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 activators
Several pyrimidines are inhibitors
• Aspartate transcarbamoylase (ATCase)
Important regulatory point in prokaryotes
Catalyzes 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 sulfhydryl
groups
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/dL
at 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