Download Nucleotide Metabolism

3/22/2013
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.
1
3/22/2013
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
•
•
•
•
•
•
•
Nucleotide
Nucleoside
Nucleobase
AMP
ADP
ATP
dAMP
2
3/22/2013
Some Examples of Nucleotides
UDP‐Glucose
ATP
NAD+
FAD
GTP
CoA
S‐AM
De Novo Synthesis
3
3/22/2013
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…
4
3/22/2013
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!
5
3/22/2013
Purines
• Two distinct strategies for amination
– Mechanisms
• Regulation
– Feedback to 5‐phospho ribosylamine
– Branchpoint regulation
6
3/22/2013
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
7
3/22/2013
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
8
3/22/2013
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
9
3/22/2013
Regulation of Reductase
• One enzyme balances needs of cell via regulation of activity and selectivity
• Be able to explain why this table makes sense
10
3/22/2013
Methylation
•
•
•
•
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
11
3/22/2013
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
O
NH
O
-
O
P
N
O
O
O
O-
H
H
OH
OH
H
H
O
F
NH
O
-
O
P
N
O
O
O
O-
H
H
OH
OH
H
H
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
12
3/22/2013
Review of Purines
• Knowing blue in figure will help with chapter summary
13
3/22/2013
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
14
3/22/2013
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
*
Answers
1.
2.
3.
4.
5.
6.
B
C
B
D
D
C
15