Nucleotide Metabolism Flashcards
nucleoSIDE
ribose ring + nitrogenous base at 1’
nucleoTIDE
nucleoside + phosphate esterified at 5’
deoxyribonucledotides
have H instead of OH at 2’ of ribose ring
pyrimidine
one ring w/ 2 nitrogens
cut of py
purine
two ring structure each w/ 2 N’s
pur as gold
roles of nucleotides
- energy metabolism (ATP, GTP)
- nucleic acids (NTPs and dNTPs)
- coenzymes (NAD, FAD, coA)
- activated intermediates (UDP-glucose, GDP-mannose, SAM)
- allosteric modulators (AMP, ATP, cAMP, cGMP)
- physiological modulators (ADP, adenosine)
salvage pathways
what it do
allow generation of nucleotides from free bases and nucleosides from diet
since de novo synthesis energetically expensive
de novo synthesis
purines-basic
ring strucutre assembled on a base (PRPP)
de novo synthesis
purine pathway
- assemble PRPP, start w/ ribose-5-phosphate from PPP, w/ PRPP synthetase
- committed step, convert to PRA using glutamine
- make IMP w/ carbons and nitrogens from amino acids, carbon and oxygen from HCO3-, carbon from N10formyl THF, ATP hydrolysis
- convert IMP to AMP or GMP
- convert to ATP and GTP w/ NMPs aka adenylate kinase or guanylate kinase
GTP and ATP are readily interconverted
regulation of purine synthesis
- PRPP synthetase, feedback inhibition by ADP and GDP
- commited step (amidophosphoribosyltransferase) aka PRA by allosteric effectors
-inhibitors: AMP and GMP bc end prodcuts
-activators: PRPP
ADP and GDP are the purine nucleotides
purine degradation
GMP
@ liver
1. GMP to guanosine via 5’nucleotidase removing P
2. convert to free base (guanine) via purine nucleoside phosphorylase removing ribose
GMP>guanosine>guanine
purine degradation
AMP
route 1:
1. AMP to adenosine via 5’-nucleotidase remove P
2. convert to inosine via adenosine deaminase
3. convert to free base hypoxanthine via pruine nucleoside phosphorylase removing ribose
route 2:
1. AMP to IMP via AMP deaminase
2. IMP to inosine via 5’-nucleotidase dephosphorylating
3. convert to hypoxanthine via purine nucleoside phosphorylase
adenosine deaminase = lymphocytes
AMP deaminase = muscle
purine degradation
final steps
- hypoxanthine and guanine converted to xanthine
- xanthine oxidase converts it to uric acid
- excreted via kidneys
hyperuricemia
excess uric acid in blood
from reduced excretion (renal insufficiency, metabolic acidosis)
OR
inc production (inc nucleotide turnover aka sickle cell, G6DP deficiency, chemo, or purine rich diet)
alcohol does both
treat w/ allopurinol
gout
crystals of sodium urate deposit in joints of extremities (esp first toe)
bc uric acid is relatively insoluble and close to limit of solubility normally
treat with allopurinol
kidney stones
high concentrations of uric acid in urine lead to deposition of stones in kidney aka urolithiasis
10-25% ppl with gout also have stones
allopurinol
effective treatment for hyperuricemia
metabolizes to oxypurinol that competitively inhibits xanthine oxidase
=more hypoxanthine and xanthine that are more soluble than uric acid so excreted easier
de novo synthesis
pyrimidines-basic
start as synthesis of free bases then get transferred to ribose
synthesis of UMP
first pyrimidine made
- syn carbamoyl phosphate w/ glutamine + bicarb
- carbamoyl aspartate by adding aspartate
- cyclization = dihydroorotate
- dehydrogenation = orotate
- syn OMP w/ PRPP
- decarboxylate = UMP
CAD
single multifunctional protein w/ 3 distinct active sites for steps 1/2/3 of pryrimidine syn
Carbamoyl phosphate synthetase II
Aspartate transcarbamoylase
Dihydroorotase
UMP synthase
single multifunctional protein for last two steps of pyrimidine syn
orotate phosphoribosyltransferase
OMP decarboxylase
orotic aciduria
defect in UMP synthase so orotic acid accumlates in urine
symptoms of megaloblastic anemia and maybe cellular immunity
treat w/ pyrimidine supplement (uridine)
synthesis of CMP
- UMP to UTP w/NMP
- UTP to CTP w/CTP synthase and glutamine
regulation of pryimidine
- carbamoyl phosphate synthetase II
-inhibited by UTP
-activated by PRPP - CTP synthase
-inhibited by CTP
-activated by UTP
deoxyribonucleotide synthesis
triggered by cell division bc require DNA
NDP to dNDP via ribonucleotide reductase and thioredoxin oxidation
regulated by allosteric effectors
-specific NTP or dNTP for reduction of a NDP= positive effector
-some dNTPs potent negative effectors aka dTNP
NTP and dNTPs regulate ribonucleotide reductase and dATP key inhibitor
hydroxyurea
pharmacological inhibitor of ribonucleotide reductase
blocks activity so no dNTP synthesis so no cell division
chemotherapy
SCID
severe combined immuodeficiency
mutations in adenosine deaminase (purine degradation) = build up adenine metabolites aka dATP
ribonucleotide reductase activity inhibited so all purine/pyrimidine conversion to dNDPs blocked
immune response requires cell proliferation so inhibited from lack of dNTPs
adenine metabolites toxic for immune system
synthesis of dTMPs
must come from dUMP via thymidylate synthase = dTMP
-uses N5N10-methylene THF
dTMP kinase converts to dTDP
cytotoxic therapy
inhibit thymidylate synthase = prevent cell proliferation bc dec dTTP available
5-fluorouracil>FUMP>FUDP>FUTP or FdUDP>FdUMP = irreversible inhibition
lack of dTTP = dUTP and FdUTP in DNA instead = strand breakage and death
pyrimidine degradation
UMP, CMP, dCMP > beta-alanine (>acteyl CoA)
dTMP > beta-aminoisobutyrate (>succinyl CoA)
both betas water soluble so eliminated in urine OR further metabolized
beta-amino unique to thymine degradation so measures DNA turnover
salvage pathways
liver
converts nucleotides to nucleosides and free bases > enter blood > tissues via RBCs
salvage pathways
diet
- nucleic acids DNA/RNa in food digested by pancreatic nucleases = free nucleotide
- convert to nucleosides via phosphatases and taken up by intestinal epithelial cells
nucleosides and free basees precursors for nucleotide synthesis
salvage of purines
requires
1. HGPRTase for hyposxanthine and guanine
2. APRTase for adenine
plus PRPP = IMP (hypox) or GMP (guanine and adenine)
product inhibition
only adenosine only nucleoside directly phos to nucleotide AMP
lesch-nyhan syndrome
hyperuricemia, uric acid stones, intellectual diability, self-injurious/destructive biting of fingers and lips
severe or complete deficiency of HGPRTase activity
salvage of pyrimidines
pyrimidine phosphoribosyltransferase
uracil + PRPP = UMP
orotate + PRPP = OMP
conversion to nucleotides via uridine-cytidine kinase
