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Roles of nucleotides
- monomeric units of nucleic acids
- energy currency (ATP), activated intermediates
- regulation - adenosine controls coronary blood flow, cAMP&cGMP signal, allosteric regulators
- coenzyme components (NAD+, FAD, CoA)
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Purine metabolism
- synthesized on PRPP
- first nucleotide: IMP
- required energy
- degraded to uric acid
- salvaged thru base
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Pyrimidine metabolism
- synthesized then added to PRPP
- first nucleotide: UMP
- requires energy
- degraded to aa, NH3, CO2
- salvaged thru nucleoside
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pyrimidine salvage
- while ingested purines are degraded to uric acid, pyrimidines are absorbed as nucleosides
- de novo synthesis is obligatory for Toxo, P. falciparum and HIV
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PRPP
- Phosphoribosylpyrophosphate - used for purine and pyrimidine synthesis
- Ribose-5-P + ATP -PRPP synthetase-> PRPP
- regulated step
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de novo synthesis pathway
- dietary supply inadequate
- LIVER (other cells too)
- Purine ring built on PRPP
- Pyrimidine built then added to PRPP
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Purine de novo biosynthesis committed step
- PRPP + Gln -amidophosphoribosyl transferase-> 5-phosphoribosylamine + Glu + PPi
- cancer cells love Gln - this is why
- regulated by final product/feedback inhibition
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How are ribose v. deoxyribose nucleotides made?
- ribose made first - IMP -> GMP or AMP -> GDP or ADP -> deoxy forms
- in other words, the diphosphate form of ribose is converted into deoxyribose
- remember aa synth is directly tied to nucleotide synth
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GOUT
- primary: familial, overactivity of PRPP synthetase, Lesch-Nyhan
- secondary: meds, incr. cell death (chemo), chronic kidney failure
- hyperuricemia
- arthritis: usually occurs in one joint at a time - common in big toe
- urinary tract stones
- tophi - visible crystals
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Prevention and treatment of gout
- avoid foods rich in purines: seafood, gravy, red meat, EtOH
- NSAIDs, corticosteroids - prevent inflammation
- allopurinol - blocks xanthine oxidase, decrease uric acid formation, buildup of xanthine ok b/c soluble
- colchicine - microtubule inhibition, decreases inflammation
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Pseudogout
- Deposition of calcium pyrophosphate crystals
- Caused by excessive Ca2+ in blood - overactive thyroid gland, hemochromatosis
- treat with colchicine, NSAIDs, corticosteroids (not allopurinol!)
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Lesch-Nyhan syndrome
- HGPRT deficiency: no purine salvage
- uric crystals, gout-like
- neuro defects (HGPRT expressed in brain)
- aggressive behavior, compulsive self-mutilation
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HGPRT
- purine salvage
- hypoxanthine or guanine + PRPP -hypoxanthine phosphoribosyl transferase-> IMP or GMP
- deficiency - Lesch-Nyhan, gout-like symptoms due to uric acid buildup
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SCID
- severe combined immunodeficiency
- B and T cell defects due to lack of ADA (adenosine deaminase)
- d-adenosine can't be converted to d-inosine, dATP builds up and inhibits synth of DNA
- treatment: BMT, stem cell
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UMPS
- pyrimidine de novo synth
- UMP synthetase
- activates 5-FU, chemo agent, and allows it to inhibit DNA synth
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Oroticaciduria
- orotate + PRPP -> UMP
- UMPS deficiency
- orotate accumulates, very rare
- anemia, retarded growth, immunodeficiency
- treatment: uridine (uracil doesn't help b/c defective enz would be needed to attach ribose)
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PNP
- purine nucleoside phosphorylase
- deficiency leads to incr. levels of dGTP
- inhibits RNR, toxic to immune system
- not as severe as SCID/ADA deficiency
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RNR
- ribonucleotide reductase
- converts NDPs -> dNDPs
- NADPH is e- source - comes from pentose phosphate shunt
- lack of any dNTP = lethal
- excess of dNTP = mis-incorporation
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essential aa's
PVT TIM HALL
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non-essential aa synthesis
- transamination of alpha-ketoacids
- most use B6 as a cofactor
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positive and negative nitrogen balance
- positive: intake > excretion => growth, pregnancy, tissue repair
- negative: excretion > intake => starvation, malnutrition, illness, surgery, burns
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Thymine
- dUMP + CH3 -thymidylate synthase-> dTMP
- THF = source of methyl, oxidized to DHF
- need to minimize [dUTP] in cell b/c DNA pol doesn't efficiently differentiate between dUTP and dTTP
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thymidylate synthase
- dUMP->dTMP
- blocked by FdUMP (5FU activated by UMPS), suicide inhibitor
- cancer cells will upregulate this pathway, be more affected
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DHFR
- dihydrofolate reductase
- DHF -> THF (uses NADPH)
- blocked by antifolates: methotrexate, aminopterin, trimethoprim
- latter 2 are antibiotics that bind to bacterial DHFR best
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Metabolic roles of folate
- One-carbon unit metabolism
- Purine (formyl-THF) and pyrimidine synthesis (methylene-THF)
- Synthesis of the methyl donor S-adenosylmethionine (SAM)
- Needed for the conversion of homocysteine to methionine (N5-methyl-THF)
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PABA
- bacteria can synthesize folic acid using para-amino benzoic acid (PABA)
- sulfonamide is an analog that inhibits folic acid synth = antibiotic
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Only rxn in humans that requires THF and vit B12
- methionine synthase
- homocysteine -> methionine
- vit B12 deficiency: megaloblastic anemia; pernicious anemia; paresthesias; peripheral neuropathy; irritability; dementia; depression; psychosis
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6-Mercaptopurine (6-MP)
- inhibits amidophosphoribosyltransferase
- 6MP + PRPP -HGPRT-> 6MP-ribose-P +PPi
- (converted to the nucleotide by HGPRT)
- childhood leukemia
- other derivs are immunosuppressants
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Cytosine Arabinose (araC)
- analog of CTP
- incorporates into DNA, inhibits chain elongation
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AZT
- azido-3-deoxythymidine
- pyrimidine analog
- viral pol more error prone and sensitive than host DNA pol
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Acyclovir
- only activated by viral thymidine kinase
- acts as analog, stops replication
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