Lecture 64 Flashcards
Nucleotide Metabolism Pt II
1
Q
synthesis of deoxyribonucleotides
A
- ribonucleotides (GDP, ADP, CDP, UDP) converted to deoxyribonucleotides (dGDP, dADP, dCDP, dUDP)
- utilizes ribonucleotide reductase for ALL reactions
- this reduces the -OH group and releases water
pg 1629-1630
2
Q
ribonucleotide reductase structure
A
- 2 large subunits → R1 and R2 (sometimes α and β)
- cysteine residues donate the H (but then form disulfide bonds causing the enzyme to stop working)
- 2 substrate specificity sites with tight regulation → ATP, dATP, dTTP, dGTP regulation the reduction of specific ribonucleotides
- activity sites: ATP activates the enzyme (high energy), dATP inhibits the enzyme (feedback → no more need)
- sites are all on R1 subunit
pg 1631, 1633-1636
3
Q
synthesis of deoxyribonucleotides pt 2
A
- thioredoxin is a coenzyme for ribonucleotide reductase which allows the disulfide bond to be broken
- when oxidized, the disulfide bond exists
- when reduced by thioredoxin reductase (using NADPH), the disulfide bond is broken and it can partake in the reaction again
pg 1632
4
Q
inhibitors of dNDP synthesis: hydroxyurea
A
- aka hydroxycarbamide
- inhibits ribonucleotide reductase and therefore inhibits generation of substrates for DNA synthesis
- antineoplastic agent used in the treatment of cancers such as melanoma
- also used in treatment of sickle cell anemia, but the increase in fetal Hb seen is because of changes in gene expression and NOT due to ribonucleotide reductase inhibition
pg 1637
5
Q
conversion of uracil to thymine
A
- our bodies have no need for dUDP since uracil is not in DNA
- need to add the methyl group to form dTDP
- dUDP first must be converted to dUMP (release a free phosphate) and then thymidylate synthase converts to dTMP
- dTMP phosphorylated to form dTDP
- thymidylate synthase requires a 1-carbon donor (N5, N10-methylene-FH4) to form dTMP and leaves behind FH2
- FH2 is converted back to FH4 by dihydrofolate reductase using NADPH
pg 1638-1639
6
Q
inhibitors of thymidylate synthase
A
- thymine analogs like 5-fluorouracil which serve as antitumor agents
- metabolically converted to 5-FdUMP which becomes permanently bound to the active site of thymidylate synthase, making the drug a suicide inhibitor
- methotrexate inhibits dihydrofolate reductase therefore preventing the 1-carbon donor from forming
pg 1640-1641
7
Q
digestion of nucleic acids
A
- several pancreatic enzymes play a role: ribonucleases, deoxyribonucleases, and phosphodiesterases
- nucleosides are taken into enterocytes by sodium-dependent transporters and degraded to free bases and the respective sugar 1-phosphate
- dietary purine bases are NOT used for the synthesis of tissue nucleic acids, but are degraded to uric acid in the enterocytes
- most of the uric acid enters the blood and is excreted in the urine
- dietary pyrimidine bases enter the circulation
- nucleosidases in intestinal cells break the nucleosides down into (deoxy)ribose 1-phosphate and their purine/pyrimidine base
pg 1643
8
Q
degradation and salvage of pyrimidines
A
- no medical importance as the process causes no diseases
- the pyrimidine ring is opened and degraded into highly soluble products: CMP and UMP → β-alanine, TMP → β-aminoisobutyrate, ammonia, and CO2
- pyrimidine bases can be salvaged to nucleosides, which are phosphorylated to nucleotides
- because of their high solubility, pyrimidine salvage is less significant clinically than purine salvage → easily removed or shuttled into different pathways
- salvage of pyrimidine nucleosides is the basis for using uridine in the treatment of hereditary orotic aciduria
pg 1644
9
Q
degradation of purine nucleotides
A
- GMP releases Pi to become guanosine, guanosine releases ribose 1-phosphate to become guanine, guanine releases free ammonia to become xanthine
- AMP releases free ammonia to become IMP, IMP releases Pi to become inosine, inosine releases ribose 1-phosphate to become hypoxanthine, hypoxanthine uses oxygen and xanthine oxidase to release H2O2 and become xanthine (in an oxidation reaction)
- accumulation of xanthine causes clinical symptoms (insoluble)
- xanthine uses oxygen and xanthine oxidase to release H2O2 and become uric acid which is soluble and excreted in urine
- uric acid is the final product of purine degradation
pg 1645-1646
10
Q
salvage of purine nucleotides
A
- recycle of purine bases from normal turnover and diet
- very important in the brain
- principal source of nucleotides in resting T-lymphocytes
- important for many intracellular microbes
- AMP and IMP can be interconverted
- IMP can be converted to GMP
pg 1647
11
Q
purine salvage deficiencies: Lesch-Nyhan syndrome
A
- rare, X-linked recessive disorder
- complete deficiency of HGPRT (converts guanine to GMP and hypoxanthine to IMP)
- results in: increase of PRPP, decrease of IMP, GMP; increase of de novo synthesis; increased degradation/turnover; accumulation of large amounts of uric acid
pg 1648
12
Q
Lesch-Nyhan syndrome clinical presentation
A
- hyperuricemia: formation of uric acid stones in the kidneys (urolithiasis) and deposition of urate crystals in joints (gouty arthritis) and fatty tissues
- motor dysfunction
- cognitive deficits
- behavioral disturbances that include self-mutilation (biting of lips and fingers)
pg 1649
13
Q
purine salvage deficiencies: severe combined immunodeficiency syndromes (SCID)
A
- integrity of the purine salvage pathway is critical for normal differentiation and function of immmunocompetent cells
- about 15% of SCID cases are due to deficiency of adenosine deaminase (converts adenosine to inosine) → rare AR
- about 4% of cases are due to deficiency of purine nucleoside phosphorylase (PNP) (converts guanosine to guanine and inosine to hypoxanthine) → only 70 cases described
pg 1650-1651
14
Q
adenosine deaminase (ADA) deficiency
A
- causes a type of SCID involving T-cell, B-cell, and natural killer cell depletion (lymphocytopenia)
- untreated ADA-deficient children usually die before age 2 from overwhelming infection
- treatments include BMT, ERT, and gene therapy
pg 1651
15
Q
purine nucleoside phosphorylase (PNP) deficiency
A
- rarer and less severe than ADA deficiency
- primarily affects T cell development
- PNP-deficient individuals have recurrent infections and neurodevelopmental delay
pg 1651
