Flashcards in Lecture 51 Deck (25)
What are four pyrimidines?
Cytosine (2 oxy, 4 amino)
Thymine (2,4 dioxy 5-methyl)
Uracil (2,4 dioxy)
Orotic Acid (2,4 dioxy 6 carboxy)
What are the three forms of pyrimidines?
1) Base [cytosine]
2) Nucleoside (Base + Ribose) [cytidine]
3) Nucleotide (Base + Ribose + PO4 ester) [cytidine monophosphate]
What is the 1st step in pyrimidine synthesis de novo?
1) Glutamine + HCO3- + 2ATP → Glutamate + Carbamoyl phosphate + 2 ADP + Pi
2) Uses Carbamoyl phosphate synthetase 2
3) Occurs in the cytosol
What are activators and inhibitors of the 1st step of pyrimidine synthesis (of Carbamoyl phosphate synthetase 2 in the cytosol)?
1) Inhibited by UTP
2) Activated by ATP
3) Activated by PRPP (5-phosphoribosylpyrophosphate)
Describe the reactions catalyzed by CAD
1) Glutamine + 2 ATP + CO2 → Carbamoyl phosphate + 2 ADP + Pi + Glutamate (uses Carbamoyl phosphate synthase 2)
2) Carbamoyl phosphate + Aspartate → Carbamoyl aspartate + Pi (uses aspartate transcarbamylase)
3) Carbamoyl aspartate → Dihydroorotate + H2O (uses Dihydroorotase)
4) CAD = Carbamoyl phosphate synthase 2 + Aspartate transcarbamylase + Dihydroorotase (1 polypeptide, 3 domains, 3 activities)
What is different between pyrimidine and purine synthesis?
Pyrimidines are synthesized as a free ring while purines are not
What happens to Dihydroorotate once it is formed?
1) It is oxidized to Orotate by reducing NAD+ to NADH
2) Uses Dihydroorotate dehydrogenase
What contributes to the different parts of a pyrimidine base?
1) Glutamine donates N
2) Aspartate donates 2/3rds of the ring with 3 Cs and 1 N
3) CO2 donates a C
What happens to Orotate once it is formed?
1) Orotate + PRPP → Orotidine 5’-monophosphate (OMP) + PPi (uses Orotate phosphoribosyl transferase
2) OMP → Uridine 5’-monophosphate (UMP) + CO2 (uses OMP decarboxylase)
What does UMP synthase consist of?
1) Orotate phosphoribosyl transferase
2) OMP decarboxylase
3) 1 polypeptide, 2 domains, 2 activities
What is low UMP synthase activity indicative of?
Orotic aciduria (abnormal growth, megaloblastic anemia, & treated with uridine-rich diet)
How is CTP made from UTP?
UTP + ATP + Glutamine → CTP + ADP + Pi + Glutamate
How are ribonucleotides converted to deoxyribonucleotides?
1) Ribonucleoside diphosphate + Thioredoin (2 SH) (reduced) → Deoxyribonucleoside diphosphate + H2O + Thioredoxin (S-S) (oxidized) (uses ribonucleotide reductase)
2) Thioredoxin (S-S) (oxidized) + NADPH + H+ → Thioredoxin (2SH) (reduced) + NADP+ (uses Thioredoxin reductase)
What inhibits & activates the conversion of ribonucleotides to deoxyribonucleotides?
1) Inhibited by dATP
2) Activated by ATP
How does the rate limiting step differ between prokaryotes and eukaryotes in pyrimidine synthesis?
1) In Eukaryotes: The Rate limiting step is the reaction catalyzed by Carbamoyl phosphate synthetase 2
2) In Prokarotes: The rate limiting step is the reaction catalyzed by aspartate transcarbamylase
What inhibits and activates the rate limiting steps in pyrimidine synthesis?
1) UTP inhibits the rate limiting step in eukaryotes (using carbamoyl phosphate synthetase 2)
2) UTP inhibits the rate limiting step in prokaryotes (using aspartate transcarbamylase)
3) ATP activates CPS2
4) PRPP activates CPS2
What inhibits Aspartate transcarbamylase (ATCase)?
The endproducts of pyrimidine biosynthesis inhibit ATCase (e.g. CTP)
What is ADA deficiency?
ADA deficiency is due to a lack of the enzyme adenosine deaminase. This deficiency results in an accumulation of deoxyadenosine, which, in turn, leads to a buildup of dATP in all cells, which inhibits ribonucleotide reductase and prevents DNA synthesis, so cells are unable to divide. Since developing T cells and B cells are some of the most mitotically active cells, they are highly susceptible to this condition.
What reaction does ribonucleotide reductase catalyze?
1) ADP → dADP → dATP
2) GDP → dGDP → dGTP
3) CDP → dCDP → dCTP
4) UDP →dUDP → dUTP → dTTP
How is thymidine produced?
1) dUMP + reduced N5,N10-methylene-tetrahydrofolate → TMP + oxidized Dihydrofolate (Thymidylate synthase)
2) Dihydrofolate + NADPH → Tetrahydrofolate + NADP+ (Dihydrofolate reductase)
3) Tetrahydrofolate + Serine → N5,N10-methylene-tetrahydrofolate + Glycine (Serine transhydroxymethylase)
What is the salvage reaction for a pyrimidine?
Uridine-cytidine kinase (deoxycytidine kinase; thymidine kinase) converts nucleosides to nucleotides
What is the degradation reaction for a pyrimidine?
Pyrimidine rings are cleaved and degraded to soluble structures (contrast to purines)
What is an example of drug therapy to reduce over producton of thymidine (TMP)?
1) Fluorodeoxyuridylate (5-FU) down regulates Thymidylate synthase. 5FU is a simple derivative of Uracil (5-Fluoro-Uracil)
2) Methotrexate & Aminopterin down regulates Dihydrofolate reductase
What is the function of AZT?
AZT inhibits HIV reverse transcriptase (RNA-dependent DNA polymerase). This class of compounds (chemotherapeutics, viral inhibitors, etc.) are called nucleoside analogs