Download MBBS (Bachelor of Medicine and Bachelor of Surgery) Latest Pyrimidine Lecture PPT
Lesch-Nyhan Syndrome
? Inability of the body to salvage hypoxanthine and
guanine due to the complete deficiency of HGPRTase
(Hypoxanthine-Guanine phosphoribosyl transferase)
? It is an X-linked inherited disorder of purine
metabolism, the disease is limited to males only
? Different types of mutations in HGPRTase gene have
been identified in patients with Lesch Nyhan
syndrome.
? Incidence is 1:10,000 males.
? HGPRT deficiency results in the accumulation of
PRPP and decrease in GMP and IMP.
? Increased level of Hypoxanthine and Guanine
in degradation to uric acid
? Also PRPP accumulates
stimulates production of Purine nucleotides
increases their degradation to uric acid
? Leads to hyperuricemia---Gout-like symptoms
Nephrolithiasis ( Renal stones)
Neurological symptoms
? self mutilation
? spasticity,
? aggressiveness,
? mental retardation
DIAGNOSIS
? Increase urinary urate / creatinine ratio
? Absent / reduced enzyme activity in
lymphocytes or fibroblast
? Mutation analysis of Hypoxanthine-Guanine
phosphoribosyl transferase (HGPRT) gene.
Severe combined immunodeficiency (SCID)
? The deficiency of adenosine deaminase (ADA) causes
severe
combined
immunodeficiency
(SCID)
involving T-cell and usually B-cell dysfunction.
? ADA deficiency results in the accumulation of dATP.
? dATP is an inhibitor of ribonucleotide reductase
which causes reduced synthesis of other dNTPs and
therefore DNA synthesis and cell replication is
inhibited.
? Thus proliferation and differentiation of immune cells
is compromised.
SCID
? Lymphocytes usually contain high levels of ADA.
? Therefore, ADA deficiency is mainly manifested as
reduced lymphocytes.
? This leads to impaired cellular and humoral immunity.
? Hypouricemia is due to defective breakdown of
purine nucleotides.
ADA estimation in CSF is used for the diagnosis of
tuberculous meningitis.
ADA levels can be estimated in various body fluids like
blood, CSF, pleural fluid, pericardial fluid, ascitic fluid, etc.
SCID - Treatment
? Antibiotics and periodic injections of
immunoglobulin will be lifesaving.
? Bone marrow stem cells will increase both T
and B cells in the patients.
? Enzyme replacement therapy with ADA-
Polyethylene glycol ( the first successful
application of enzyme replacement therapy for
an inherited disease.
? Gene therapy- recently, ADA gene has been
successfully transfected into stem cells of
ADA deficient children.
Purine Nucleoside Phophorylase
Deficiency
? Less severe than ADA deficiency
? Associated with severe deficiency of T- cells
but apparently normal B- cell function.
? Immune dysfunction appear to result from
accumulation of dGTP, and dATP, which
inhibit ribonucleotide reductase and thereby
deplete cells of DNA precursors.
METABOLISM OF
PYRIMIDINE
DR. SUDHANSHU SHEKHAR
ASSOCIATE PROFESSOR
DEPT. OF BIOCHEMISTRY
Pyrimidine is a heterocyclic ring.
4
? 5 5
3
5
2
1
6
Sources of different atoms of pyrimidine rings
Synthesis of pyrimidine nucleotides
A. Denovo synthesis
B. Salvage pathway
Denovo synthesis of
Formation
of
pyrimidine nucleotide
pyrimidine nucleotides
refers to the formation of
from pyrimidine bases
pyrimidine ring structure
followed by the addition
of ribose phosphate
Denovo synthesis
? The synthesis of pyrimidines is a much simpler
process compared to that of purines.
? Aspartate, Glutamine and bicarbonate contribute to
atoms in the formation of pyrimidine ring.
? Pyrimidine ring is first synthesized and then attached
to ribose 5-phosphate.
? This is in contrast to purine nucleotide synthesis
where in purine ring is built upon a pre-existing
ribose-5-phosphate.
Synthesis of pyrimidine nucleotides
Tissue and site of synthesis
Mainly occurs in the liver.
The reaction occurs in cytosol and mitochondria.
The formation of orotate from dihydroorotate
occurs ie mitochondria and all other reactions
occur in the cytosol.
Step 1: Carbamoyl Phosphate Synthesis
? The reaction occurs in
cytoplasm
(in
urea
synthesis, the reaction is
in mitochondria).
? The
nitrogen
of
glutamine, ATP and
bicarbonate react to form
carbamoyl
phosphate
(step 1).
? The enzyme is carbamoyl
phosphate synthetase II
(CPS II). N
Step 2: Rate Limiting Step :Condensation
? Carbamoyl phosphate and
aspartate combine to form
carbamoyl aspartate
? The enzyme is aspartyl trans
carbamoylase (ATC), which
is allosterically regulated
? The atoms C2 and N3 are
derived from carbamoyl
phosphate and the rest are
from aspartate.
Step 3: Formation of Pyrimidine Ring
? The 3rd nitrogen and 4th
carbon are joined by a
covalent
bond
and
carbamoyl aspartate is
cyclized.
Ring closure:
? Dihydo orotic acid is
produced.
? The enzyme is dihydro
orotase (DHOase)
Step 4: Oxidation
O
? Hydrogen
atoms
are
removed from C5 and C6
positions, so that orotic
acid is produced
? Enzyme is dihydro orotate
dehydrogenase (DHODH).
? It requires NAD as co-
enzyme.
Step 5:Transfer of ribose phosphate & Formation of OMP
? Ribose-5-phosphate
is
added to orotic acid, so as to
?
produce orotidylic acid or
orotidine
monophosphate
(OMP).
? PRPP is the donor of ribose-
5-P.
O
? The enzyme is orotate
phosphoribosyl transferase
(OPRTase)
Step 6: Decarboxylation
O
? The C7 of OMP is removed
as carbon dioxide, so that
uridine
monophosphate
(UMP) is produced
? This is the first pyrimidine
that is synthesized.
? The
enzyme
is
OMP-
decarboxylase (OMPDC).
? 6-aza-uridine inhibits this
step, and so used as an
anticancer drug
Uridine monophosphate (UMP)
Step 7: Synthesis of
Triphosphates
? UMP is phosphorylated to
form
UDP
(uridine
diphosphate) with the help of
ATP
? The enzyme is nucleoside
monophosphatekinase (UMP
kinase).
? The UDP is phosphorylated to
UTP (uridine triphosphate)
with the help of ATP
? The enzyme is nucleoside
diphosphate kinase
Step 8: Formation of CTP
? UTP is converted to
CTP by adding an
amino group from
glutamine catalyzed
by CTP synthetase.
? It needs ATP
Step 8
Step 9 .Reduction of ribonucleoside diphosphates
to their corresponding dNDP's
Step10.Formation of TMP
from UDP
? dUMP is substrate for
Step9
TMP synthesis.
? dUDP
is
dephosphorylated
to
dUMP
? Methylation of dUMP
occurs at C5 by
N5,N10methyleneTHF,
forming TMP.
? This reaction is catalysed
Step10
by Thymidylate synthase.
SALVAGE PATHWAY OF PYRIMIDINE
SYNTHESIS
Pyrimidine base
PRPP
Pyrimidine Phosphoribosyl
Transferase
PPi
Pyrimidine nucleotide
Regulation of pyrimidine synthesis
CPSII and Aspartate transcarbomylase are main
regulatory enzymes.
There is feedback regulation to maintain optimal
pyrimidine nucleotide concentrations.
CPS II ?
- inhibited by UTP .
- activated by PRPP
Aspartate transcarbomylase :
- inhibited by CTP
- activated by ATP
Cross Regulation of Purine and pyrimidine
synthesis:
? PRPP is required for the synthesis of both purines and
pyrimidines, so its regulation by both ensures
coordinated purine and pyrimidine synthesis
? PRPP stimulates the purine and pyrimidine synthesis
through amidotransferase and carbamoyl phosphate
synthase respectively.
? So both purine and pyrimidine feedback inhibit PRPP
synthase
? Increase synthesis of pyrimidines (TDP) leads to
allosteric inhibition of PRPP synthase.
? Purine (ADP) also inhibit PRPP synthase.
Degradation
of
pyrimidine
nucleotides
?The
pyrimidine
nucleotides
undergo
similar
reactions
(dephosphorylation, deamination
and cleavage of glycosidic bond) to
liberate the nitrogenous bases
cytosine, uracil and thymine.
?The bases are then degraded to
highly soluble products -alanine
and -aminoisobutyrate.
?These are the amino acid which
undergo transamination and other
reactions to finally produce acetyl
CoA and succinyl CoA
DISORDERS OF PYRIMIDINE METABOLISM
1.OROTIC ACIDURIA
Features :
? Orotic aciduria type I ?
? Due to lack of feedback
deficiency of
inhibition orotic acid production
a) Orotate phosphoribosyl
is excessive.(UMP inhibits
transferase and
OMP decarboxylase)
b ) OMP ?decarboxylase.
Rapidly growing cells are
? Orotic aciduria type II :
affected ?
- Rare
a)
anemia
- deficeincy of ONLY
b)
Retarded growth
OMP decarboxylase.
c)
Crystals excreted in urine
causing urinary obstruction.
? Both types are inherited as
autosomal recessive disorders.
? Both types respond to uridine ,
as it is converted to UTP . This
acts as feed back inhibitor
Other causes of orotic aciduria
1.Deficeincy
of
liver
mitochondrial
ornthine
?
trancarbomylase (X-linked).
under utilised substrate carbomyl phosphate enters cytosol
Stimulates pyrimidine nucleotide biosynthesis
Leading to orotic aciduria
2. Drugs may precipitate orotic aciduria:
ALLOPURINOL , a purine analog is a substrate for
Orotate phosphoribosyl transferase.
It competes for phosphoribosylation with natural substrate,
orotic acid.
The resulting nucleotide product inhibits
OMP DECARBOXYLASE
leading to Orotic aciduria and orotiduniria
Reye's syndrome
? This is considered as a secondary orotic
aciduria.
? It is believed that a defect in ornithine
trascarbamoylase (or urea cycle ) causes the
accumulation of carbamoyl phosphate.
? This is then diverted for the increased synthesis
and excretion of orotic acid.
Anti-Folate Drugs
? Cancer cells consume dTMP quickly for DNA
replication
? Interfere with thymidylate synthase reaction to
decrease dTMP production
? Fluorodeoxyuridylate ? irreversible inhibitor ? also
affects rapidly growing normal cells (hair follicles,
bone marrow, immune system, intestinal mucosa)
? Dihydrofolate reductase step can be stopped
competitively (DHF analogs)
? Anti-Folates:
Aminopterin,
methotrexate,
trimethoprim
Inhibitors
5-Flurouracil and methotrexate
This post was last modified on 30 November 2021