9. nucleotide metabolism

ribose
2’C attached to OH
deoxyribose
2’C attached to H
PRPP
common sugar/phosphate compound to which all the major bases are attached. formed from ribose 5p by PRPP synthetase, which is activated by Pi and inhibited by purine nucleotides. purines form directly on it; pyrimidines form elsewhere and then attach to it.
synthetase
requires ATP
synthase
does not require ATP
all newly synthesized nucleotides are
ribonucleoside monophosphates, NMP
conversion of NMP to NDP
specific kinases that typically use ATP as a phosphate donor (same for deoxy) = adenylate kinase, guanylate kinase, etc.
conversion of NDP to NTP
less specific enzyme than NMP->NDP (same for deoxy) = nucleoside diphosphate kinase
ribonucleotide reductase
converts NDP to dNDP. donates necessary H, which it gets from thioredoxin, which gets H from NADPH.
ribonucleotide reductase regulation
allosterically by ATP (increased), dATP (decreased), dTTP, and dGTP (these two are specific)
purine synthesis (precursors and step 1)
CO2, several AAs (glycine, glutamine, aspartate), and N10-formylTHF donate carbons. first step = transamination: glutamine donates an amine group to PRPP, synthesizing 5’phosphoribosylamine (committed step). >>> hypoxanthine>IMP>AMP or GMP.
regulation of synthesis of 5’phosphoribosylamine
regulated by availability of PRPP. inhibitory feedback from GMP, AMP and IMP. energy from ATP is needed, carbons from N10formylTHF are needed.
regulation of IMP>GMP or AMP
both require 2 steps. both 1st steps end-product inhibited. ATP is converted to AMP to make GMP and vice versa.
HGPRT
salvages IMP and GMP purine bases, attaching them to PRPP and converting them to NTP.
APRT
salvages AMP purine base, attaching it to PRPP and converting it to ATP.
Lysch-Nyhan syndrome
x-linked, nearly complete deficiency of HGPRT. PRPP, hypoxanthine and guanine increase, IMP and GMP decrease. this stimulates purine synthesis = more hypoxanthine and guanine = uric acid = kidney stones, gouty arthritis and soft tissue deposition. motor dysfunction, cognitive defects, self-mutilation.
purine degradation
dietary purines are typically degraded in intestine, not used to make nucleic acids. AMP>IMP. IMP and GMP lose phosphate groups, ribose is removed yielding hypoxanthine and guanine, both converted to xanthine and then uric acid.
Gout / diagnosis
hyperuricemia = acute and chronic arthritis (MTP joint of big toe, soft tissues = tophi). aspiration of synovial fluid is needed for diagnosis, shows crystals under polarized light microscopy.
gout causes
most often caused by underexcretion (bad kidney function = secondary OR specific idiopathic problem = primary). may also be caused by overproduction of uric acid. most often idiopathic, but gene defect = lysch-nyhan. patients with high rates of cell turnover or metabolic disease may also become hyperuricemic.
gout treatment
acute – antiinflammatory = ASA, colchicine (depolymerizes microtubules and thus decreases neutrophil movement). allopurinol and xanthine oxidase inhibit uric acid synthesis. probenecid and sulfinpyrazone increase renal excretion of uric acid.
ADA deficiency
adenosine deaminase (converts to inosine to initiate degradation pathway) defect leads to build-up of adenosine. increased dATP inhibits ribonucleotide reductase. B and T and NK cells compromised = SCID (severe combined immunodeficiency disease). death by age 2 without treatment.
pyrimidine synthesis
1. regulating step: cytosolic carbamoyl phosphate synthetase II. inhibited by UTP, activated by PRPP and ATP. next convert carbamoyl phosphate to orotate which is attached to PRPP to make OMP > UMP. UTP>CTP, dUMP>dTMP (with help of N5N10formylTHF).
pyrimidine salvage
mostly degraded into highly soluble products
dietary folate>
dihydrofolate (DHF) > tetrahydrofolate (THF) via dihydrofolate reductase (DHFR).
THF
capable of reversibly accepting a carbon: purine synthesis, dTMP synthesis, methionine regeneration
methotrexate
DHFR inhibitor = cancer treatment
sulfonamides
antimicrobial: competitively inhibits bacterial DHPS, which is necessary for bacterial THF synthesis
trimethoprim
inhibits bacterial DHFR = anti-microbial
pentose monosaccharide
forms sugar backbone of nucleotide
CPS I vs. II: cellular location, pathway, nitrogen source
I: mitochondria, urea cycle, ammonia. II: cytosol, pyrimidine synthesis, glutamine
pernicious anemia
when b12 deficient, n5methylTHF can’t make THF and homocysteine can’t make Met. system wants to make THF so everything backs up at n5methylTHF. is a type of megaloblastic anemia. mechanism? rbcs can’t replicate, no dTMP?
inadequate folate can be caused by
dietary insufficiency, increased demand by lactation and pregnancy, poor absorption in the intestine, drugs like methotrexate
spina bifida
low THF in first few weeks of fetal life; comes with anencephaly. is defective neural tube closing.
megaloblastic anemia
low THF halts DNA synthesis in bone marrow – b12 required, causes pernicious anemia when defective which is similar to megaloblastic anemia, but won’t respond to folate supplementation!
5-fluorouracil
thymidine analog, permanently binds and inhibits thymidylate synthase, blocking dUMP>dTMP
cAMP, NAD, FAD, ATP
all nucleotides! remember the formula for this
what is the common NMP precursor to the pyrimidines
OMP, then UMP
major clinical consequences of insufficient folate
megaloblastic anemia in adults, anencephaly and spina bifida in developing fetus

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