate synthetase II to form carbamoyl phosphate
What enzyme links carbamoyl phosphate to aspartate?
Aspartate transcarbamoylase
What enzyme catalyzes pyrimidine ring closure?
Dihydroorotase
What do carbamoyl phosphate synthetase II, aspartate transcarbamoylase, and dihydroorotase have in common?
All encoded by the same polypeptide
How is OMP converted to UMP?
Decarboxylation
How is CTP synthesized?
UMP + ATP -> UDP + ADP
UDP + ATP -> UTP + ADP
CTP is generated from UTP by accepting amide nitrogen from glutamine
How does CTP regulate pyrimidine synthesis?
Negative feedback
CTP allosterically inhibits aspartate transcarbamoylase
Remember…what does this enzyme do?
How does the pyrimidine salvage pathway differ from de novo?
No common precursor with denovo
Use different sugars based on what you want to synthesize
ribose-1-phosphate (for C or U)
deoxyribose-1-phosphate (for T)
pyrimidine base reacts to form nucleoside
kinase phosphorylates nucleoside to make nucleotide
How are deoxyribonucleotides synthesized?
Reduce ribonucleotides to deoxyribonucleotides using ribonucleotide reductase
What are the substrates and products for ribonucleotide reductase?
NDPs (e.g. CDP) and products are dNDPs (e.g. dCDP)
Where do the electrons used to reduce NDPs come from?
NADPH
How is deoxyribonucleotide production regulated?
dATP binds ribonucleotide reducatse to inhibit
other dNTPs bind to ribonucleotide reductase to alter substrate specificity–mechanism to balance nucleotide production
What is oxidized when ribonucleotides are reduced?
Thioredoxin
How is thioredoxin reduced? What is oxidized during this process?
Thioredoxin reductase
NADPH is oxidized
How are dNTPs produced?
Phosphorylation of dNDPs
How is uracil prevented from being incorporated into DNA?
dUDP is dephosphorylated to form dUMP
dUMP is methylated to form dTMP, which is phosphorylated to form dTDP and dTTP
What enzyme generates dTTP from dUMP? Where does the carbon that is added come from?
Thymidylate synthase
THF
What is 5-fluorouracil? How does it work?
Structure similar to uracil–can be converted into a nucleotide
Inhibits thymidylate synthase (what does this do?)
Anti-cancer drug–prevents DNA synthesis
What is methotrexate? How does it work?
Another anti-cancer drug
Inhibits DHFR–this prevents formation of folate–which prevents DNA synthesis
What is gemcitabine? How does it work?
Pyrimidine ring analogue–can be converted into nucleotide diphosphate mimic
Suicide inhibitor of ribonucleotide reductase–makes enzyme nonfunctional
Another anti-cancer drug–prevents DNA synthesis
Methotrexate reduces the production of which nucleotides? Why?
AMP, GMP, and dTMP
Inhibits DHFR, which prevents 1-carbon transfers
