WEEK 12 (Pyrimidine Metabolism) Flashcards
What is the goal of Pyrimidine Synthesis?
To create CMP (Cytidine Monophosphate), UMP (Uracil Monophosphate) & TMP (Thymidine Monophosphate)
What are the ingredients of Pyrimidine Synthesis?
- Ribose phosphate (HMP Shunt)
- Amino acids
- Carbons (Tetrahydrofolate, CO2)
Describe the beginning of Pyrimidine synthesis
1) CARBAMOYL PHOSPHATE is formed
2) CARBAMOYL ASPARTATE is formed from CARBAMOYL PHOSPHATE and ASPARTATE catalysed by ATCase (ASPARTATE TRANSCARBAMOYLASE)
How does ATCase represent feedback control in enteric bacteria?
ATCase is inhibited by the end product CTP and is activated by ATP
Where does Pyrimidine synthesis take place?
The cytoplasm of the cell
What is the importance of Carboxyl phosphate?
- Found in the cytoplasm
- Catalyses 3 steps therefore contains 3 different active sites
- Utilises Bicarbonate, ATP molecules & Glutamate to form Carbamoyl phosphate
Describe the stages of Pyrimidine synthesis
1) Bicarbonate molecule is phosphorylated via CARBAMOYL PHOSPHATE SYNTHETASE II to form CARBOXYPHOSPHATE which has an additional negative charge making it more reactive
2) ASPARTATE is added onto CARBOXYPHOSPHATE to form OROTIC ACID
3) OROTIC ACID is phosphorylated via OROTATE PHOSPHORIBOSYLTRANSFERASE and decarboxylated via OROTIDYLIC ACID DECARBOXYLASE (UMP SYNTHASES) to form UMP
4) CTP SYNTHASE converts UMP to CTP and THYMIDYLATE SYNTHASE converts UMP to TMP
What is the activator and inhibitor of CPS II (Carbamoyl phosphate synthetase II)?
Activator = PRPP
Inhibitor = UTP (Uridine Triphosphate)
How is Carbamoyl Phosphate synthesised?
Carbamoyl Phosphate is synthesised from GLUTAMINE and CO2 and is catalysed by CARBAMOYL PHOSPHATE SYNTHETASE (CPS) II
Where is CPS II found?
Cytosol
What are the differences between CPS I and CPS II
CPS I
LOCATION: Mitochondria
PATHWAY INVOLVED: Urea cycle
SOURCE OF NITROGEN: Ammonia
REGULATORS: Activator is N-acetyl-Glutamate
CPS II
LOCATION: Cytosol
PATHWAY INVOLVED: Pyrimidine synthesis
SOURCE OF NITROGEN: gamma amide group of glutamine
REGULATORS: Activator is PRPP & Inhibitor is UTP
What are the sources of the atoms in the pyrimidine ring?
- Glutamine
- CO2
- Aspartate
Describe the De novo synthesis of Pyrimidines
De novo synthesis of pyrimidines occurs in the cytoplasm and begins with aspartate, carbon dioxide and glutamine. A CPS (Carbamoyl Phosphate Synthetase) creates Carbamoyl phosphate which aspartate is added to create orotic acid. The principal end product is UMP which is modified to form CTP and TMP
What is the difference in functions of Nucleoside monophosphate kinase and Nucleoside diphosphate kinase?
NUCLEOSIDE MONOPHOSPHATE KINASE catalyses transfer of an inorganic phosphate to UMP to form UDP
NUCLEOSIDE DIPHOSPHATE KINASE catalyses transfer of an inorganic phosphate from ATP o UDP to form UTP
Describe how CTP is formed from UTP
CTP (CYTIDINE TRIPHOSPHATE) is produced by AMINATION of UTP by CTP SYNTHETASE with glutamine providing the nitrogen
What happens to some CTP molecules once formed?
CTP is dephosphorylated to CDP which is a substrate for RIBONUCLEOTIDE REDUCTASE. dCDP product can be phosphorylated to dCTP for DNA SYNTHESIS or dephosphorylated to dCMP that is deaminated to dUMP
Describe Ribonucleotide Reductase
- Required for the formation of all four deoxyribonucleotides for DNA synthesis
- The nucleotide substrates must be diphosphate
- Inhibited by dADP and dATP
What are the properties of Hydroxyurea?
- Inhibits ribonucleotide reductase
- Blocks formation of deoxynucleotides
- Rarely used for malignancy
- Used for Polycythemia Vera (blood cancer that causes bone marrow to make too many red blood cells)
- Used in Sickle cell anemia
- Causes increased foetal haemoglobin
What is dUMP converted to?
dUMP is converted to DEOXYTHYMIDINE MONOPHOSPHATE (dTMP) by THYMIDYLATE SYNTHASE which uses N5, N10 METHYLENE-THF as the source of the methyl group
What are the properties of Methotrexate?
- Chemotherapy agent & Immunosuppressant
- Mimics DHF
- Inhibits dihydrofolate reductase
- Blocks synthesis of dTMP
- Rescues with Leucovorin
What are the properties of the inhibition of Thymidylate Synthase?
- Includes thymine analogs such as 5-FLUOROURACIL which serve as antitumor agents
- 5-fluouracil is metabolically converted to 5-fluorodeoxyuridine monophosphate (5-FdUMP) which become permanently bound to the inactivated thymidylate synthase, making the drug a SUICIDE INHIBITOR
Can Pyrimidine nucleotides also be synthesised by salvage pathways?
Yes
Why are the catabolic pathways for pyrimidines simpler than those for purines?
Because the intermediates are relatively soluble, there are few known derangements of pyrimidine breakdown
B-alanine is used in the biosynthesis of which enzyme?
Coenzyme A
Describe the Salvage pathway for UMP, CMP, TMP and dCMP
Uridine + ATP + Uridine Cytidine Kinase = UMP
Cytidine + ATP = CMP
Deoxythymidine + Thymidine Kinase + ATP = TMP
Deoxycytidine + ATP = dCMP
What does the low activity of Orotate Phosphoribosyltransferase and OMP Decarboxylase cause?
- Poor growth
- Megaloblastic anemia
- Excretion of large amounts of rotate in the urine
What does the administration of Uridine result in?
Improvement of the anaemia and decreased excretion of orotate
