Priyanshi mishra (6)

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Dr. Harisingh gour central university sagar
(M.P. )
Microbial synthesis of pyrimidine bases of DNA
and RNA
Presented to - Dr. yogesh Bhargav (department
of applied microbiology)
PRESENTED BY - PRIYANSHI MISHRA MSC 2ND
SEMESTER
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Introduction

• nucleotides are Basic building blocks of nucleic
  acid.
• Nucleotides purine and pyrimidine.
• The pathway for biosynthesis of nucleotides fall
  into two classes
• De novo pathway (means a new)
• Salvage pathway
• 1) De novo pathway biochemical pathway where
  nucleotides are synthesized from new simple
  precursor molecules.
• 2) Salvage pathway used to recover bases on
  nucleotides formed during the degradation of RNA
  and DNA.
• Both these pathways leads to the biosynthesis of
  ribonucleotides.
• All the deoxyribonucleotides are synthesized from
  corresponding ribonucleotides.

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pyrimidine biosynthesis introduction

• Biosynthesis is a multistep, enzyme catalyzed
  process where substrates are converted into more
  complex products in living organisms.
• Pyrimidine- cytosine, thyamine and uracil.
• Pyrimidine nucleotides are
• Cystidine monophosphate (CMP)
• Uridine monophosphate (UMP)
• Thymidin monophosphate (TMP)
• The biosynthesis of pyrimidine is simpler process
  than the purine.
• Pyrimidine ring is synthesized as free pyrimidine
  and it is incorporated into the nucleotide.
• Aspartate, glutamine (amide group) and CO2
  contribute to atoms in the formation of
  pyrimidine ring.

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enzyme involved in pyrimidine synthesis

• phase 1 synthesis pathway from Aspartate and
  cytosolic cardamoyl Phosphate to UMP
• CAD enzyme (located in cytosol) contains three
  domain each with their own enzyme activity.
• Glutamine dependent Carbamoyl phosphate synthase
  2 (CPS2) rate limiting enzyme for de novo
  synthesis of pyrimidine nucleotides.
• Aspartate Carbamoyl transferase,
• Dihydroorotase.
• 2. Dihydroorotate dehydrogenase (located in
  inner mitochondrial membrane).
• UMP synthase (located in cytosol).
• Phase 2 synthesis of UTP and CTP
• Different kinase
• CTP synthetase.
• Phase 3 synthesis of thymine containing
  deoxyribonucleotide
• Thymidylate synthase

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De novo pathway

• In de novo synthesis of pyrimidine, the six
  members ring is synthesized first and then it is
  attached to ribosome- 5 phosphate to form
  pyrimidine nucleotide.
• UMP is synthesized as a pyrimidine base to which
  ribose- 5 phosphate is added.
• CTP and UTP are derived from UMP.
• Pyrimidine rings is synthesized as free
  pyrimidine and incorporated into the nucleotide.
  .
• Out of 6 enzymes involved in this pathway, 5 are
  present on the outer surface of inner
  mitochondrial membrane.
• Pyrimidine is an heterocyclic ring.

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Reactions
step 1 Formation of cardamoyl Phosphate
carbamoyl Phosphate is formed from ATP, glutamine
and CO2. The reaction is catalyzed by CPSII.
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step 2 - condensation carbamoyl phosphate
condenses with aspartate to form carbamoyl
aspartate, catalyzed by aspartate
transcarbomylase.
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Step 3 - Ring closure this occurs via loss of
water. This reaction is catalyzed by
dihydroorotase, forming dihydroorotic acid.
Step 4 dehydration removal of hydrogen atoms
from C5, C6 by dihydroorotate dehydrogenase
(mitochondrial).
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Step - 6 transfer of ribose phosphate this
is transferred from PRPP, forming OMP
(Orotidylate) catalyzed by orotate
phosphoribosyl transferase.
Step 5 ribose 5 phosphate get converted to
PRPP (phosphoribosyl - 1 pyrophosphate)
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Step 7 decarboxylation OMP is decarboxylated
forming UMP. UMP is the first true pyrimidine
ribonucleotide.
Step 8 Phosphorylation of UMP Forms UDP and
UTP, with help of ATP.
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Step - 9 Formation of CTP - UTP is a minaret
by glutamine and ATP, catalyzed by CTP synthesis.
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De novo pathway
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Salvage pathway

• similar to purine,pyrimidine are also recovered
  from the derivative intermediate of nucleic acid
  such as , DNA and RNA.
• The pyrimidines can also serves as precursor in
  the salvage pathway to be converted to the
  respective nucleotide.
• This reaction is catalyzed by pyrimidine
  phosphoribosyl transferase which utilizes PRPP as
  the source of ribose-5- phosphate.

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Regulation of pyrimidine synthesis

• CPSII, Aspartate transcarbomylase and
  dihydroorotase are present as multienzyme complex
  .
• Orotate phosphoribosyl transferase and OMP
  decarboxylase are present as single functional
  enzyme. Due to clustering of these enzymes, the
  synthesis is well coordinated.
• Dihydroorotate dehydrogenase is mitochondrial
  enzyme.
• CPSII and Aspartate transcarbomylase and OPR
  transferase and OMP decarboxylase are sensitive
  to allosteric regulation.
• CPSII is main regulatory enzyme in mammalian
  cells.
• CPSII INHIBITED BY UTP,
• ACTIVATED BY PRPP.
• Aspartate transcarbamylase main regulatory
  enzyme in prokaryotes.
• Inhibited by CTP, activated by ATP
• Requirement of ATP for CTP synthesis and
  stimulatory effect of GTP and CTP synthase
  ensures a balanced synthesis of pyrimidines.

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Cont.
Regulation of pyrimidine synthesis.
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Degradation of pyrimidine nucleotides

• The pyrimidine nucleotides undergo similar
  reactions (dephosphorylation, destination, and
  cleavage of glycosdiic bond) like that of purine
  nucleotides to liberate the nitrogenous bases,
  cytosine, uracil and thymine.
• The bases are then degraded to highly soluble
  products beta-amino-isobutyrate.
• These are the amino acid which undergo
  transamination and other reactions to finally
  produced Co-A and succinyl CoA.

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Disorder of pyrimidine metabolism
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