L27: Nucleotide Metabolism

Question 1

LO1: Name the digestion products of polynucleotides

Answer 1

• pyrimidines
• purines
• ribose
• phosphate

Question 2

LO2: Compare and contrast the metabolic pathways leading to the de novo synthesis of purines vs. pyrimidines

Answer 2

PURINE SYNTHESIS

1. ribose-5-phosphate activated to PRPP by PRPP synthase (regulated step, + by Pi and - by purine ribonucleotides, aka negative feedback inhibition)
2. IMP synthesized through a series of 10 enzymes, including catalysis by PPAT (committed step, + by PRPP and - by purine ribonucleotides, also negative feedback inhibition)
3. IMP converted to adenosine monophosphate and guanosine monophosphate

PYRIMIDINE SYNTHESIS

1. Orotate is synthesized; carbamoyl phosphate formed (catalyzed by cytosolic CPSII), then combined with aspartate to form carbamoyl aspartate which is dehydrated and oxidized to form orotate
2. Orotate then joined to PRPP to form OMP which is decarboxylated to generate UMP
3. UMP is phosphorylated to UTP, which can be converted to CTP by CTP synthase (glutamine=N source)

Question 3

LO3: What metabolites contribute carbon and nitrogen atoms to the purine and pyrimidine rings?

Answer 3

contribution to purines: N-formyl-tetrahydrofolate (unique), CO2, glutamine, glycine (unique), aspartate

contribution to pyrimidines: CO2, glutamine, aspartate

common to both: CO2, glutamine, aspartate

Question 4

LO4: Describe the primary regulatory mechanism controlling the rate of nucleotide metabolism

Answer 4

RIBONUCLEOTIDE REDUCTASE
-uses thioredoxin as a conenzyme; synthesizes deoxyribonucleotides from ribonucleoside diphosphates
+ by substrates (ATP), - by products (dATP)

Question 5

LO5: Name the co-factors needed for conversion of ribonucleotides to deoxyribonucleotides

Answer 5

thioredoxin: regenerated by thioredoxin reductase and NADPH, needed by ribonucleotide reductase

Question 6

LO6: What property of cells sensitizes them to inhibitors of nucleotide metabolism?

Answer 6

-they’re rapidly dividing (nucleotide metabolism inhibitors work where nucletotides synthesis is high)

Question 7

LO7: Name three enzymes in nucleotide metabolism that are targeted by therapeutics

Answer 7

5-FU works on thymidylate synthase, which converts dUMP to dTMP

Mycophenolic acid inhibits inosine monophosphate DH, which synthesizes GMP

Sulfa drugs inhibit dihydropteroate synthetase, which is needed for synthesis of THF in bacteria

Methotrexate inhibits dihydrofolate reductase, which is needed to regenerate THF (depletion inhibites purine and DNA synthesis)