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