1. Chapter 8. Nucleotide Metabolism
Functions of nucleotites:
They are precursors of DNA and RNA
ATP is a universal currency of energy
Physiological mediators (cAMP, cGMP)
They are activated intermediates in many biosynthesis (UDP-glucose, CDP-choline)
Adenine nucleotides are components of some coenzymes (NAD+, NADP+ and FAD )

2. 1. Biosynthesis of purine nucleotites:
1) The origins of the atoms in the purine ring:
One C unit
One C unit

3. 2) Formation of phosphoribosylamine
Ribose 5-phosphate PRPP
ATP
ADP
PRPP synthase
PRPP: 5-Phosphoribose-1-pyrophosphate

4. Glutamine glutamate + PPi
PRPP phosphoribosyl-1-amine
Glutamine glutamate + PPi
Gln PRPP
Amidotransferase

5. 3) Formation of inosinate (IMP)

6. 4) Formation of AMP and GMP

7. 5) Formation of ATP and GTP
AMP ADP ATP
GMP GDP GTP
Kinase
Phosphorylation Pi
ATP ADP
Kinase
Kinase
ATP ADP
ATP ADP

8. 6) Regulation of purine nucleotite synthesis
Feedback inhibitors: AMP, ADP, GMP, GDP, IMP
AMP ADP ATP
R-5-P PRPP PRA IMP
GMP GDP GTP
PRA: 5-phosphoribosyl-1-amine activation inhibition

9. 7) Salvage pathways for purine nucleotite synthesis
PRPP Purine ribonucleotide
Adenine + PRPP AMP + PPi
Hypoxanthine + PRPP IMP + PPi
Guanine + PRPP GMP + PPi
Adenosine + ATP AMP +ADP

10. 8) Biosynthesis of deoxyribonucleotides (At the NDP level)
Ribonucleoside deoxyribonucleoside
diphosphate diphosphate
Base=purine or pyrimidine

11. The mechanism for ribonucleotide reduction:
NDP
dNDP
Ribonucleotide SH
reductase SH
Ribonucleotide S
reductase S
Thioredoxin S S
Thioredoxin SH SH
Thioredoxin
reductase
FADH2
FAD
NADPH + H+
NADP+

12. 8) Inhibition of purine nucleotide biosynthesis by some anticancer drugs
Reaction Inhibitor
PRPP PRA mercaptopurine(6MP)
Glycinamide ribonucleotide Methotrexate(MTX)
Formylglycinamide ribonucleotide
Formylglycinamide ribonucleotide Azaserine
Formylglycinamidine ribonucleotide
IMP AMP MP
IMP GMP MP, Azaserine
Adenine AMP MP
Guanine GMP MP

14. 2. Degradation of purine nucleotides:
AMP IMP Hypoxanthine Xanthine Uric acid
GMP Guanine Xanthine Uric acid

16. Gout is induced by high serum levels of urate, a disease that affects the joints and kidneys.
Allopurinol is an analog of hypoxanthine. It is extensively used to treat gout.
In the body, allopurinol is converted into alloxanthine, which then remains tightly bound to the active site of xanthine oxidase and thus inhibits the production of urate.

17. 3. Synthesis of pyrimidine nucleotides:
1) Origins of the atoms in the pyrimidine ring C
N C
C C N
Carbamoyl
phosphate
Aspartate

18. 2) Pathway of pyrimidine nucleotide synthesis
Formation of carbamoyl phosphate
2ATP + HCO ADP+ Pi
Gln Glu

19. Differences between carbamoyl phosphate biosynthesis in the pyrimidine pathway and that in the urea cycle
In pyrimidine pathway In urea cycle
Location Cytosol Mitochondria
-NH2 from: Gln NH4+
Enzyme Carbamoyl phosphate Carbamoyl phosphate
synthase-II synthase-I
N-acetyl-Glu No effect Activator

20. B) Formation of orotate

21. C) Formation of UMP

22. C) Formation of other pyrimidine nucleotides
UMP UDP UTP
ATP ADP
ATP ADP
Kinase
Kinase

23. UDP dUDP dUTP dUTP dUMP NADPH NADP+, H2O ATP ADP Ribonucleotide
reductase
Kinase
H2O PPi
dUTPase

24. - - D) Regulation of pyrimidine nucleotide synthesis CO2 + Gln + ATP
Carbamoyl phosphate
N-Carbamoylaspartate
UMP -
Inhibited by UMP -
Inhibited by CTP

25. 3) Salvage pathway for pyrimidine nucleotide biosynthesis
Pyrimidine Pyrimidine ribonucleoside
monophosphate
Orotate Orotidylate UMP
PRPP PPi
Pyrimidine
Phosphoribosyl
transferase
PRPP PPi
H CO2
Orotate
Phosphoribosyl
transferase
Orotidylate
decarboxylase

26. 4) Anticancer drugs that block the pyrimidine nucleotide biosynthesis
A) 5-Fluorouracil (5-FU): is converted in vivo into fluorodeoxyuridylate (F-dUMP), which is an analog of dUMP and irreversibly inhibits thymidylate synthase.

27. B) Aminopterin and methotrexate (MTX): both are analogs of dihydrofolate. They are potent competitive inhibitors of dihydrofolate reductase and thus block the reactions using one-carbon units.

28. C) Cytosine arabinoside and azaserine: Cytosine arabinoside is an analog of cytosine riboside and thus inhibits the reduction of CDP to dCDP, while azaserine inhibits the conversion of UTP to CTP.

29. - - - - Azaserine UMP UTP CTP CDP dCDP UDP dUDP dUMP dTMP 5-FU
Cytosine
arabinoside
Azaserine - -
5-FU - -
Aminopterin

30. 4. Degradation of pyrimidine nucleotide
The pyrimidine ring can be completely degraded in humans. The products include: NH3, CO2, b-alanine, and b-aminoisobutyrate. Both b-alanine, and b-aminoisobutyrate can be further converted into acetyl-CoA and succinyl-CoA, respectively, or are excreted in the urine.

33. 5. Dysmetabolism of nucleotides
Disease Deficiency Symptoms
Gout PRPP synthetase hyper uricemia
Lesch-Nyhan HGPRT purine and uric
syndrom acid, cerebral
paralysis
Immunodeficiency adenosine deaminase deoxyadenosine
Diseases uria, dysostosis
Kidney stone APRT kidney stone,
urodynia, hematuria
Xanthinuria xanthine oxidase kidney stone,
no symptoms