WEEK 12 (Pyrimidine Metabolism)

Question 1

What is the goal of Pyrimidine Synthesis?

Answer 1

To create CMP (Cytidine Monophosphate), UMP (Uracil Monophosphate) & TMP (Thymidine Monophosphate)

Question 2

What are the ingredients of Pyrimidine Synthesis?

Answer 2

• Ribose phosphate (HMP Shunt)
• Amino acids
• Carbons (Tetrahydrofolate, CO2)

Question 3

Describe the beginning of Pyrimidine synthesis

Answer 3

1) CARBAMOYL PHOSPHATE is formed
2) CARBAMOYL ASPARTATE is formed from CARBAMOYL PHOSPHATE and ASPARTATE catalysed by ATCase (ASPARTATE TRANSCARBAMOYLASE)

Question 4

How does ATCase represent feedback control in enteric bacteria?

Answer 4

ATCase is inhibited by the end product CTP and is activated by ATP

Question 5

Where does Pyrimidine synthesis take place?

Answer 5

The cytoplasm of the cell

Question 6

What is the importance of Carboxyl phosphate?

Answer 6

• Found in the cytoplasm
• Catalyses 3 steps therefore contains 3 different active sites
• Utilises Bicarbonate, ATP molecules & Glutamate to form Carbamoyl phosphate

Question 7

Describe the stages of Pyrimidine synthesis

Answer 7

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

Question 8

What is the activator and inhibitor of CPS II (Carbamoyl phosphate synthetase II)?

Answer 8

Activator = PRPP
Inhibitor = UTP (Uridine Triphosphate)

Question 9

How is Carbamoyl Phosphate synthesised?

Answer 9

Carbamoyl Phosphate is synthesised from GLUTAMINE and CO2 and is catalysed by CARBAMOYL PHOSPHATE SYNTHETASE (CPS) II

Question 10

Where is CPS II found?

Answer 10

Cytosol

Question 11

What are the differences between CPS I and CPS II

Answer 11

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

Question 12

What are the sources of the atoms in the pyrimidine ring?

Answer 12

• Glutamine
• CO2
• Aspartate

Question 13

Describe the De novo synthesis of Pyrimidines

Answer 13

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

Question 14

What is the difference in functions of Nucleoside monophosphate kinase and Nucleoside diphosphate kinase?

Answer 14

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

Question 15

Describe how CTP is formed from UTP

Answer 15

CTP (CYTIDINE TRIPHOSPHATE) is produced by AMINATION of UTP by CTP SYNTHETASE with glutamine providing the nitrogen

Question 16

What happens to some CTP molecules once formed?

Answer 16

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

Question 17

Describe Ribonucleotide Reductase

Answer 17

• Required for the formation of all four deoxyribonucleotides for DNA synthesis
• The nucleotide substrates must be diphosphate
• Inhibited by dADP and dATP

Question 18

What are the properties of Hydroxyurea?

Answer 18

• 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

Question 19

What is dUMP converted to?

Answer 19

dUMP is converted to DEOXYTHYMIDINE MONOPHOSPHATE (dTMP) by THYMIDYLATE SYNTHASE which uses N5, N10 METHYLENE-THF as the source of the methyl group

Question 20

What are the properties of Methotrexate?

Answer 20

• Chemotherapy agent & Immunosuppressant
• Mimics DHF
• Inhibits dihydrofolate reductase
• Blocks synthesis of dTMP
• Rescues with Leucovorin

Question 21

What are the properties of the inhibition of Thymidylate Synthase?

Answer 21

• 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

Question 22

Can Pyrimidine nucleotides also be synthesised by salvage pathways?

Answer 22

Yes

Question 23

Why are the catabolic pathways for pyrimidines simpler than those for purines?

Answer 23

Because the intermediates are relatively soluble, there are few known derangements of pyrimidine breakdown

Question 24

B-alanine is used in the biosynthesis of which enzyme?

Answer 24

Coenzyme A

Question 25

Describe the Salvage pathway for UMP, CMP, TMP and dCMP

Answer 25

Uridine + ATP + Uridine Cytidine Kinase = UMP

Cytidine + ATP = CMP

Deoxythymidine + Thymidine Kinase + ATP = TMP

Deoxycytidine + ATP = dCMP

Question 26

What does the low activity of Orotate Phosphoribosyltransferase and OMP Decarboxylase cause?

Answer 26

• Poor growth
• Megaloblastic anemia
• Excretion of large amounts of rotate in the urine

Question 27

What does the administration of Uridine result in?

Answer 27

Improvement of the anaemia and decreased excretion of orotate