Lesch-Nyhan Syndrome
? Inability of the body to salvage hypoxanthine and
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guanine due to the complete deficiency of HGPRTase(Hypoxanthine-Guanine phosphoribosyl transferase)
? It is an X-linked inherited disorder of purine
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metabolism, the disease is limited to males only? Different types of mutations in HGPRTase gene have
been identified in patients with Lesch Nyhan
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syndrome.? Incidence is 1:10,000 males.
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? HGPRT deficiency results in the accumulation of
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PRPP and decrease in GMP and IMP.? Increased level of Hypoxanthine and Guanine
in degradation to uric acid
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? Also PRPP accumulatesstimulates production of Purine nucleotides
increases their degradation to uric acid
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? Leads to hyperuricemia---Gout-like symptomsNephrolithiasis ( Renal stones)
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Neurological symptoms? self mutilation
? spasticity,
? aggressiveness,
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? mental retardationDIAGNOSIS
? Increase urinary urate / creatinine ratio
? Absent / reduced enzyme activity in
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lymphocytes or fibroblast
? Mutation analysis of Hypoxanthine-Guanine
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phosphoribosyl transferase (HGPRT) gene.Severe combined immunodeficiency (SCID)
? The deficiency of adenosine deaminase (ADA) causes
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severe
combined
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immunodeficiency(SCID)
involving T-cell and usually B-cell dysfunction.
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? ADA deficiency results in the accumulation of dATP.
? dATP is an inhibitor of ribonucleotide reductase
which causes reduced synthesis of other dNTPs and
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therefore DNA synthesis and cell replication isinhibited.
? Thus proliferation and differentiation of immune cells
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is compromised.SCID
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? Lymphocytes usually contain high levels of ADA.
? Therefore, ADA deficiency is mainly manifested as
reduced lymphocytes.
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? This leads to impaired cellular and humoral immunity.
? Hypouricemia is due to defective breakdown of
purine nucleotides.
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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.
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SCID - Treatment? Antibiotics and periodic injections of
immunoglobulin will be lifesaving.
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? Bone marrow stem cells will increase both T
and B cells in the patients.
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? Enzyme replacement therapy with ADA-Polyethylene glycol ( the first successful
application of enzyme replacement therapy for
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an inherited disease.
? Gene therapy- recently, ADA gene has been
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successfully transfected into stem cells ofADA deficient children.
Purine Nucleoside Phophorylase
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Deficiency
? Less severe than ADA deficiency
? Associated with severe deficiency of T- cells
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but apparently normal B- cell function.
? Immune dysfunction appear to result from
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accumulation of dGTP, and dATP, whichinhibit ribonucleotide reductase and thereby
deplete cells of DNA precursors.
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METABOLISM OFPYRIMIDINE
DR. SUDHANSHU SHEKHAR
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ASSOCIATE PROFESSOR
DEPT. OF BIOCHEMISTRY
Pyrimidine is a heterocyclic ring.
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4
? 5 5
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35
2
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1
6
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Sources of different atoms of pyrimidine ringsSynthesis of pyrimidine nucleotides
A. Denovo synthesis
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B. Salvage pathwayDenovo synthesis of
Formation
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of
pyrimidine nucleotide
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pyrimidine nucleotidesrefers to the formation of
from pyrimidine bases
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pyrimidine ring structure
followed by the addition
of ribose phosphate
Denovo synthesis
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? The synthesis of pyrimidines is a much simpler
process compared to that of purines.
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? Aspartate, Glutamine and bicarbonate contribute toatoms in the formation of pyrimidine ring.
? Pyrimidine ring is first synthesized and then attached
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to ribose 5-phosphate.
? This is in contrast to purine nucleotide synthesis
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where in purine ring is built upon a pre-existingribose-5-phosphate.
Synthesis of pyrimidine nucleotides
Tissue and site of synthesis
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Mainly occurs in the liver.
The reaction occurs in cytosol and mitochondria.
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The formation of orotate from dihydroorotateoccurs ie mitochondria and all other reactions
occur in the cytosol.
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Step 1: Carbamoyl Phosphate Synthesis
? The reaction occurs in
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cytoplasm
(in
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ureasynthesis, the reaction is
in mitochondria).
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? The
nitrogen
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ofglutamine, ATP and
bicarbonate react to form
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carbamoyl
phosphate
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(step 1).? The enzyme is carbamoyl
phosphate synthetase II
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(CPS II). N
Step 2: Rate Limiting Step :Condensation
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? Carbamoyl phosphate and
aspartate combine to form
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carbamoyl aspartate? The enzyme is aspartyl trans
carbamoylase (ATC), which
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is allosterically regulated
? The atoms C2 and N3 are
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derived from carbamoyl
phosphate and the rest are
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from aspartate.Step 3: Formation of Pyrimidine Ring
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? The 3rd nitrogen and 4thcarbon are joined by a
covalent
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bondand
carbamoyl aspartate is
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cyclized.Ring closure:
? Dihydo orotic acid is
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produced.
? The enzyme is dihydro
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orotase (DHOase)Step 4: Oxidation
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O? Hydrogen
atoms
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are
removed from C5 and C6
positions, so that orotic
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acid is produced? Enzyme is dihydro orotate
dehydrogenase (DHODH).
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? It requires NAD as co-
enzyme.
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Step 5:Transfer of ribose phosphate & Formation of OMP
? Ribose-5-phosphate
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isadded to orotic acid, so as to
?
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produce orotidylic acid or
orotidine
monophosphate
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(OMP).
? PRPP is the donor of ribose-
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5-P.O
? The enzyme is orotate
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phosphoribosyl transferase
(OPRTase)
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Step 6: DecarboxylationO
? The C7 of OMP is removed
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as carbon dioxide, so that
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uridinemonophosphate
(UMP) is produced
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? This is the first pyrimidine
that is synthesized.
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? Theenzyme
is
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OMP-
decarboxylase (OMPDC).
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? 6-aza-uridine inhibits thisstep, and so used as an
anticancer drug
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Uridine monophosphate (UMP)Step 7: Synthesis of
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Triphosphates? UMP is phosphorylated to
form
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UDP
(uridine
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diphosphate) with the help ofATP
? The enzyme is nucleoside
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monophosphatekinase (UMP
kinase).
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? The UDP is phosphorylated toUTP (uridine triphosphate)
with the help of ATP
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? The enzyme is nucleoside
diphosphate kinase
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Step 8: Formation of CTP
? UTP is converted to
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CTP by adding an
amino group from
glutamine catalyzed
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by CTP synthetase.? It needs ATP
Step 8
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Step 9 .Reduction of ribonucleoside diphosphates
to their corresponding dNDP's
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Step10.Formation of TMP
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from UDP
? dUMP is substrate for
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Step9TMP synthesis.
? dUDP
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is
dephosphorylated
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todUMP
? Methylation of dUMP
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occurs at C5 by
N5,N10methyleneTHF,
forming TMP.
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? This reaction is catalysedStep10
by Thymidylate synthase.
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SALVAGE PATHWAY OF PYRIMIDINESYNTHESIS
Pyrimidine base
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PRPPPyrimidine Phosphoribosyl
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TransferasePPi
Pyrimidine nucleotide
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Regulation of pyrimidine synthesis
CPSII and Aspartate transcarbomylase are main
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regulatory enzymes.There is feedback regulation to maintain optimal
pyrimidine nucleotide concentrations.
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CPS II ?
- inhibited by UTP .
- activated by PRPP
Aspartate transcarbomylase :
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- inhibited by CTP- activated by ATP
Cross Regulation of Purine and pyrimidine
synthesis:
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? PRPP is required for the synthesis of both purines and
pyrimidines, so its regulation by both ensures
coordinated purine and pyrimidine synthesis
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? PRPP stimulates the purine and pyrimidine synthesis
through amidotransferase and carbamoyl phosphate
synthase respectively.
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? So both purine and pyrimidine feedback inhibit PRPP
synthase
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? Increase synthesis of pyrimidines (TDP) leads toallosteric inhibition of PRPP synthase.
? Purine (ADP) also inhibit PRPP synthase.
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Degradationof
pyrimidine
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nucleotides
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?Thepyrimidine
nucleotides
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undergo
similar
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reactions(dephosphorylation, deamination
and cleavage of glycosidic bond) to
liberate the nitrogenous bases
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cytosine, uracil and thymine.?The bases are then degraded to
highly soluble products -alanine
and -aminoisobutyrate.
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?These are the amino acid which
undergo transamination and other
reactions to finally produce acetyl
CoA and succinyl CoA
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DISORDERS OF PYRIMIDINE METABOLISM1.OROTIC ACIDURIA
Features :
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? Orotic aciduria type I ?
? Due to lack of feedback
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deficiency ofinhibition orotic acid production
a) Orotate phosphoribosyl
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is excessive.(UMP inhibits
transferase and
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OMP decarboxylase)b ) OMP ?decarboxylase.
Rapidly growing cells are
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? Orotic aciduria type II :
affected ?
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- Rarea)
anemia
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- deficeincy of ONLY
b)
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Retarded growthOMP decarboxylase.
c)
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Crystals excreted in urine
causing urinary obstruction.
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? Both types are inherited asautosomal recessive disorders.
? Both types respond to uridine ,
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as it is converted to UTP . This
acts as feed back inhibitor
Other causes of orotic aciduria
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1.Deficeincy
of
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liver
mitochondrial
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ornthine?
trancarbomylase (X-linked).
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under utilised substrate carbomyl phosphate enters cytosol
Stimulates pyrimidine nucleotide biosynthesis
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Leading to orotic aciduria2. Drugs may precipitate orotic aciduria:
ALLOPURINOL , a purine analog is a substrate for
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Orotate phosphoribosyl transferase.It competes for phosphoribosylation with natural substrate,
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orotic acid.The resulting nucleotide product inhibits
OMP DECARBOXYLASE
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leading to Orotic aciduria and orotiduniria
Reye's syndrome
? This is considered as a secondary orotic
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aciduria.
? It is believed that a defect in ornithine
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trascarbamoylase (or urea cycle ) causes theaccumulation of carbamoyl phosphate.
? This is then diverted for the increased synthesis
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and excretion of orotic acid.Anti-Folate Drugs
? Cancer cells consume dTMP quickly for DNA
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replication? Interfere with thymidylate synthase reaction to
decrease dTMP production
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? Fluorodeoxyuridylate ? irreversible inhibitor ? also
affects rapidly growing normal cells (hair follicles,
bone marrow, immune system, intestinal mucosa)
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? Dihydrofolate reductase step can be stopped
competitively (DHF analogs)
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? Anti-Folates:Aminopterin,
methotrexate,
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trimethoprim
Inhibitors
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5-Flurouracil and methotrexate
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