Lecture 30 Pyrimidine Metabolism/Disease Raymond B. Birge, PhD.
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Transcript of Lecture 30 Pyrimidine Metabolism/Disease Raymond B. Birge, PhD.
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Lecture 30Pyrimidine Metabolism/Disease
Raymond B. Birge, PhD
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Pyrimidine metabolism (Overview)
1. Nomenclature/nucleotide structure
2. Synthesis pathways
3. Synthesis of deoxy-ribonucleotides
4. Salvage & degradation pathways
5. Metabolic disease of pyrimidine metabolism (orotic aciduria) Suggested reading: Lippencott’s Chapter 22
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Structure of Pyrimidines
C= 2 oxy, 4 aminoT= 2,4 dioxy 5-methyl
U= 2,4 dioxyO= 2,4 dioxy 6 carboxy
O
O
O
O
Orotic acid
Uracil
NH2
O
Thymine
O
O
Cytosine
CH3
Uracil
Orotic Acid
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Nomenclature of Pyrimidines
* when the base is purine, then the nucleoside ends in OSINE (AdenOSINE, GuanOSINE, InOSINE) when the base is pyrimidine, then the nucleoside ends in IDINE (UrIDINE, CytIDINE, ThymIDINE)
PO4 is an acid: cytidylic acid/cytidylate; note thymidine only deoxyribose
Cytidine MonophosphateCytosine Cytidine
Base Nucleotide Base + ribose + P04 ester
Nucleoside*Base + ribose
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Inhibited by UTP; Activated by ATP & PRPP
(occurs in cytosol)
1st Step is Regulated
5-phosphoribosylpyrophosphate
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Multifunctional enzyme synthesis: CAD
Contrast to purines: Pyrimidines synthesized as free ring
C arbamoyl phosphate synthetase II: by UTP; by ATP & PRPP;
A spartate transcarbamylase
D ihydroorotase1 polypeptide, 3 domains, 3 activities
From Figure 22.21 in Lippincott
Carbamoyl phosphate synthase II
Aspartatetranscarbamylase
Dihyroorotase
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Making a pyrimidine
From Figure 22.21 in Lippincott
Desaturating the ring gives the pyrimidine,
OROTATE.
Carbamoyl phosphate synthase II
Aspartatetranscarbamylase Dihyroorotase
Dihyroorotatedehydrogenase
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Precursors
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Which of the following contributes nitrogen atoms DIRECTLY to both purine and pyrimidine rings?
1. Aspartate
2. Carbamoyl phosphate
3. Bicarbonate
4. Glutamate
5. Tetrahydrofolate
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Making UMP: another multifunctional enzyme
From Figure 22.21 in Lippincott
UMP synthase: Orotate phosphoribosyl transferase OMP decarboxylase
1 polypeptide, 2 domains, 2 activities
OMP decarboxylase Orotate phosphoribosyltransferase
Low UMP activity Orotic aciduria (abnormal growth; megaloblastic anemia; treat with uridine-rich diet)
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CTP Synthetase
ATP, Gln ADP, Pi, Glu
Synthesis of CTP
Ribose-Tri-P04
UTP
(Uracil)
Ribose-Tri-P04
CTP
(Cytosine)
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Clinical Significance-pyrimidine metabolism
ID: A 2 year old female referred to a pediatric clinic
Chief Complaint: My baby doesn’t play, sleeps all the time and is weak.
History Present Illness: Baby was treated for anemia by family doctor but didnot respond to vitamin B12, folic acid, iron or vitamin C. She is the third-born child of a healthy white couple; her mother had an uneventful pregnancy and a eutopic delivery. Both brothers are healthy.
Physical Exam: Low weight and height for age, marked pallor; flacidity & lethargy; sleepiness
Pathology: CBC: megaloblastic anemia; UA: increased orotic acid excretion with formation of orotic acid crystals.
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Pyrimidine Biosynthesis-IV
CPS II ATC DHO DHOD OA OPRT ODC UMP
AMP
PRPP
PRPP
Purine biosynthesis
+_
UTP_ _
Committed Steps
Eukaryote Prokaryote
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C02 + Glutamine + ATP
Carbamoyl Phosphate
CTP
ATP
Rat
e
[Aspartate]
Carbamoyl Asparate
Inhibited by CTP
UMP
CTP
UTP
ATCase is feedback inhibited by the end-products of pyrimidine biosynthesis
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Ribonucleotides to Deoxyribonucleotides
1. Ribonucleotide Reductase
2. Thymidylate Synthase: (prevent incorporation into RNA)
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Ribonucleotide reductase
Thioredoxin reductase
Inhibited by dATP; Activated by ATP
Ribonucleotides to Deoxyribonucleotides
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ADP UDPCDPGDP
Ribonucleotide Reductase
dADP dGDP dCDP dUDP
ATP
ADP
ATP
ADP
ATP
ADP
dUTP
ATP
ADP
dUMP
H20
PPi
dTDP
dTTP
ATP
ADP
ATP
ADP
5,10 THF
DHF
dATP dGTP dCTP
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dUMP TDP
N5,N10-methylene-tetrahydrofolate
Dihydrofolate
Tetrahydrofolate
Thymidylate synthase
Dihydrofolate reductase
NADPH
NADP+
reducedoxidized
Serine transhydroxymethylase
Thymidine biosynthesis
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Salvage & degradation of pyrimidines
uridine-cytidine kinase: nucleoside to nucleotide(deoxycytidine kinase)
(thymidine kinase)
Salvage:
Degradation: pyrimidine rings cleaved and degraded to soluble structures
(contrast to purines)
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F
Uracil 5-Fluoro-Uracil (5FU)
5FU is a simple derivative of Uracil
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Targets of drug therapy
NADP+
N5,N10-methylene-tetrahydrofolate
dUMP dTMP
Dihydrofolate
Tetrahydrofolate
Thymidylate synthase
Dihydrofolate reductase
NADPH
reducedoxidized
MethotrexateAminopterin
Fluorodeoxyuradylate (5-FU)
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Conversion of Serine to Glycine
N
N
N
NH2N
OH
CH2NHR
H
H
NH
N CH2
NH2C
Folate
Tetrahydrofolate (FH4)
Dihydrofolate reductase
N5, N10-Methylene FH4
NH3+H
CH2OH
CO 2-
C Serine
NH3+H
H
CO 2-
CGlycine
Serine hydroxymethyltransferase (PLP-dep.)
Key intermediatein biosynthesis ofpurines andformation ofthymine Important in
biosynthesis of heme,porphyrins, and purines
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Using nucleotides for selecting hybrid cells
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3’ AZido-2’3’ dideoxyThymine (AZT)
AZT inhibits HIV reverse transcriptase (RNA-dependent DNA polymerase)
This class of compounds (chemotherapeutics, viral inhibitors, etc.) are called nucleoside analogs.
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Bottom Line
Recognize names and structures of pyrimidines; NMPs/dNTPs Orotate, Uracil, Cytosine, Thymine; CTP/dCTP, TTP
Name the sources of atoms in the pyrimidine ring: carbamoyl phosphate (C,N: from Gln, CO2 ); Aspartate (C,N)
Recognize the regulated reaction: Carbamoyl phosphate synthase II: UTP; ATP, PRPP Contrast the synthesis of purines & pyrimidines
Explain the cause of Orotic aciduria; Contrast with hyperuricemia Explain mechanisms of the following treatments: sulfonamides, methotrexate, 5-Fluorouracil