Metabolism of
Nucleotides
Guided by
Dr. D V Bhale
( prof. & HOD)
By
Roshan Kumar Mahat
(PG Student)
Department Of Biochemistry

Objectives to know
1.
2.
3.
4.
5.
6.
7.
Nitrogen bases i.e. purines & pyrimidines
Nucleosides
Nucleotides
Synthesis of purine nucleotides
Regulation of purine nucleotides synthesis
Inhibitors of purine nucleotide synthesis
Disorders of purine metabolism

8. Synthesis of pyrimidine nucleotides
9. Inhibitors of pyrimidine nucleotides
synthesis
10. Disorders of pyrimidine nucleotide
synthesis

Pyrimidines and Purines
Pyrimidine and purine are the names of the
parent compounds of two types of nitrogencontaining heterocyclic aromatic
compounds.
N
N
N
N
Pyrimidine
N
Purine
N
H

Important Pyrimidines
• Pyrimidines that occur in DNA are cytosine
and thymine. Cytosine and uracil are the
pyrimidines in RNA.
NH
O
2
O
CH3
HN
O
HN
HN
N
H
Uracil
O
O
N
H
Thymine
N
H
Cytosine

Important Purines
• Adenine and guanine are the principal
purines of both DNA and RNA.
NH2
O
N
N
N
Adenine
N
H
N
HN
H2N
N
Guanine
N
H

Caffeine and Theobromine
• Caffeine (coffee) and theobromine (coffee
and tea) are naturally occurring purines.
O
H3C
N
N
O
O
CH3
N
CH3
Caffeine
N
N
HN
O
CH3
N
CH3
Theobromine
N

Nucleosides
• Is a structure formed by the combination of
nitrogen base and sugar.
N2 base
Sugars
Nucleoside
Adenine
Deoxyribose/Ribose
Adenosine
Guanine
Deoxiribose/Ribose
Guanosine
Thymine
Deoxyribose
Thymidine
Cytosine
Deoxyribose/Ribose
Cytidine
Uracil
Ribose
Uridine

Nucleotides
• Nucleotides are phosphoric acid esters of
nucleosides.
Nucleoside
Phosphoric acid
Nucleotides
Adenosine
Phosphoric acid
Adenylate (AMP)
Guanosine
Phosphoric acid
Guanylate ( GMP)
Thymidine
Phosphoric acid
Thymidylate ( TMP)
Cytidine
Phosphoric acid
Cytidylate (CMP)
Uridine
Phosphoric acid
Uridylate (UMP)

Synthesis of purine nucleotides
Denovo
synthesis
Synthesis of
purine base step
by step on the
ribose 5phosphate
Salvage
pathway
Addition of ribose
5-phosphate to the
preformed purine
bases or addition
of phosphate to
the purine
nucleosides
Synthesis
of purine
nucleotides

Sources of different atoms of
purine ring

Denovo synthesis of purine
nucleotides
Tissue and site of synthesis
Tissues- major tissue is liver
Site- cytosol

Inhibitors
1. Sunfonamide
 Are structural analogues of PABA
 Act as competitive inhibitors of synthesis of folic
acid from PABA in bacteria.
 They inhibit the reactions of purine nucleotide
synthesis requiring folic acid ( GAR
transformylase and AICAR transformylase)
 Used as bacteriostatic drugs to control bacterial
infection.

2. Methotrexate and Aminopterin
Are structural analogue of folic acid.
They act as a competitive inhibitors of
dihydrofolate reductase thus blocking the
biosynthesis of tetrahydrofolic acid.
They inhibit the reaction requiring folic acid
for purine nucleotide synthesis.
Used in t/t of cancers like leukemia
choriocarcinoma.

3. Trimethoprim
structural analogue of folic acid.
Acts as a competitive inhibitors of
dihydrofolate reductase in bacteria thus
blocking the biosynthesis of tetrahydrofolic
acid.
Inhibit the reaction requiring folic acid to
purine nucleotide synthesis.
Used in the t/t of bacterial infections and
UTI.

4. 6-mercaptopurine
 is a structural analogue of purine bases.
 is converted to 6- thioionosine
monophosphate by the enzyme HGPRT,
called lethal synthesis.
 6-thio IMP inhibits the conversion of IMP
to AMP and GMP.
 6-thio IMP also feed back inhibits
glutamine PRPP amidotransferase.
Used as an anticancer drug.

5. Thioguanine
Is a guanine analogue.
 It is converted to 6-thio GMP by the
enzyme HGPRT.
 6-thio GMP inhibits the conversion of IMP
to GMP.
 Also inhibits glutamine PRPP
amidotransferase.
 Used as an anticancer drug.

6. Azaserine
 is a structural analogue of glutamine.
 is a glutamine antagonist.
 inhibits the enzyme reactions in purine and
pyrimidine nucleotide synthesis that utilize
glutamine as a substrste.
 it is highly toxic to the cells so it is not
used clinically as a drug.

Regulation
1. Intracellular conc. Of PRPPdepends upon 2 factors i.e. its synthesis &
utilization.
Synthesis depends on Availability of R-5-P.
 Action of enzyme PRPP synthetase.
Utilization depends on Denovo synthesis.
 Salvage pathway.

2. Activity of enzyme PRPP amidotransferase.
Increased activity of PRPP
amidotransferase leads to Increased
synthesis of AMP amd GMP, which
feedbackly inhibit the enzyme PRPP
amidotransferase.
3. Both AMP and GMP inhibit their own
formation by feedback inhibition of
adenylosuccinate synthetase and IMP
dehydrogenase.

Salvage pathway
It refers to the formation of purine nucleotides
by the
1. Addition of ribose phosphate ( from PRPP)
to the preformed purine bases.
2. Addition of phosphate to the preformed
purine nucleosides.

Significance
Salvage pathway provide a pathway for the
utilization of purine bases derived from diet
(exogenous) and normal turnover of the
nucleic acids.
In erythrocytes, denovo syntheis of purine
nucleotides does not occur because of
absence of PRPP amidotransferase. The
requirement of purine nucleotides is met by
the salvage pathway.

Synthesis of purine nucleotides from purine
bases
Catalyzed by HGPRT and APRT.
Adenine + PRPP
AMP + PPi
APRT
Hypoxanthine + PRPP
Guanine + PRPP
HGPRT
HGPRT
IMP + PPi
GMP+ PPi

Synthesis of purine nucleotides from
purine nucleosides
Adenosine + ATP
AMP+ ADP
Adenosine kinase

Degradation of purine nucleotides

Disorders of purine metabolism
1. Gout
2. Lesch nyhan syndrome
3. Immunodeficiency associated with purine
metabolism
4. Infantile autism

Gout
Metabolic disorders associated with
overproduction of uric acid.
At physiological form, uric acid is found in
more soluble form as sodium urate.
In severe hyperuricemia, crystal of sodium
urate get deposited in the soft tissues,
particularly in joints. Such deposits are
commonly known as tophi.

This causes inflammation of joints resulting
in gouty arthritis.
The prevalence of gout is about 3 per 1000
persons, mostly affecting males.
Post menopausal women, however are as
susceptable as men for this disease.
Historically, gout was found to be
associated with high living, over eating and
alcohol consumption.
Lead poisoning also causes gout by
decreasing uric acid excretion.

Types of gout
Gout
Primary
gout
Metabolic
gout
Renal gout
Secondary
gout
Metabolic
gout
Renal gout

Primary metabolic gout
It is an inborn error of purine metabolism
due to overproduction of uric acid.
Causes:
1. Increased activity of PRPP synthetase
2. Overactivity of PRPP amidotransferase
3. HGPRT deficiency
4. Glucose 6-phosphatase deficiency
5. Elevation of glutathione reductase

Primary renal gout
It is due to failure of uric acid excretion
from the body so that uric acid level in the
body gets increased.

Secondary metabolic gout
Secondary gout is due to secondary to
certain diseases like leukemia,
polycythemia, lymphoma, psoriasis and
increased tissue breakdown like in trauma,
starvation etc.

Secondary renal gout
It is due to secondary to defective
glomerular filtration of urate due to
generalized renal failure.

Tratment of gout
Is by
1. Use of colchicine & uricosuric drugs.
 To remove urates from the joint, colchine
is the drug of choice.
 To remove the urates from the body,
urocosuric drugs such as probenecid,
sulfinpyrazole, salicylates etc are used.
2. Use of allopurinol-inhibits the activity of
enzyme xanthine oxidase as a result of
which uric acid is not produced.

Lesch Nyhan syndrome
 Fist described tn 1964 by Michael Lesch( a
medical student) and William L. Nyhan (his
teacher).
 It is X linked metabolic disorder since the
structural genes for HGPRT is located on the X
chromosome.
 It affects only males and is characterized by
excessive uric acid production and neurological
abnormalities such as mental retardation,
aggressive behaviour, learning disability etc.
 The patients of this disorder have an irresistible
urge to bite their fingers and lips,ofen causing
self-mutilation.

Biochemical basis
HGPRT deficiency spares the utilization of
PRPP through salvage and the accumulated
PRPP takes part in the purine biosynthesis
by the denovo pathway finally leading to
hyperuricemia.
The biochemical basis for neurological
abnormalities are big enegma till date.
Indeed, it is surprising that the deficiency of
a single enzyme can cause such an
abnormal behavioural changes.

 few explanations are putforth in this regard.
Neurological symptoms may be due to
decreased availability of purines to the
developing brain which has a low capacity
for denovo purine synthesis and hence
depends on purine salvage pathway for the
supply of purine nucleotides it requires.

Treatment
 allopurinol is used to treat hyperuricemia
but it has no effect on the neurological
menefestation in theses patients.
Treatment for the neuro-behavioural
features are limited to behavioural therapy
and providing protective physical device to
prevet self-mutilation.

Immunodeficiency diseases associated
with purine metabolism
Two different immunodeficiency disorders
associated with degredation of purine
nucleotides are known.
The enzyme defects are adenosine
deaminase and purine nucleoside
phosphorylase, involved in uric acid
synthesis.

The deficiency of ADA causes SCID
involving T- cell and usually B- cell
dysfunction.
It is explained that ADA deficiency results
in the accumulation of Datp which is an
inhibitor of ribonucleotide reductase and
thus DNA synthesis, replication are
adversely affected.
Different modes of t/t such as blood
transfusion, bone marrow transplantation
are tried to cure the diseases but with
limited effects.

But, like in any other inborn error, the real hope
for the future is only gene therapy.
In 1990, a 5 year old girl suffering from SCID
was successfully cured by transfecting the ADA
gene into stem cells of the patients.
This is considered as landmark in the history of
trating inborn errors of metabolism.
The deficiency of purine nucleoside
phosphorylase is associated with impairement
of T cell function but has no effect on B cell
function.
It is believed that d GTP inhibits the
development of normal T-cells.

Infantile Autism
Recently it was observed that children
suffering from infantile autism exihibited
increased excretion of uric acid but
surprisingly the serum concentrations are
within normal limits.
The biochemical basis for this is unknown.
An oral dose of uridine is tried in the t/t.

Synthesis of pyrimidine
nucleotides
Synthesis
of
pyrimidine
nucleotides
Denovo
synthesis
Synthesis of
pyrimidine
nucleotide refers
to the formation
of pyrimidine ring
structure followed
by the addition of
ribose phosphate.
Salvage
pathway
Formation of
pyrimidine
nucleotides from
pyrimidine bases

Sources of different atoms of prrimidine rings

Denovo 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.

Pyrimidine Synthesis
O
-
2 A T P + H C O 3 + G lutam in e + H 2 O
C
2 ADP +
O
C arbam oyl
Phosphate
Synthetase II
G lutam ate +
Pi
C
C
CH
PRPP
PPi
C
O
C
N
H
O
PO 3
N
COO
2-
O 3P
O
CH2
NH2
C
C
O
HN
O
CH
HN
O
H
H
OH
O ro tate P h o sp h o rib o sy l
T ran sferase
OH
H
H
COO
-2
O ro tid ine-5'-m onophosphate
(O M P )
O rotate
C arbam oyl P hosphate
R educed
Q uinone
A spartate
A spartate
T ranscarbam oylase
(A T C ase)
O
C
CH2
CH
N
H
C
CH2
HN
D ihydroorotase
O
2-
CH
N
H
CH
N
O
H 2O
C
O
CH
HN
C
C
C
O
O
NH2
CO2
Q uinone
Pi
HO
OM P
D ecarboxylase
D ihydroorotate
D ehydrogenase
O 3P
O
CH2
O
COO
H
OH
COO
H
OH
H
H
D ih y d ro o ro tate
C arb am o y l A sp artate
U ridine M o nophosphate
(U M P )

UMP  UTP and CTP
• Nucleoside monophosphate kinase catalyzes
transfer of Pi to UMP to form UDP; nucleoside
diphosphate kinase catalyzes 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

UDP
Ribonucleotide reductase
dUDP
dUMP
dUDP
dUMP
Thymidylate synthetase
N5,10 formyl THF
dTMP
formyl THF

Regulation of pyrimidine synthesis
 In bacteria, aspartate transcarbamoylase catalyses
a committed step in pyrimidine biosynthesis.
 Aspartate transcarbamoylase is a good example of
an enzyme controlled by feedback mechanism by
the end product CTP.
 In certain bacteria, UTP also inhibits aspartate
transcarbamoylase. ATP, however stimulates
aspartate transcarbamoylase activity.

Carbamoyl phosphate synthase II is the
regulatory enzyme of pyrimidine synthesis
in animals.
It is activated by PRPP and ATP and
inhibited by UDP and UTP.
OMP decarboxylase inhibited by UMP and
CMP, also controls pyrimidine formation.

Inhibitors of pyrimidine synthesis
Sulfonamides
Methotrexate
Trimethoprim
5-fluorouracil
Fluorocytosine

Salvage pathway
Salvage pathway of pyrimidine nucleotide
synthesis refers to the formation of
pyrimidine nucleotides from pyrimidine
bases.
Significance
Salvage pathway provide a pathway for the
utilization of pyrimidine bases derived from
diet(exogenous) and normal turnover of
nucleic acids.

Enzymes and reactions
There are 2 enzymes that catalyze the
reactions of salvage pathway. They are
uracil phosphoribosyl transferase (UPRT)
and thymidine kinase.
Uracil + PRPP
UMP + PPi
UPRT
Thymidine + ATP
Thymidine
kinase
TMP+ ADP

Disorders of pyrimidine metabolism
Disorders of pyrimidine metabolism includes:
Orotic aciduria
Reye’s syndrome

Orotic aciduria
Is a rare metabolic disorder characterized by the
excretion of orotic acid in urine, severe anemia
and retarded growth.
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 domains (bifunctional enzyme).

Treatment
Feeding diet rich in uridine or cytidine is an
effective t/t of orotic aciduria.
These compounds provide pyrimidine
nucleotides required for DNA and RNA
synthesis.

Reye’s syndrome
Is considered as a secondary orotic aciduria.
It is believed that a defect in ornithine
transcarbamoylase (of urea cycle) causes
the accumulation of carbamoyl phosphate.
This is then diverted for the increased
synthesis and excretion of orotic acid.

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