Synthesis of nucleotides, DNA, RNA; metabolism, urate production
1. Distinguish between nucleotides and deoxynucleotides, purines and pyrimidines;
-Nucleotides (RNA) and deoxynucleotides (DNA) – differ at position #2 : –OH or –H
respectively
-Purines (A/G; 2 cyclic rings)
-Pyrimidines (C/T/U; single cyclic ring)
2. Describe general steps in the overall synthesis of purine and pyrimidine
nucleotides and identify differences in the pathways.
Purine bio synthesis:
Ribose 5 phosphate-> PPRP -> 5- Phosphribosylamine-> IMP
IMP in the branching point to create the purine nucleotides
GMP<-IMP->AMP
Purines are biologically synthesized as nucleotides and in particular as ribotides, i.e.
bases attached to ribose 5-phosphate. A key regulatory step is the production of 5phospho-α-D-ribosyl 1-pyrophosphate (PRPP) by PRPP synthetase, which is
activated by inorganic phosphate and inactivated by purine ribonucleotides. It is not
the committed step to purine synthesis because PRPP is also used in pyrimidine
synthesis and salvage pathways. The first committed step is the reaction of PRPP,
glutamine and water to 5'-phosphoribosylamine, glutamate, and pyrophosphate catalyzed by pyrophosphate amidotransferase, which is activated by PRPP and
inhibited by AMP GMP and IMP.
The salvage pathways converts the adenine and guanine bases back into the
nucleotide form. Used when there is a deficiency in PRPP.
Pyrimidine Biosynthesis
Asparate-> Orotate->UMP
CMP(amine group added)<-UMP->TMP(acetyl group added)
Purine Degradation
3. Outline steps in the formation of deoxynucleotides from nucleotides;
•
Cells usually contain 5-10 times more RNA than DNA
•
Majority of nucleotide biosynthesis is rNTPs
•
But... proliferating cells need dNTPs
•
These are produced by the reducing rNDPs by Ribonucleotide reductase (RR),
followed by phosphorylation of dNTPs by the same nucleotide diphosphate kinases
that phosphorylate rNDPs, using ATP as donor.
4. Describe briefly how nucleotides are metabolised and how urate is formed.
Catabolism of purines-> uric acid
May 2012
Copyright UTS
Uric acid
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5. Describe the effects of methotrexate and mercaptopurine on nucleotide metabolism.
Chemotherapy
• Methotrexate
– A competitive inhibitor of DHFR
–
Also aminopterin and trimethoprim – Resistance due to accumulation of
point mutations
5-fluorouracil and 5-fluorodeoxyuridine
–
A irreversible inhibitor of thymidylate synthase
–
Both are converted to FdUMP which inhibits the enzyme
Folic acid is required in the cell for:
–
biosynthesis of purines
–
conversion of dUMP to dTMP (thymidylate synthase)
–
Regeneration of reduced folic acid depends on dihydrofolate reductase
(DHFR).