1
Q
Purine bases
A
- Adenine
- Guanine
- Hypoxanthine
- Xanthine
2
Q
Pyrimidine bases
A
- Uracil
- Cytosine
- Thymine
3
Q
PRPP Synthesis
A
- From ribose-5-phosphate and ATP
- Catalyzed by PRPP Synthetase
4
Q
Sources of ribose-5-phosphate
A
- Glucose metab, pentose phosphate shunt
- Nucleoside degradation creates R1P which can be converted
5
Q
PRPP is required for:
A
- Purine and pyrimidine nucleotide de novo synthesis
- Salvage pathways for purine nucleotides
- Biosynthesis of NAD and FAD
6
Q
Synthesis of purine nucleotides
A
- Purine ring built on PRPP
- Precursors are glutamine, glycine, CO2, aspartate, and two 1 carbon fragments from one-carbon folate pool (THF)
7
Q
Sulfonamides
A
- Block the biosynthesis of folate by bacteria
- Blocks two steps in the production of IMP that require folate
8
Q
Components of IMP nucleotide ring
A
-Glutamine, glycine, CO2, aspartate, two one carbon fragments from the one-carbon folate pool
9
Q
Azaserine
A
- Analog of glutamine that blocks amide transfer from glutamine
- Interferes with two steps in IMP production that require glutamine transfer reactions
10
Q
Energy cost of making IMP
A
-Six high energy phosphate bonds
11
Q
Formation of AMP from IMP
A
- Adenylosuccinate formed by addition of aspartate to IMP
- Fumarate cleaved to yield AMP
- GTP cleaved in the process
12
Q
Formation of GMP from IMP
A
- XMP formed by oxidation of IMP
- Amino group from glutamine added to form GMP
- ATP cleaved in the process
13
Q
HGPRT
A
- Catalyzes formation of NTs from hypoxanthine or guanine (forms IMP and GMP respectively)
- Salvage pathway
- Uses PRPP
14
Q
APRT
A
- Catalyzes formation of AMP from adenosine
- Salvage pathway
- Uses PRPP
15
Q
Nucleoside kinase
A
- Salvage pathway only for adenosine
- Forms AMP and ADP using ATP
16
Q
Synthesis of pyrimidine nucleotides
A
- NOT built on PRPP. Synthesized first and then reacts with PRPP to form nucleotide
- Precursors are carbamoylphosphate and aspartate
17
Q
Formation of carbamoylphosphate in the production of UMP
A
-Carbamoylphosphate synthesized in the cytosol from glutamine and CO2. Also synthesized in liver as an intermediate in urea synthesis (takes place in mitochondria with a different enzyme).
18
Q
Orotic aciduria
A
- Genetic disorder of pyrimidine biosythnesis (the first enzyme)
- Orotic acid accumulates in blood and excreted in urine.
- Alleviated by feeding of uridine or cytidine: supplies the body with needed pyrimidine
19
Q
Pyrimidine synthesis
A
- Conversion of UMP to UTP Two steps
1. UMP Kinase
2. Nucleoside diphosphate kinase - Amino group from glutamine then donated to UTP to form CTP (activated by GTP)
20
Q
Uracil salvage pathway
A
- Uracil converted to UMP by two sequential reactions
1. Uridine phosphorylase: Uracil + Ribose-1-phospate –> Uridine + Pi
2. Uridine kinase: Uridine + ATP –> UMP + ADP
21
Q
Cytidine salvage pathway
A
-Can also be phosphorylated by uridine kinase
22
Q
Enzymes inhibited by azaserine
A
- Any that use glutamine as an amino donor
- CPSII
- CTP synthase
- FGAR aminotransferase
- GMP synthase
- PRPP aminotransferase
- Asparagine synthase
23
Q
When are deoxyribonucleotides sythesized?
A
-When cells are actively dividing and in S phase
24
Q
Formation of deoxynucleotides
A
- Formed from reduction of ribonucleoside diphosphates (ONLY)
- Reduction catalyzed by ribonucleotide reductase
25
Products of ribonucleotide reductase
-dCDP, dADP, dGDP, dUDP
26
Fate of dUDP
- Not incorporated into DNA so dephosphorylated to dUMP state
- dTMP formed from dUMP via thymidylate synthase
27
Thymidylate synthase reaction
- Forms dTMP from dUMP
| - Methylene group from N5N10 methylenetetrahydrofolate transferred and reduced to a methyl group
28
Regeneration of THF
- Via dihydrofolate reductase
- Requires NADPH
- Only made by cells in S phase
29
5-Fluorouracil
- Anticancer agent
- Converts to F-UMP in cells
- Converted to F-dUMP which is the critical form!
- F-dUMP interacts with thymidylate synthase and N5N10 methylenetetrahydrofolate
- Transfer of a methylene group to pyrimidine ring is blocked by fluorine so thymidylate synthase is trapped and can't synthesize dTMP
30
Methotrexate
- Anticancer agent
- Analog of folic acid
- Inhibits dihydrofolate reductase
- Regeneration of THF is blocked and synthesis of dTMP is inhibited
31
Salvage pathway for synthesis of thymine deoxyribonucleotides
- Thymine phosphorylase converts thymine to thymidine
| - Thymidine kinase converts thymidine to dTMP
32
Nucleotide degradation via dephosphorylation
- Nucleotidases remove 5' phosphates from ribo and deoxyribo nucleotides converting them to nucleosides
- AMP deaminated to IMP and then hydrolyzed to inosine or hydrolyzed to adenosine and then deaminated to inosine
- GMP hydrolyzed to guanosine
- CMP deaminated to UMP then hydrolyzed to uridine or hydrolyzed to cytidine then deaminated to uridine
- UMP hydrolyzed to uridine
33
Nucleotide degradation via phosphorolysis
- Nucleoside phosphorylases
- A bond is cleaved by adding Pi
- Form free base and R1P or dR1P
34
Purine degradation
- Final product is uric acid
- Hypoxanthine oxidized to xanthine by xanthine oxidase
- Guanine deaminated to xanthine
- Xanthine oxidized to uric acid by xanthine oxidase--O2 required and superoxide formed
35
Components of purine degradation
- Dephosphorylation
- Deamination
- Separation of base from ribose
- Oxidation to uric acid
36
Degradation of pyrimidines
- Final produces B-alanine (from uracil) and B-aminoisobutyrate from thymine
- Undergo same components as purine degradation
37
Cause of gout
- Precipitation of sodium urate crystals in joints and kidneys
- PRPP synthetase may be too active
- Partial deficiency of HGPRT leads to increased PRPP which leads to increased purine synthesis
38
Treatment of gout
- Allopurinol
- Blocks production of uric acid
- Oxidized by xanthine oxidase to oxypurinol which binds tightly to xanthine oxidase inhibiting its function
- Colchine may also be used
39
Allopurinol in cancer treatment
-Can help deal with the large quantities of nucleotides released from killed cancer cells
40
Lesch-nyhan syndrome
- Hereditary X linked recessive condition
- Severe or complete deficiency of HGPRT
- Increased synthesis of purines
- Hyperuricemia, gout, neurological symptoms (not sure why)
- Allopurinol won't relieve neurological symptoms
41
ADA deficiency (SCID)
- Adenosine deaminase deficiency
- deoxyadenosine and adenosine are abundant because cannot be degraded to inosine and deoxyinosine
- Inhibits ribonucleotide reductase which inhibits DNA synthesis
- White blood cells cannot proliferate
