The document discusses nucleotide metabolism, specifically focusing on pyrimidine synthesis, including de novo synthesis and the salvage pathway, and highlights the key substrates and products involved in this process. It details the steps of pyrimidine synthesis from carbamoyl phosphate and aspartate, clarifying enzymatic actions and regulatory mechanisms. The significance of pyrimidines is also noted, as they play crucial roles in DNA and RNA synthesis, as well as various biological activities.

1. NUCLEOTIDE
METABOLISM
PREPARED BY; MISS RABIA KHAN BABER
COURSE TITLE : BIOCHEMISTRY IV

2. LEARNING OBJECTIVES OF
CLASS
PYRIMIDINE
SYNTHESIS
DENOVO
SYNTHESIS
REGULATION SIGNIFICANCE

3. PYRIMIDINE SYNTHESIS

5.  Thymine,cytosineanduracilarepyrimidinebases.
 Thepyrimidine ring issynthesized as free
pyrimidine &it isincorporated into the nucleotide.
 Aspartate, glutamine (amide group) &CO2
contribute to atoms in the formation of pyrimidine
ring.
PYRIMIDINE BASES

6. DE NOVO PYRIMIDINE
SYNTHESIS
 Biosynthesis of pyrimidine nucleotides can occur by
a de novo pathway or by the reutilization of
preformed pyrimidine bases or ribonucleosides
(salvage pathway).
 The pyrimidine synthesis is a similar process than
that of purines. In the de novo synthesis of
pyrimidines, the ring is synthesized first and then it is
attached to a ribose-phosphate to for
a pyrimidine nucleotide.

7. LOCATION
 De novo pyrimidine synthesis occurs in the cytosol of
cells in all tissues.
SUBSTRATES
 CO2; glutamine; ATP; Aspartate; H2O; NAD+;
Phosphoribosyl pyrophosphate (PRPP).
PRODUCTS
 UTP; CTP; glutamate; NADH; CO2

8. STEPS OF SYNTHESIS
Pyrimidines are synthesized from carbamoyl
phosphate and aspartate. Ribose-5-phosphate is then
attached to yield pyrimidine ribonucleotides.
Cytosine is found in both DNA and RNA. Uracil is
found only in RNA

9. Synthesisofcarbomylphosphate
 Thereaction occursin cytoplasm
 Glutamine transfers its amido nitrogen to CO2 to
produce carbamoyl phosphate.
 This reaction is catalyzed by ATP dependent enzyme
carbamoyl phosphate synthetase II (CPS II).
 CPS-Iisinvolved in urea cycle.
STEP#1

10. Rate limiting step
 Carbamoyl phosphate condenses with aspartate
to form carbamoyl aspartate.
 This reaction is catalysed by aspartate
transcarbamoylase.
 The atoms C2 & N3 are derived from
carbamoyl phosphate & the rest are from aspartate.
STEP#2

11. Formation of pyrimidine ring
 Dihydroorotase catalyses the pyrimidine ring
closure with a loss of H2O.
Oxidation
 NAD+ dependent dehydrogenation, leading to the
formation of orotate.
 The enzyme is dihydro orotate
dehydrogenase (DHODH).
STEP#3 & STEP#4

12. Formation of OMP
 Ribose 5-phosphate isnow added to orotate to
produce orotidine monophosphate (OMP).
 Thisreaction iscatalysed by orotate
phosphoribosyltransferase.
 PRPPisthe donor of ribose 5-phosphate.
STEP#5

13. Decarboxylation
 OMP undergoes decarboxylation to uridine mono-
phosphate (UMP).
 The enzyme isOMP decarboxylase (OMPDC).
 Thisisthe first pyrimidine that issynthesized.
 6-aza-uridine inhibits this step &used as an
anticancer drug.
STEP#6

14.  Orotate phosphoribosyl transferase &OMP
decarboxylase are domains of a single protein.
 A defect in this bifunctional enzyme causes orotic
aciduria.
CLINICAL CORELATION

15. STEP#7
Synthesis of triphosphates
 By an ATP-dependent UMP kinase reaction, UMP
is converted to UDP which serves as a precursor for
the synthesis of dUMP, dTMP, UTP&CTP.
 UDPisphosphorylated to UTP(uridine
triphosphate) with the help of ATP.
 Theenzyme isnucleoside diphosphate kinase.

16. STEP#8
Formation of CTP
 Cytidine triphosphate (CTP)issynthesized from UTP
byamination.
 CTPsynthetase isthe enzyme &glutamine provides
the nitrogen.
 CTPsynthetase needsATP.

17. REGULATION OF PYRIMIDINE
SYNTHESIS
 In bacteria, aspartate transcarbamoylase (ATCase)
catalyses a committed step in pyrimidine biosynthesis.
 ATCaseisa good example of an enzyme controlled by
feedback mechanism by the end product CTP.
 Carbamoyl phosphate synthetase II (CPSII) is the
regulatory enzyme of pyrimidine synthesis in animals.
 It isactivated by PRPPand ATP&inhibited by UDP
& UTP.
 OMP decarboxylase, inhibited by UMP & CMP,
also controls pyrimidine formation.

18. SIGNIFICANCE OF
PYRIMIDINE SYNTHESIS
 Pyrimidine nucleotides, in common with purine
nucleotides, are required for the synthesis of DNA
and RNA.
 They also participate in intermediary metabolism. For
example, pyrimidine nucleotides are involved in the
biosynthesis of glycogen and of phospholipids.
 Pyrimidines have diverse biological activities such as
antimicrobial, CNS depressant, anti-inflammatory,
analgesic, anti-convulsant, anticancer, antihelmentic,
antioxidant and herbicidal.

19. REFERENCES
 Textbook of Biochemistry-U Satyanarayana
 Textbook of Biochemistry-DM Vasudevan
 Text book of medical biochemistry, MN Chatterjee
 Fundamentals of biochemistry, J.L Jain, Sunjay Jain,
Nitin Jain