nucleotide metabolism. bases/nucleosides/nucleotides base= base base + sugar= nucleoside base +...
TRANSCRIPT
Nucleotide Metabolism
Bases/Nucleosides/Nucleotides
Base=
Base Base + Sugar=
Nucleoside
Base + Sugar + Phosphate=
Nucleotide
Adenine DeoxyadenosineDeoxyadenosine 5’-triphosphate
(dATP)
HN
CHN
C
CN
CN
C
NH2
H
NCH
N
NHCN
NH2
O
H
H
HHO
H
H
HOH2C
NCH
N
NHCN
NH2
O
H
H
HHO
H
H
OCH2PO
O
PO
O
P
O- O-O-
O-
O
• Energy metabolism (ATP)*• Monomeric units of nucleic acids*• Regulation of physiological processes
– Adenosine controls coronary blood flow– cAMP and cGMP serve as signaling molecules
• Precursor function-GTP to tetrahydrobiopternin• Coenzyme components- 5’-AMP in FAD/NAD+
• Activated intermediates- UDP Glucose• Allosteric effectors- regulate themselves and others
Cellular Roles of Nucleotides
How I hope to make this at least bearable if not mildly interesting
• Purines and Pyrimidines
– Synthesis (de novo and salvage pathways)
– Degradation
– Relevant disease states
– Relevant clinical applications (Friday)
You are not responsible for any structures
Purines and Pyrimidines
Adenine Guanine
Thymine/Uracil Cytosine
TwoPurines
TwoPyrimidines
HN
CHN
C
CN
CN
C
NH2
H
NC
CC
HN
C
O
CH3
HO
HN
C
CC
N
CH
O
H
H
NH2
HN
CHN
C
CN
CN
C
O
H2N
H
Synthesis Pathways
• For both purines and pyrimidines there are two means of synthesis (often regulate one another)– de novo (from bits and parts)
– salvage (recycle from pre-existing nucleotides)
Salvage Pathwayde novo Pathway
Many Steps Require an Activated Ribose Sugar (PRPP)
5’
de novo Synthesis
• Committed step: This is the point of no return– Occurs early in the biosynthetic pathway– Often regulated by final product (feedback inhibition)
X
Purine Biosynthesis (de novo)
• Atoms derived from:– Aspartic acid– Glycine– Glutamine– CO2
– Tetrahydrofolate
• Also requires– 4 ATP’s
Purines are synthesized on the Ribose ring
Committed Step
Inhibited byAMP, GMP, IMP X
Purine Biosynthesis (de novo)
ATP GTP
(A bunch of steps you don’t need to know)
(Inosine Monophosphate)NCH
NC
CN
CN
C
O
H
NCH
NC
CN
CN
C
O
H2N
H
NCH
NC
CN
CN
C
NH2
HFeedbackInhibition
Purine Degradation
• Sequential removal of bits and pieces
• End product is uric acid• Uric acid is primate-specific
Other species further metabolize
uric acid
Excreted inUrine
XanthineOxidase
Excess Uric Acid Causes Gout
• Primary gout (hyperuricemia)– Inborn errors of metabolism that lead to overproduction of Uric Acid
• Overactive de novo synthesis pathway– Leads to deposits of Uric Acid in the joints– Causes acute arthritic joint inflammation
Offal foods such as liver, kidneys, tripe, sweetbreads and tongueAvoid:
XanthineOxidase
Allopurinol
X
Immunodeficiency Diseases Associated with Purine Degradation
• Defect in adenosine deaminase– Removes amine from adenosine
• SCID- severe combined immunodeficiency
• “Bubble Boy” Disease• Defect in both B-cells and T-
cells (Disease of Lymphocytes)• Patients extremely susceptible to
infection - hence the Bubble
Lymphocyte
Therapies for SCID
• Can be diagnosed in infants through a simple blood test (white cell count)
• Bone marrow transplant for infants – Familial donor
• Continued administration of adenosine deaminase (ADA-PEG)
• Gene therapy- repair defective gene in T-cells or blood stem cells
Salvage Pathway for Purines
Hypoxanthineor
Guanine
+ PRPP = IMP or GMP + PPi Hypoxanthineguanosylphosphoribosyl transferase
(HGPRTase)
Adenine + PRPP = AMP + PPi Adeninephosphoribosyl transferase
(APRTase)
Lesch-Nyhan Syndrome
• Absence of HGPRTase• X-linked (Gene on X)
– Occurs primarily in males
• Characterized by:– Increased uric acid
– Spasticity
– Neurological defects
– Aggressive behavior
– Self-mutilation
Total Aside on X-linked Diseases
• Why are X-linked diseases generally found only in males?
• Females have two X chromosomes - would need to mutate both copies to see a recessive phenotype
• Males have a single X chromosome
XY XX
Think about Fragile X Syndrome
Biosynthesis of Pyrimidines
• Pyrimidine rings are synthesized independent of the ribose and transferred to the PRPP (ribose)
• Generated as UMP (uridine 5’-monophosphate)
• Synthesized from:– Glutamine
– CO2
– Aspartic acid– Requires ATP
NC
CC
HN
C
O
CH3
HO
H
NC
CC
N
CH
O
H
H
NH2
Uracil Cytosine
Regulation of Pyrimidine Biosynthesis
• Regulation occurs at first step in the pathway (committed step)
• 2ATP + CO2 + Glutamine = carbamoyl phosphate
Inhibited by UTPIf you have lots of UTP around this means you won’t
make more that you don’t need
X
Hereditary Orotic Aciduria
• Defect in de novo synthesis of pyrimidines
• Loss of functional UMP synthetase– Gene located on chromosome III
• Characterized by excretion of orotic acid
• Results in severe anemia and growth retardation
• Extremely rare (15 cases worldwide)
• Treated by feeding UMP
Why does UMP Cure Orotic Aciduria?
CarbamoylPhosphate Orotate
UMPSynthetase
X
FeedbackInhibition
• Disease (-UMP)– No UMP/excess orotate
• Disease (+UMP)– Restore depleted UMP– Downregulate pathway via feedback inhibition (Less orotate)
Biosynthesis: Purine vs Pyrimidine
• Synthesized on PRPP
• Regulated by GTP/ATP• Generates IMP• Requires Energy
• Synthesized then added to PRPP
• Regulated by UTP• Generates UMP/CMP• Requires Energy
Both are very complicated multi-step process whichyour kindly professor does not expect you to know in detail
Pyrimidine Degradation/Salvage
• Pyrimindine rings can be fully degraded to soluble structures (Compare to purines that make uric acid)
• Can also be salvaged by reactions with PRPP– Catalyzed by Pyrimidine phosphoribosyltransferase
Degradation pathways are quite distinct for purines and pyrimidines, but salvage pathways are quite similar
Wait a minute:So far we’ve only made GMP, AMP, and UMP
We need the dNTPs according to the Know-it-All Professor who
taught us that a couple of months ago
Two Problems
• These are monophosphates (i.e. GMP)- we need triphosphates (i.e. GTP) for both DNA and RNA synthesis
• These are ribonucleotides- that’s fine for RNA but we also need to make DNA
Synthesis of ribonucleotides first supports the RNA world theory
Specific Kinases Convert NMP to NDP
NucleosideMonophosphates
NucleosideDiphosphates
MonophosphateKinases
• Monophosphate kinases are specific for the bases
AMP + ATP 2ADP
GMP + ATP GDP + ADP
Adenylate Kinase
Guanylate Kinase
Conversion of Ribonucleotides to Deoxyribonucleotides
OH
HHO
H
H
HOCH2
OH
OH
1´
2´3´
4´5´O
H
HHO
H
H
HOCH2 OH
H
1´
2´3´
4´
5´BASE BASE
Deoxyribonucleoside Ribonucleoside
Somehow we need to get rid of this oxygen
RibonucleotideReductase
Ribonucleotide Reductase
• Catalyzes conversion of NDP to dNDP
• Highly regulated enzyme
• Regulates the level of cellular dNTPs
• Activated prior to DNA synthesis
• Controlled by feedback inhibition
dNDP to dNTP (the final step)
• Once dNDPs are generated by ribonucleotide reductase a general kinase can phosphorylate to make the dNTP’s
• So far we’ve made GTP, ATP, and UTP (which can be aminated to form CTP)
• What about TTP?
You’ll have to tune in tomorrow
Plan for Tomorrow
• Brief Explanation of how dUMP is converted to dTMP
• Some clinically relevant treatments based on these pathways that are used to combat:– Cancer– Bacterial Infections– Viral Infections
Take Home Concepts from Today’s Lecture
• Nucleotides can be made through two pathways – (de novo and salvage)
• Pathways are regulated by feedback inhibition
• Specific degradation pathways exist
• Molecular basis of metabolic diseases mentioned
• What steps are necessary to generate a dNTP from the initial NMP made