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Metabolism of Purine and Pyrimidine
Prof.Orhan CANBOLAT ; MD , PhD
There are two major purines,
adenine (A) and guanine (G),
 and three major pyrimidines,
cytosine (C), uracil (U), and thymine (T).

Purine and Pyrimidine
A nucleoside is formed from the linkage of a sugar with a
nitrogen-containing base.,
1. The bases that make up the physiologically relevant
nucleosides all have ring structures.
a. The purines adenine, guanine, and inosine have a doublering system.
b. The pyrimidines cytosine, thymine, and uracil have sixmembered ring structures.
2. Ribose and 2-deoxyribose are the main sugars found in
nucleosides and nucleotides.
Nucleotides ; one, two, or three phosphate groups have been
added to the sugar.
Nucleosides - Nucleotides
Biomedical Importance

DNA – RNA ( Nucleic acid metabolism )

ATP – GTP (Energy Metabolism )

NADH – FADH( Co- enzymes )

S- Adenosyl – methionine (donor for methyl group )

UDP – glucronic acids ( donor for conjugation ; bilirubin glucronid )

cAMP – cGMP ( Hormone Signal )

Anticancer Drugs ( Mtx , 5Flu, 6-TG , 6-MG )

Antiviral drugs (Aciclovir , Lamivudine )

Gout Disease Treatment ( Allopurinol )
Human diseas
that involve purine
and pyrimidine (Gout Disease ,
Lesh Nyhan Synrome ,
ADA Deficiency , Orotic aciduria )
Purines & Pyrimidines Are Dietarily Nonessential
Human tissues can synthesize purines and pyrimidines from
amphibolic intermediates.
Ingested nucleic acids and nucleotides, which therefore are
dietarily nonessential, are degraded in the intestinal tract to
mononucleotides, which may be absorbed or converted to purine
and pyrimidine bases.
GLİSİN
CO2
C
ASPARTAT
N
C
N10-
1
2
N
6
5
3
4
C
C
N
7
8
C
N5, N10 methenilH4folat
9
N
Formiltetrahidrofolat
GLUTAMAT
The sources of nitrogen and carbon atoms of purine ring
Biosynthesis of Purine Nucleotides ; De- Novo

Ribose 5-phosphate derived from the pentose phosphate
pathway or from dietary sources is the starting material
that eventually gives rise to inosine monophosphate
(IMP)
ATP
Mg
AMP
Ribose 5-Phosphate PRPP Synthetase
PRPP
key regulatory enzyme
The first step creates the multi-purpose intermediate
5-phosphoribosyl-1-pyrophosphate (PRPP).
In prokaryotes, is catalyzed by a different polypeptide by contrast, in eukaryotes,
the enzymes are polypeptides with multiple catalytic activities
FOLIC ACID DEFICIENCY
• Decreased levels of folate coenzymes needed for various reactions of de novo
purine synthesis and thymine synthesis produce shortages of
deoxyribonucleotides and consequent impaired DNA synthesis in many
tissues.
• Blood levels of folic acid may become inadequate due to dietary insufficiency or
poor absorption due to intestinal problems or alcoholism.
• Folate coenzyme concentrations may also decline as a result of treatment with
drugs that inhibit dihydrofolate reductase, eg, methotrexate
.
• Patients with folic acid deficiency may have diarrhea and nausea, but the
principal symptoms are weakness and easy fatigability due to megaloblastic
anemia arising from impaired cell division in the bone marrow.
• Folate deficiency during pregnancy is a major contributor to neural tube
defects because of the critical role of folate in neuronal development.
• Folate supplementation of food in the United States is expected to reduce folateassociated birth defects by up to 70%.
Biosynthesis of Purine Nucleotides ; Salvage
Salvage Reactions“ Convert Purines & Their Nucleosides to Mononucleotides
Salvage of purine nucleosides
Free purines may be joined with PRPP to produce mononucleotides
by one of two enzymes.
1. The reaction shown below is catalyzed by
adenine phosphoribosyltransferase (APRT).
Adenine + PRPP → AMP + Ppi
2. The following reactions are catalyzed by ,
hypoxanthine-guanine phosphoribosyltransferase (HGPRT).
Hypoxanthine + PRPP → IMP + PPi
Guanine + PRPP → GMP + Ppi
Salvage of purine nucleosides is achieved by phosphorylation with
ATP as the phosphate donor.
LESCH-NYHAN SYNDROME
• Lesch-Nyhan syndrome is an X-linked disorder arising from deficiency
of HGPRT, which results in failure to salvage hypoxanthine and guanine
to the corresponding nucleotides IMP and GMP.
• Inability to utilize PRPP in the salvage pathway leads to PRPP
accumulation, which, in conjunction with low levels of IMP and GMP,
causes chronic allosteric activation of PRPP glutamyl amidotransferase
and excessive purine synthesis.
• The excess purines are degraded to uric acid causing increased blood
levels of this metabolite (hyperuricemia) and deposition of sodium
urate crystals in the joints and kidneys.
• Patients with Lesch-Nyhan syndrome experience gout-like episodes of
joint pain and kidney stones as well as severe neurologic problems,
including self-mutilation, spastic movements, and mental retardation.
•
Degradation of purine nucleotides to uric acid
GMP
Ksantin
Oksidaz
ADP
PNP
GUANOZİN
GUANİN
R-1P
H2O2
.
O2
Pi
5’NT
ADA
Ksantin
NH3
Oksidaz
Feed back Regulation
AMP & GMP Feedback-Regulate Their Formation from IMP
GMP , ADP feedback-inhibits PRPP glutamyl amidotransferase
ADP ,GDP feedback-inhibits PRPP synthetase
AMP and GMP also inhibit
hypoxanthine-guanine phosphoribosyltransferase,
Hyperuricemia
GOUT
•Hyperuricemia and chronic or episodic joint pain due to
deposition of sodium urate crystals and consequent
inflammation (gouty arthritis) are the hallmarks of gout.
• Uric acid is minimally water-soluble and most cases of gout
arise from inadequate excretion by the kidneys ,leading to
build-up of uric acid and precipitation of urate stones in the
kidneys and extremities.
• The joints of the hands and feet are prone to accumulation
of crystals because of reduced solubility of
sodium urate at the slightly cooler temperature of the
extremities.
1.PRPP synthetase
ATP
AMP
Riboz 5-P
PRPP
PRPP
sentetaz
ÜRİK ASİT
Ischemia - Reperfusion
ATP YIKIMI ARTIŞI
ADP
HİPOKSANTİN
KO
KO
H2.O2
O2
ÜRİK ASİT
3. Von Gierke’s Disease
G-6-Paz
G-6-P
glukoneogenez
HMŞ
GLUKOZ
R-5-P üretiminde artış
•
ÜRİK ASİT ARTIŞI
5’AMP
AMP
dATP
5’NT
ADENOZİN
ADENOZİN
ADA
RNdiP
Ribonükleotid
İNOZİN
İNOZİN
redüktaz
PNP
HİPOKSANTİN
DNA sentezinin
bozulması
T ve B
hücre
Immun system deficiency
dRNdiP
HİPOKSANTİN
KSANTİN
OKSİDAZ
KSANTİN
KSANTİN
OKSİDAZ
ÜRİK ASİT
KSANTİNÜRİ
Ve
KSANTİN TAŞI
HİPOÜRİSEMİ
Ribonucleotide Reductase
Biosynthesis of Pyrimidine Nucleotides
ASPARTAT
GLUTAMİN
C
N
HCO3
4
3
5
C 2
1 6
N
C
C
The pyrimidine ring is synthesized first and is then attached to ribose 5phosphate to eventually produce the nucleotide uridine 5′-monophosphate
(UMP).
Biosynthesis of pyrimidine nucleotides
Carbamoyl phosphate synthetase II ; CPS-II

An ammonium ion contributed by glutamine is
combined with bicarbonate (derived from
dissolved CO2) in a two-step reaction that
requires hydrolysis of two molecules of ATP.

b. This complex reaction is catalyzed by
carbamoyl phosphate synthetase II(CPS-II).

c. CPS-II, the critical enzyme regulating the
pyrimidine synthetic pathway, is activated by
ATP and PRPP and feedback-inhibited by the
end product UTP.
CPS-II, a cytosolic enzyme, is different from the
mitochondrial enzyme of the urea cycle CPS-I.
Synthesis of dTTP
Mechanism of Tymidylate synthesis
Deoxythymidylate (dTMP) is formed from 2′-deoxyuridylate (dUMP) in
a one carbon transfer by thymidylate synthetase
1. The donor coenzyme for the one-carbon transfer is N5,N10-methylene
tetrahydrofolate (N5,N10-methylene THF); simultaneous reduction to a
methyl group leaves dihydrofolate (DHF) as byproduct.
2. N5, N10-methylene THF is regenerated from DHF by a series of
reactions, one of which involves dihydrofolate reductase.
INHIBITORS OF dTMP SYNTHESIS AS ANTICANCER
AGENTS

• Several drugs that interfere with production
of dTMP by blocking the reaction catalyzed by
thymidylate synthetase are inhibitors of DNA
synthesis and cell proliferation.

The thymine analog 5-fluorouracil (5-FU) is
converted to 5-fluoro-dUMP, which acts as a
suicide inhibitor of thymidylate synthetase.

• Methotrexate is a folate analog that acts as a
potent competitive inhibitor of dihydrofolate
reductase,causing a decreased supply of THF
coenzymes needed by thymidylate synthetase.
Control of Pyrimidine Nucleotide


İnhibition of CPSII by UMP and UTP
Activation of PRPP and ATP
CO2 + Glutamin + ATP
UMP ,UTP
Aspartat
Karbomoil fosfat
CTP
N-karbomoil aspartat
UMP
UTP
CTP
Catabolism of pyrimidine
Orotic aciduria

Mutation of one of the two enzyme activities of UMP
synthase leads to orotic aciduria, characterized by
accumulation of its first substrate orotic acid and
insufficient levels of the product UMP,

which reduces availability of uridine triphosphate (UTP)
and cytidine triphosphate (CTP) for use in nucleic acid
synthesis.

• Patients with orotic aciduria excrete large amounts of
orotic acid in their urine, and they exhibit lethargy,
weakness, severe anemia, and growth retardation.

• This autosomal recessive disorder can be treated by
feeding a diet rich in uridine, which is salvaged to UMP
and finally to UTP.
İnhereted disorders of pyrimidine metabolism
Clinical
Disorder
Defective
enzmyme
Orotic
aciduria
Type I
Orotate
phosphoribosyl
transferase
Orotic acid
crystallüria ,
megaloblastic
anemia ,immu
deficiency
Orotic
aciduria
Tip II
orotidate
decarboxilase
Orotic acid
crystallüria ,
megaloblastic
anemia ,immu
deficiency
Ornitihine
transcarba
moylase
deficiency
Ornitihine
transcarbamoylase
Mild orotic aciduria