Download MBBS Pyrimidine Lecture PPT

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