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Nucleotide Metabolism II
• Biosynthesis of deoxynucleotides
• Salvage Pathway• Catabolism: Purines• Catabolism:
Pyrimidines
• Feedback inhibition in purine nucleotide biosynthesis
CPS II
• Cytosolic CPS II uses glutamine as the nitrogen donor to carbamoyl phosphate
Regulation of pyrimidine synthesis
• CPS II is allosterically regulated:PRPP and IMP are activatorsSeveral pyrimidines are inhibitors
• Aspartate transcarbamoylase (ATCase) Important regulatory point in prokaryotesCatalyzes the first committed pathway step Allosteric regulators:
CTP (-), CTP + UTP (-), ATP (+)
• Regulation of pyrimidine nucleotide synthesis in E. coli
Biosynthesis of deoxynucleotides
• Uses diphosphates (ribo) • Ribonucleotide
reducatase• 2 sub-units • R1- reduces, active and
two allosteric sites (activity and specificity site)
• R2- tyrosine radical carries electrons
• removes 2' OH to H
Ribonucleotide reductase reaction
• removes 2' OH to H • Thioredoxin and
NADPH used to regenerate sulfhydrylgroups
Thymidylate synthesis
• UDP ------> dUMP• dUMP --------> dTMP• required THF • methylates uracil
Regulation
THF• Mammals cannot conjugate rings or synthesize PABA.• So must get in diet.• Sulfonamides effective in bacteria due to competitive
inhibition of the incorporation of PABA
Cancer Drugs
• fluorouracil-- suicide inhibitor of Thy synthase
• aminopterin• Methotrexate --
inhibits DHF reductase
Salvage of Purines and Pyrimidines
• During cellular metabolism or digestion, nucleic acids are degraded to heterocyclic bases
• These bases can be salvaged by direct conversion to 5’-mononucleotides
• PRPP is the donor of the 5-phosphoribosyl group
• Recycling of intact bases saves energy (reduced nitrogen sources are scarce)
Salvage Pathway
• extra-hepatic tissues • free purines• APRT • HGPRT • Uracil• Salvage via Purine
Nucleoside phosphorylase
Lesch-Nyhan syndrome
• Lack of HGPRT • x- linked • elevates PRPP • Increase de novo
purine biosynthesis • overproduction of
urate
Catabolism: Purines
• Dietary purines: mostly degraded • Purines produce urate• excreted in urine in mammals
Degradation of uric acid
Gout results from excess sodium urate
• Gout is caused from overproduction or inadequate excretion of uric acid
• Sodium urate is relatively insoluble and can crystallize in tissues
• Gout can be caused by a deficiency of hypoxanthine-guanine phosphoribosyltransferase or defective regulation of purine biosynthesis
Problems
• Urate soluble 7 mg/dLat 37C
• cooler extremities, crstallizes
• synovial fluid • Gout
Allopurinol is a treatment for gout
• Allopurinol is converted in cells to oxypurinol, an inhibitor of xanthine dehydrogenase
• Allopurinol prevents high levels of uric acid
• Hypoxanthine, xanthine are more soluble
Treatment
• Allopurinol • inhibit xanthine oxidase• Chelates Mo 4+ • suicide inhibitor • deplete PRPP • secrete hypoxanthine/Xanthine, more soluble
Pyrimidine Metabolism
• Pyrimidine nucleotides are hydrolyzed to the nucleosides and Pi
• Then thymine, uracil and (deoxy) ribose 1-phosphate are produced
• Catabolism of the thymine and uracil bases ends with intermediates of central metabolism
Catabolism: Pyrimidines
• No problems • produce B-alanine,
NH3, and CO2
Nucleotide Catabolism: disorders
• SCID • ADA deficiency • Both B/T lymphocytes
affected • elevated dATP 50-100x • inhibit ribonucleotide
reductase • inhibit synthesis of other
deoxynucleotides• pyrimidine starvation • no cell division
Nucleotide Catabolism: disorders
• PNP deficiency • T lymphocytes • elevated dGTP• inhibit ribonucleotide
reductase • inhibit synthesis of
other deoxynucleotides
• no cell division