7-15 Nucleotide Metabolism Flashcards
What are nucleotides, purines, and pyrimidines?
Purines: Adenine, guanine, hypoxanthine, xanthine
Pyrimidines: Uracil, cytosine, thymine
What is PRPP and why is it important?
PRPP is an intermediate in nucleotide metabolism - activated ribose ready for base attachment.
Formed from Ribose-5 phosphate + ATP
(R5P comes from glucose metabolism, pentose phosphate shunt, nuc degradation)
- PRPP required in: de novo synthesis of purines and pyrimidines, salvage pathway, biosynthesis of NAD and FAD
How are purines put together, and how is their synthesis regulated?
- Purine ring built on molecule of PRPP
- Synthesis of inosine 5’ monophosphate (IMP): 10 steps, uses 6 high energy bonds (first compound in pathway with completely formed purine ring system)
- Synthesis regulated by feedback inhibition (enzymes in IMP synthesis inhibited by IMP, GMP, AMP)
How do sulfinamides and azaserine inhibit nucleotide synthesis?
These drugs inhibit IMP synthesis, which is part of purine synthesis.
- Azaserine blocks amide transfer from glutamine
- Sulfonamides block biosyn of folic acid in bacteria, prevents formation –> blocks nucleotide synthesis
What are salvage pathways? What are the 3 enzymes important in this?
Recover bases and nucleosides that are formed during DNA/RNA degradation (~90% in body).
- HGPRT: forms nucs (IMP and GMP) from hypoxanthine and guanine (defects = gout)
- APRT: form AMP from adenine
- Nucleoside kinase: adenosine + ATP -> AMP + ADP
How are pyrimidines assembled?
- Not built on PRPP like purines
- Pyrimidine ring is formed then reacts with PRPP to form the nucleotide
- Precursors of the ring: carbamoyl phopsphate (syn in cytosol and liver- intermediate in urea syn, reason OTC deficiency leads to high levels of pyrimidines) and aspartate
- UMP (UMP kinase) UDP (nucleoside diphsophate kinase) UTP
- Amino group from glutamine makes UTP–> CTP
When are deoxyribonucleotides made and how?
Only made by cells that are actively dividing in S phase (0.01%)
- Formed by reduction of ribonucleoside diphosphates (only diphopshates, dUTP bad!) by ribonucleotide reductase enzyme
- Products include dADP, dCDP, dGDP, dUDP
What is thymidylate synthase and ribonucleotide reductase, how do these enzymes work and why are inhibitors against them important chemo agents?
- Ribonucleotide reductase is inhibited by hydroxyurea (chemo agent)
- dUMP (Thymidylate synthase) dTMP (uses THF)
What is 5-fluorouracil and methotrexate and how are they related to THF?
- 5-fluorouracil: anticancer agent, traps thymidylate synthase in complex with F-dUMP, inhibiting synthesis of dTMP
- Methotrexate: inhibits dihydrofolate reductase –> regeneration of THF is blocked –> synthesis of dTMP inhibited
How are purines and pyrimidines degraded? What is xanthine oxidase?
- Nucleosidases remove 5’ phosphates
- Nucleoside phosphorylases: nucleosides –> free bases and R1-P/dR1-P (Reverisble in salvage pthwy)
- Xanthine oxidase: xanthine –> uric acid (final product of purine deg)
What is the final end product of purine degradation, and why is it a problem in gout? What is allopurinol?
- Uric acid is final product
- When levels high, causes gout be precipitating into crytstals in joints/kidneys
- Tx of gout: allupurinol - blocks production of uric acid by inhibiting zanthine oxidase
What is Lesch-Nyhan Syndrome?
- Hereditary x-linked recessive
- Loss of HGPRT activity
- Increased synthesis of purines (not salvaged, PRPP levels increase, levels of IMP and GMP decrease)
- Symptoms: hyperuricemia, gout, urinary stones, neuro issues
- Tx: allupurinol to reduce uric acid formation, doesn’t alleviate neuro problems
What is adenosine deaminase deficiency?
- Abundance of deoxyadenosine and adenoside
- Converted to nucletides in WBC
- High levels of dATP inhibit riboneucleotide reductase –> inhibits DNA synthesis –> WBC can’t prolferate
Tx: bone marrow transplant, ERT, GT
Key points of IMP biosynthesis
- First step is rate limiting and regulated step
- 2 steps require folate, blocked by drugs that block folate biosyn in bacteria (sulfa)
- Nuc ring is made from glutamine, glycine, CO2, aspartate, and 2 one-carbon fragments
- 2 steps rquire glutamine amino transfer rxns (inhib by azaserine)
- Expensive process - better to salvage nucleotides (90%)
How is pyrimidine biosynthesis regulated?
- Enzyme: single protein with 3 subunites catalyzes 1st 3 steps, last two by another enzyme
- Pyrimidine nucleotides decrease activities of enzymes
