Metabolism 11 Flashcards
What are the sources of the atoms in the purine ring structure?
The sources of the atoms in the purine ring structure are three amino acids, CO2,
and a tetrahydrofolate (FH4) derivative.
What is the precursor for purine biosynthesis? How is this precursor produced?
The precursor for purine biosynthesis is ribose-5-phosphate, produced by the
pentose-phosphate pathway.
How is the purine built?
What is the first formal step?
Second step? Enzyme?
When is the nucleotide IMP formed? What is its significance?
When is the committed and major regulation step?
The purine ring is built up atom by atom on top of the ribose.
The first formal step (step 1 below) is the activation of ribose-5-phosphate to give
PRPP, with the pyrophosphate coming from ATP. (PRPP: 5-phospho-alpha-Dribose-
1-pyrophosphoric acid)
The second step is the formation of 5-phosphoribosylamine, with the amino group
coming from the side-chain of glutamine. This is the committed step and the
major regulated step. Enzyme: glutamine PRPP amidotransferase.
The nucleotide IMP is formed (step 11), and becomes the precursor for both AMP
and GMP.
Describe purine biosynthetic pathway.
p 5/6
Draw a summary and regulation of purine biosynthesis.
p 7
What happens to AMP and GMP that are generated in purine biosynthesis ?
How does the concentration of its products compare to concentrations of monophosphates, nucleosides or free bases?
AMP and GMP are phosphorylated to give nucleoside 5-diphosphates (ADP, GDP) and nucleoside 5-triphosphates (ATP, GTP), which are present in higher concentrations than monophosphates, nucleosides, or free bases.
How might an antitumor agent work? Why?
Tumors are very dependent on the purine biosynthetic pathway to produce purine nucleotides for DNA and RNA synthesis. In contrast, most normal tissues are less dependent on de novo synthesis and instead recycle existing purine bases (see salvage pathway below). A drug that inhibits purine biosynthesis would be more toxic to tumor cells than to most normal cells.
Tumors are very dependent on the purine biosynthetic pathway to produce purine nucleotides for DNA and RNA synthesis. In contrast, most normal tissues are less dependent on de novo synthesis and instead recycle existing purine bases (see salvage pathway below). A drug that inhibits purine biosynthesis would be more toxic to tumor cells than to most normal cells.
Describe the mechanism of antitumor agent, 6-mercaptopurine.
6-Mercaptopurine is converted to a nucleotide (by the salvage pathway). The nucleotide (monophosphate) then inhibits the enzymes in the purine biosynthetic pathway that catalyze steps 2, 12a, 12b, and 13a. It is used to treat acute lymphocytic leukemia (ALL) and other cancers.
p 7
What are the sources of the atoms in the pyrimidine ring structure?
The sources of the atoms in the pyrimidine ring structure are glutamine, aspartate,
and CO2:
Describe pyrimidine nucleotide biosynthesis.
What is the regulated step? What catalyzes it?
What happens first?
What is the role of UMP?
The regulated step (1) is catalyzed by carbamoyl phosphate synthetase II.
The pyrimidine ring structure is formed first and then attached to a ribose-5-
phosphate. PRPP is utilized as the activated form of ribose-5-phosphate.
UMP is the precursor for synthesis of the other pyrimidine nucleotides.
What is Leflunomide? What condition does it treat? How?
Leflunomide, an immunosuppressive drug used to treat rheumatoid arthritis,
blocks pyrimidine biosynthesis by inhibiting dihydroorotate dehydrogenase.
Describe the synthesis of deoxythymidylate (dTMP) and mechanism of action of the antitumor drugs 5-fluorouracil and methotrexate:
From what is dTMP synthesized?
What happens to 5-fluorouracil?
Describe the dihydrofolate produced in this reaction. What happens to it?
dTMP is synthesized from dUMP and N5,N10-methylene tetrahydrofolate by thymidylate synthase.
5-Fluorouracil is converted to f-dUMP which then acts as a pseudosubstrate and covalently binds to, and inhibits, thymidylate synthase.
The dihydrofolate produced in this reaction is useless and actually toxic and must be converted back to tetrahydrofolate by dihydrofolate reductase in order to be used for purine nucleotide and deoxythymidylate synthesis.
What does methotrexate do?
Methotrexate is a potent competitive inhibitor of dihydrofolate reductase and increases dihydrofolate and causes an intracellular tetrahydrofolate deficiency.
Describe the formation of deoxyribose.
What enzyme is used as a cofactor?
- Ribonucleotide reductase acts on ribonucleoside 5-diphosphates (ADP, GDP,
CDP, or UDP) to form the corresponding deoxyribonucleoside 5-diphosphates
(dADP, dGDP, dCDP, or dUDP). - In this reaction, the 2-hydroxyl group of ribose is replaced with a hydrogen atom.
The enzyme uses thioredoxin as a cofactor.
p 10
Describe the cell cycle and nucleotide biosynthesis:
At what phase does DNA synthesis occur?
When in the cycle do the concentrations of deoxyribonucleotides increase?
When in the cycle do the enzymes involved in purine and pyrimidine biosynthesis increase?
- DNA synthesis occurs during the S phase of the cell cycle.
- The concentrations of the deoxyribonucleotides required for DNA synthesis are low during most of the cell cycle but increase greatly
during S phase. - Several of the enzymes involved in purine and pyrimidine biosynthesis
increase during late G1/early S phase. Among these are ribonucleotide reductase and thymidylate synthase. The amounts of these two enzymes correlate with tumor growth rate.
