Lecture 11 - Purine/Pyrimidine Metabolism Flashcards
What is the active, reduced form of vitamin B9 - FOLIC ACID?
Tetrahydrofolate
-THF is involved in the transfer of one carbon units in various metabolic pathways
What clinical problem does a folate deficiency result in?
Megaloblastic Anemia
What derivatives of THF are involved in purine biosynthesis? Pyrimidine (Thimidylate)
Purine:
N10 - Formyl - THF
Pyrimidine:
N5,N10 - methylene - THF
What does a dietary deficiency in folate lead too?
Reduced during an pyrimidine biosynthesis
- decreased rate of DNA synthesis
- most affected are bone marrow cells that normally divide and give rise to erythroblasts
- erythrocyte precursor cells grow large but do not divide
= MEGALOBLASTIC ANEMIA
In what form does THF accumulate in the body?
What enzyme converts it to the active THF form?
Inactive form –> N5 - methyl - THF
- converted back to usable form, THF, by VITAMIN B12 enzyme (methylcobalamin)
A deficiency of vitamin B12 causes a ________ deficiency.
FOLATE deficiency
What are the parent compounds?
Major Bases?
Nucleosides?
Nucleotides?
- Purine (2 fused rings) and Pyrimidine
- Adenine, Guanine, Cytosine, Uracil, THymine
- Adenosine, Guanosine, Cytidine, Uridine, Deoxythymidine (BASE + Sugar)
- Base + Sugar + Phosphate Group
AMP and dTMP
What are the sources of atoms in purine ring structure?
What is the precursor for purine biosynthesis?
What is the first formal step?
- 3 amino acids - Glysine, Glutamine, Aspartate
- CO2
- Tetrohydrofolate (FH4) derivative
Ribose - 5 phosphate from the pentose-phosphate pathway
- First formal step = activation of ribose - 5 phosphate to give PRPP (pyrophosphate from ATP)
What is the major regulatory step in purine biosynthesis?
What enzyme is responsible?
Where does the amino group come from?
What is formed?
formation of 5 -phosphoribosylamine
- amino group from GLUTAMINE
Enzyme: glutamine PRPP amidotransferase
- form nucleotide IMP –> precursor for AMP and GMP
How is purine biosynthesis regulated?
Via negative feedback of the products that are formed
- AMP and GMP as well as IMP!
Describe the steps to form AMP and GMP.
Which step is the only one that provides positive feedback?
- Ribose - 5- phosphate
- converted to PRPP w/ ATP
- converted to 5-phosphoribosylamine with glutamine PRPP amidotransferase
- form IMP
- Form Adenylsuccinate –> AMP
- Form XMP –> GMP
Positive feedback from PRPP to glutamine PRPP amidotransferase
How is purine biosynthesis inhibited by an antitumor agent?
How is this possible without disrupting normal purine biosynthesis?
Inhibits purine biosynthesis, and thus is more toxic to tumor cells than to most normal cells since purine bases are mostly RECYCLED
- normal tissues rely LESS on de novo synthesis and MORE ON RECYCLING
What is the function of 6-Mercaptopurine?
What is it used to treat?
6-Mercaptopurine is converted to a nucleotide
which inhibits the purine biosynthetic pathway that catalyze steps 2, 12a, 12b, and 13a
- has a SH group instead of NH2 (like normal adenine) –> replaces adenine
USED TO TREAT ACUTE LYMPHOCYTIC LEUKEMIA (ALL)
What are the sources of atoms in the pyrimadine ring structure?
- Aspartate
- Glutamine
- CO2
How is pyramiding synthesis different from purine synthesis?
Does not start with ribose - 5- phosphate
- make a pyrimidine base and then transfer to ribose - 5- phosphate
What is the regulated step catalyzed by in pyrimidine biosynthesis? (enzyme)
Carbamoyl Phosphate Synthetase II
What is Leflunomide used for?
How does it work? (what does it inhibit specifically)
Treatment of rheumatoid arthritis
- blocks pyrimidine biosynthesis by inhibiting dihydroorotate dehydrogenase
What is UMP?
Used for synthesis of the pyrimidine nucleotides
How do the anti-tumor drug 5-flurouracil work?
What specifically does it inhibit?
- 5-fluoro-uracil
dTMP is synthesized from dUMP and N5, N10 methylene tetrahydrofolate by THYMIDYLATE SYNTHASE - 5-fluorouracil is converted to f-dUMP which acts as a pseudo substrate
- inhibits THIDYLATE SYNTHASE
How do the anti-tumor drug methotrexate work?
What specifically does it inhibit?
- Dihydrofolate produced is USELESS and TOXIC and must be converted back to THF by dyhydrofolate reductase in order t be used for purine nucleotide and deoxythymidilate synthesis
- Methotrexate is a competitive inhibitor of dihydrofolate REDUCTASE and increases dihydrofolate and causes an intracellular THF deficiency
What is the purpose of dihydrofolate produced from the action of converting dUMP to dTMP with Thymidylate Synthase?
NO PURPOSE
- toxic and must be converted back to THF with Dihydrofolate Reductase
METHOTREXATE BLOCKS THIS
What enzyme works to covert
Ribonucleoside 5-diphosphate (ADP/GDP/CDP/UDP) to the corresponding deoxyribonucleoside 5-diphosphates (dADP, dGDP, dCDP, or dUDP)?
What is replaced with a hydrogen atom?
Ribonucleotide REDUCTASE
- 2’-Hydroxyl Group of Ribose is replaced with a hydrogen atom
When does DNA synthesis occur?
What increases during this phase?
what 2 enzymes specifically?
- S phase of the cell cycle
- Concentration of deoxyribonucleotides required for DNA synthesis
enzymes increase during late G1/S phase:
3.
RIBONUCLEOTIDE REDUCTASE &
Thymidylate Synthase
What degrades the DNA and RNA in the diet?
What is the end result?
Nucleases, nucleotidases, and nucleoside phosphorylases
- result is free pyrimidine bases (uracil and thymine) and purine bases (guanine & hypoxanthine)
What happens to most of the free purine bases produced in cells from degradation of mDNA?
They are REUTILIZED
- not degraded
What is the salvage pathway?
Catalyzed by enzyme: Hypoxanthine-guanine phsphoribosyltransferase (HGPRTase)
- hypoxanthine + PRPP –> IMP + PPi
- Guanin + PRPP –> GMP + PPi
A genetic deficiency in what enzyme results in Lesch-Nyhan syndrome? What problems are related to this defect?
- HGPRTase
2. Neurological problems: spasticity mental retardation compulsive self-mutilation ***affecting only males
ALSO HAVE GOUT!!!
How much of the purine bases that are produced by catabolism of mRNA are metabolized to uric acid?
What is uric acid the final product of?
10%
- Final product of PURINE DEGRADATION and is excreted
What catalyzes the conversion of hypoxanthine to xanthine and xanthine to uric acid?
XANTHINE OXIDASE (double duty)
Uric acid exists in two forms: keto and enol form
Which one is deprotonated to form urate?
Which is the predominate form at the pH of blood?
At the pH of urine?
ENOL form
- URATE
- Uric Acid (less soluble than urate)
What can excessive under excretion of uric acid lead to?
Describe what the effects of hyperuricemia are. What is elevated, urate or uric acid? WHERE
What does hyperuricosuria lead to? What is elevated: urate or uric acid? WHERE
GOUT
- elevated URATE in the BLOOD –> can lead to deposition of sodium rate crystals in joints
- the resulting TOPHI can elicit painful acute inflammatory responses - elevated URIC ACID in URINE can lead to deposition of uric acid stones in the kidney
What important drug is used for the treatment of GOUT?
What enzyme does it inhibit?
What does it inhibit from forming?
Allopurinol
- inhibitor of XANTHINE OXIDASE
- block the formation of XANTHINE from hypoxanthine and uric acid from xanthine
- this reduced rate and uric acid in blood and urine respectively
- shrinks TOPHI and renal uric acid stones
