Lecture 5: Nucleotide Metabolism

nucleoside

unphosphorylated nucleotide

purine

2 rings

pyrimidine

1 ring

where does purine de novo synthesis occur?

cytosol

what does purine de novo synthesis add carbons and nitrogens to?

preformed ribose 5-phosphate

purine de novo synthesis step 1

synthesis of PRPP

purine de novo synthesis step 2

synthesis of 5’-phosphoribosyl 1-amine

purine de novo synthesis step 3

synthesis of IMP

PRPP synthetase

converts ribose 5-phosphate → PRPP

glutamine phosphoribosyl amidotransferase

converts PRPP → IMP

what to T and B cells depend on for proliferation?

purine de novo synthesis

xanthine

purine nucleotide that causes negative feedback regulation for PRPP synthetase

azathioprine

cancer drug that inhibits glutamine phosphoribosyl amidotransferase

mycophenolic acid

inhibits IMP dehydrogenase

purine synthesis major molecule

N10-formyl-THF

THF synthesis pathwway

folate → dihydrofolate → THF

dihydrofolate reductase

synthesizes THF from folate

methotrexate

inhibits dihydrofolate reductase

ribonucleotide reductase

converts ribonucleotides → deoxyribonucleotides

what are preformed bases attached to in purine salvage pathway?

activated ribose PRPP

HGPRT

converts hypoxanthine → IMP and guanine → GMP

APRT

converts guanine → GMP

Lesch-Nyhan Syndrome

deficiency of HGPRT

Lesch-Nyhan Syndrome effects

increased PRPP and decreased IMP/GMP → increased purine breakdown → increased uric acid

adenosine deaminase (ADA)

deaminates deoxyadenosine → deoxyinosine

ADA deficiency result

accumulation of dATP

dATP

inhibits ribonucleotide reductase → impaired DNA synthesis

severe combined immunodeficiency (SCID)

T and B cell deficiency and inhibition of DNA synthesis

gout

high levels of uric acid in blood → deposition of sodium urate crystals in kidney and joint

xanthine oxidase

converts xanthine → uric acid

xanthine oxidase inhibitors

allopurinol, febuxostat

de novo pyrimidine first step

ring built first, then attached to PRPP

nitrogen source for carbamoyl phosphate in pyrimidine de novo

glutamine

CAD

trifunctional enzyme

orotate

first base synthesized during de novo pyrimidine

UMP

first nucleotide formed in de novo pyrimidine, central precursor for pyrimidine nucleotides

UMP synthase

bifunctional enzyme

thymine

product of DNA degradation used for pyrimidine salvage

thymine → TMP pathway

thymine → thymidine → TMP

thymidine phosphorylase

converts thymine → thymidine

thymidine kinase

converts thymidine → TMP

how much ATP for purine de novo?

6

how much ATP for pyrimidine de novo?

2

purine de novo committed step

glutamine-PRPP amidotransferase

pyrimidine de novo committed step

CPS-II

thymidylate synthase

methylates dUMP → dTMP

N5, N10-methylene-THF

donates methyl group to dUMP → dTMP

dihydrofolate reductase

regenerates THF using NADPH

5-FU

inhibits thymidylate synthase

methotrexate

inhibits dihydrofolate reductase

hydroxyurea

inhibits ribonucleotide reductase

what links pyrimidine synthesis to folate metabolism?

dTMP

N10-formyl-THF

provides formyl groups in purine synthesis

N5,N10-methylene-THF

donates methyl group in pyrimidine synthesis

THF in AA metabolism…

1. histidine degradation

2. serine synthesis

3. resynthesis of methionine from homocysteine

folate

vitamin B9 - directly from food sources

folic acid

synthetic vitamin B9

half of body’s folate store is in wwhich form?

fully reduced THF

bacteria folate synthesis path

PABA + pyrophosphate → folate

sulfonamides

structural analogs of PABA that competitively inhibit DHPS

DHPS

catalyze PABA → folate

major one-carbon donor in humans

serine

THF methyl trap form

N5-methyl-THF

methionine cycle methyl-folate trap

requires methyl-B12 as cofactor and N5-methyl-THF as methyl donor

THF deficiency causes…

FIGLU accumulation by disabling conversion to glutamate

vitamin B12 deficiency pathway

B12 deficiency blocks methionine synthase → 5-methyl THF trapped, THF levels fall → THF deficiency impairs FIGLU to glutamate → FIGLU accumulates in urine