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Nucleotide Metabolism
C483 Spring 2013
1. A ribose sugar is added to ________ rings after their synthesis and to ________
rings during their synthesis.
A) purine; pyrimidine
B) pyrimidine; purine
C) purine; purine
D) pyrimidine; pyrimidine
2. The first nucleotide product in the de novo biosynthetic pathway of purines is
A) AMP.
B) GMP.
C) IMP.
D) XMP.
3. Which of the following statements is false concerning purine synthesis?
A) N7 is from glycine
B) C2 is from carbon dioxide (bicarbonate)
C) N3 is from glutamine
D) C8 is from 10-formylTHF.
4. Which is a precursor in the de novo synthesize CTP?
A) CMP.
B) GMP.
C) TMP.
D) UMP.
5. Which of the following is not a role of a catalytic sulfur atom in ribonucleotide
reductase?
A) Proton donor
B) Radical stabilization
C) Redox reaction
D) Covalent catalysis
6. Dihydrofolate reductase and thymidylate synthetase are major targets for
anticancer drugs because
A) these enzymes are unique in cancer cells.
B) cancer cells lack sufficient amounts of these enzymes.
C) cancer cells grow rapidly and are very dependent upon the activities of these
enzymes.
D) they donate one-carbon groups.
E) All of the above.
Terminology of Nucleic Acids
•
•
•
•
•
•
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Nucleotide
Nucleoside
Nucleobase
AMP
ADP
ATP
dAMP
Some Examples of Nucleotides
UDP-Glucose
ATP
NAD+
GTP
CoA
S-AM
FAD
De Novo Synthesis
De Novo Synthesis of Purines
• Form activated
ribose
• Form 5-phospho
ribosylamine
• Build IMP from
precursors
• Synthesis of AMP
and GMP
PRPP
• Pentose
phosphate
pathway
• 2 ATP equivalents
• Over production
of PRPP is one
cause of gout
because PRPP
stimulates the
next step…
5-phosphoribosylamine
• First step of purine
biosynthesis
• Glutamine is N donor
• Regulated
– Activation by PPRP
– Increased purine
levels
– Degradation of
purines leads to
compound which can
cause gout
Purine Pathway
• Don’t need to know
details, order
• Know precursors
– N from Asp, Gln
– C from THF, Gly, CO2
• Cost
– 2 ATP eq for PRPP
– 5 more ATP steps
Know this figure!
Purines
• Two distinct strategies
for amination
– Mechanisms
• Regulation
– Feedback to 5-phospho
ribosylamine
– Branchpoint regulation
Compare/Contrast
• Purine biosynthesis
– Salvage is a major
pathway
– Base synthesized while
attached to ribose
– IMP is common
intermediate for AMP
and GMP, but itself is not
a typical nucleotide
• Pyrimidine biosynthesis
– De novo is a major
pathway
– Base is synthesized, then
attached to ribose
– UMP, a typical nucleic
acid, is converted into
other pyrimidines
De novo
Pyrimidine
Synthesis
• First step regulated
(compare to urea
cycle)
• Asp is different
than purine—
whole molecule is
incorporated
Further Modifications
• Interconversion of nucleotides (mono, di, tri
phosphates)
• Reduction to form deoxynucleotides
• Methylation to form dTMP
Nucleotide Interconversions
• Fast, reversible, driven by high [ATP]
• NMPNDP catalyzed by specific nucleoside
monophosphate kinase
• NDPNTP catalyzed by nonspecific kinase
• AMP + ATP  ADP + ADP important in energy
balance
Deoxyribonucleotides
• Deoxygenation occurs on diphosphates
• One enzyme affects all transformations
[dUDP]
Ribonucleotide Reductase
• Sulfur does amazing
chemistry!
– Stable radical
– Proton donor
– Redox reagent
• NADPH is ultimate
source of reducing
Regulation of Reductase
• One enzyme balances needs of cell via regulation of
activity and selectivity
• Be able to explain why this table makes sense
Methylation
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dTMP is made from dUMP
Key step in replicating cells
Therapeutic target for anti-cancer drugs
Two key enzymes
Thymidylate Synthase
• Methylene-THF acts as
a “methyl” donor
– Donates methylene
– And hydride
• Fascinating chemistry!
– Sulfur is covalent
catalyst
– Internal 1,3-hydride
shift
• THF is left as DHF
5-Fluorouracil
• Incorporated into
monophosphate
nucleotide in body
• Mechanism based
inhibitor (Trojan Horse)
• Forms covalent link to
enzyme like normal
• No elimination possible
because proton
replaced with fluorine
DHF reductase
• DHF must be reduced
back to THF to be a
viable cofactor
• Second
chemotherapy target
• Competitive inhibitor
that is structurally
similar to THF would
end methylation
process
Review of Purines
• Knowing blue in figure will help with chapter summary
Review of Pyrimidines
• Knowing blue in figure will help with chapter summary
Catabolism
• Less important than other catabolic processes
– not a major energy source
– Lots of salvage
– Serves to clear excess
• In humans, purines  uric acid (excreted)
• In humans, pyrimidines  acetyl CoA, succinyl
CoA for some energy gain
Severe Combined Immunodeficiency
Syndrome (SCIDS)
• Deficiency of
adenosine deaminase
• First step in catabolism
• High levels of dATP
lead to low levels of
dNTP
• No DNA kills fast
growing T-cells
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Answers
1.
2.
3.
4.
5.
6.
B
C
B
D
D
C