molecular biology

1. NUCLEOTIDE
METABOLISM(2)
Dr.Samah Badr
Msc.MBBS

2. OBJECTIVES:
• Introduction
• Synthesis of pyrimidine nucleotides.
• Degradation of pyrimidine nucleotides
• Regulation of pyrimidine nucleotides metabolism
• Disorders of pyrimidine metabolism.
DR.SAMAH BADR

3. INTRODUCTION:
• An aromatic nitrogen bases -containing,
a single six membered ring.
• 3 bases: cytosine(C),uracil(U) and thymine (T)
In DNA (C) & (T).
In RNA (C) & (U).
DR.SAMAH BADR

4. CONT…
DR.SAMAH BADR

5. SYNTHESIS OF PYRIMDINE
NUCLEOTIDES
DR.SAMAH BADR

6. SITE OF SYNTHESIS:
✓ Much simpler pathway compared to purine metabolism.
✓ Tissues:
• The major site is : liver.
✓ Subcellular site:
• Cytosol and Mitochondria.
• All reactions occur in the cytosol.
• The formation of orotate from dihydroorotate occurs in mitochondriand all other
reactions occur in the cytosol
✓ There are two pathways for the synthesis of pyrimidines nucleotides:
DR.SAMAH BADR

7. DE NOVO
SYNTHESIS:
Formation of pyrimidine
nucleotides from pyrimidine
bases)
Salvage
pathway
The formation of pyrimidine ring
structure followed by the addition of
ribose phosphate.
DR.SAMAH BADR

8. SOURCE OF PYRIMIDINE ATOMS:

9. DR.SAMAH BADR

10. CPS I AND CPS II ENZYMES:
CPS I CPS II
•SITE Mitochondria Cytoplasm
•Pathway Urea Pyrimidine
•Source for (N) Ammonia Glutamine
•Inhibitor -------- CTP
DR.SAMAH BADR

11. PHOSPHORYLATION:
(UMP UTP & CTP)
• Nucleoside monophosphate kinase catalyses' transfer of Pi to UMP
to form UDP
.
• Nucleoside diphosphate kinase catalyses' transfer of Pi from ATP to
UDP to form UTP
.
• CTP formed from UTP via CTP Synthetase driven by ATP
hydrolysis.
• Glutamine provides amide nitrogen for C4 in animals.
DR.SAMAH BADR

12. DR.SAMAH BADR

13. DEOXY PYRIMIDINE NUCLEOTIDES:
• Ribonucleotide reductase
converts UDP to dUDP
.
• De-oxyuridine diphosphate
(dUDP) is dephosphorylated to
dUMP which acts as a
substrate for thymidine
monophosphate (TMP).
DR.SAMAH BADR

14. METHYLATION OF DEOXY-UMP:
DR.SAMAH BADR

15. Salvage pathway
SALVAGE PATHWAY:
✓Refers to the formation of pyrimidine nucleotides from pyrimidine
bases.
✓ Significance:
provide a pathway for the utilization of pyrimidine bases derived
from diet(exogenous) and normal turnover of nucleic acids.
DR.SAMAH BADR

16. CONT…
✓ Two enzymes that catalyze
the reactions of salvage
pathway. They are:
• Uracil phosphoribosyl
transferase (UPRT).
• Thymidine kinase(T.kinase)
✓ PRPP is the source of ribose
5-phosphate.
Uracil + PRPP UPRT UMP + PPi
Thymidine + ATP T.Kinase TMP+ADP
DR.SAMAH BADR

17. REGULATION OF PYRIMIDINE SYNTHESIS:
• The first two enzymes carbamoyl
phosphate synthase-II and aspartate
transcarbamoylase are allosteric
enzymes .
• CPS II, reaction -1 is feed- back
inhibited by UTP and activated by
PRPP
.
• Aspartate transcarbamoylase, reaction-
2, is feedback inhibited by CTP and
activated by ATP
.
DR.SAMAH BADR

18. DEGRADATION OF PYRIMIDINE
NUCLEOTIDES
DR.SAMAH BADR

19. DEGRADATION STEPS:
• The pyrimidine nucleotides undergo similar
reactions:
dephosphorylation, deamination and cleavage
of glycosidic bond like that of purine
nucleotides to liberate the nitrogenous
bases(C, U & T).
• The bases are then degraded to highlyl
soluble products β-alanine and β-
aminoisobutyrate.
• These are the amino acid which undergo
transamination and other reactions to finally
produce acetyl CoA and succinyl CoA for
fatty acid synthesis.
DR.SAMAH BADR

20. DISORDERS OF PYRIMIDINE METABOLISM
• Disorders of pyrimidine metabolism includes:
✓Orotic aciduria
✓Reye’s syndrome
DR.SAMAH BADR

21. OROTIC ACIDURIA:
✓ Is a rare inherited and metabolic disorder characterized by :
• excretion of orotic acid in urine
• severe anemia
• retarded growth(mental and physical)
✓ It is due to the deficiency of the enzymes orotate phosphoribosyl transferase
and OMP decarboxylase of pyrimidine synthesis.
✓ Both these enzymes activities are present on a single protein as bifunctional
enzyme
DR.SAMAH BADR

22. TREATMENT:
✓effective t/t of orotic aciduria feeding diet rich in uridine or
cytidine
✓These compounds provide pyrimidine nucleotides required for
DNA and RNA synthesis and stop orotic acid synthesis
✓ Allopurinol can precipitate the condition , HOW ??????
DR.SAMAH BADR

23. Reye’s syndrome:
REYE’S SYNDROME:
✓ This is considered as a secondary orotic aciduria.
✓ Due to defect in ornithine trascarbamoylase ( in urea cycle )
causes the accumulation of carbamoyl phosphate.
✓ This is then diverted for the increased synthesis and excretion
of orotic acid.
DR.SAMAH BADR

24. THANK YOU