Purine Metabolism/Diseases

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Purine Metabolism/Diseases
Raymond B. Birge, PhD
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Nucleotides - key roles in cellular processes
1. Activated precursors of RNA and DNA 2.
Adenine nucleotides are components of the major
co-enzymes, NAD, NADP, FMN, FAD, and CoA 3.
Nucleotide derivatives are activated
intermediates in biosynthetic processes
(UDP-glucose, SAM) 4. Serve as metabolic
regulators (e.g cAMP and the activation of
cell signaling). 5. Serve as major currency of
energy in all cells (ATP and GTP). 6.
Several metabolic diseases have their etiology in
nucleotide metabolism.
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Purine metabolism (Overview) 1.
Nomenclature/nucleotide structure 2. De novo
synthesis pathways 3. Re-utilization (salvage)
pathways 4. Degradation pathways 5. Metabolic
diseases of purine metabolism (Gout,
Lesch-Nyhan, SCID) Suggested reading
Lippencotts Chapter 22
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Nomenclature
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Active forms of nucleotides
di-and tri-phosphates
(i)
(ii)
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Why are nucleosides and nucleotides important For
biochemists?
Purine binding proteins (the purine proteome)
comprise a family of 3-4,000 Proteins and as
much as 50 of all druggable targets in biology.
Kinases Helicases Reductases Transferases Synthe
tases Dehydrogenases Chaperones Metabolic
Enzymes DNA and RNA processing Etc
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Common Purine Bases
H 6 oxy purine X 2,6 dioxy purine
A 6 amino purine G 2 amino, 6-oxy purine
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Nucleoside Function in extracellular signal
transduction
Adenosine nucleoside-increased during ATP
degradation.
Released in cells when there is low O2
concentration Binds to purinogenic receptors A1,
A2A, A2B, A3 Slows the heart down, at the
same time increases capillary dilation Caffeine
is a adenine derivative, and antagonizes the
effects of adenine.
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Cyclic nucleotides are important mediators for
Intracellular signal transduction
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Two pathways to generate purines and pyrimidines
1. DE NOVO BIOSYNTHETIC PATHWAYS (building
the bases from non-purine molecules)
2. SALVAGE PATHWAYS (the reutilization of
bases from dietary or catabolic sources)
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De novo biosynthesis of purines (A and
G) activation of ribose-phosphate
IMP, AMP, GMP
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Major regulatory step in purine
biosynthesis PRPP to 5-Phosphoribosyl-1-amine

Glutamate
Glutamine
PPi
PRPP
Amidophosphoribosyl transferase
Inhibited by products IMP, AMP, and GMP.
FEEDBACK INHIBITION
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Purine biosynthesis intermediates
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Purines where do the atoms come from?
Key Glycine
1 C unit of N10
f-TetraHydroFolic acid (sulfonamides
methotrexate) Glutamine
Asparate
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Amino acids utilized in purine biosynthesis
Inosine monophosphate is the precursor for both
AMP and GMP
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AMP GMP synthesis
IMP
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Hypoxanthine to Adenine/Guanine.
NH2
(N source)
Aspartate
Adenine
O
(N source)
Glutamine
NH2
Guanine
The common mechanistic theme for the conversion
to A and G is the conversion of a carbonyl
oxygen to an amino group
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Regulation of purine de novo biosynthesis classic
negative feedback
Phosphoribosyl amine
Inhibited by IMP, AMP, and GMP
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Salvage pathways re-utilizate purines
There are 2 salvage enzymes with different
specificities 1. Adenine phosphoribosyl
transferase (APRT) 2. Hypoxanthine-guanine
phosphoribosyl transferase (HGPRT)
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Stages of nucleotide metabolism
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Endonuclease
Nucleic Acid Synthesis
Nucleoside triphosphate
Phosphodiesterase
H20
Nucleotidases
PPi
Pi
ADP
Phosphoribosyl transferases
Nucleoside kinase
Nucleosides
ATP
Pi
PRPP
Phosphorylases
Ribose-1-P
Nucleobases (A, G)
Uric Acid (purines)
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Getting Back to Basics
Adenine
Adenosine monophosphate (AMP)
Adenosine
NUCLEOTIDE
NUCLEOSIDE
BASE
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Purines in humans are degraded to urate
(ADA)
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Gouty Arthritis
The Gout James Gilray 1799
By Royal Authority King George IV George
Cruickshank 19th C.
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Some famous people who had gout Henry
VIII Kublai Khan Nostradamus John Milton Isaac
Newton Frederick the Great John Hancock Thomas
Jefferson Benjamin Franklin David Wells
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Gout results from HYPERURICEMIA
Decreased URIC ACID excretion 80 of gout -
ideopathic, renal disease, diabetes
insipidus, hypertension, Downs
syndrome, many others Increased URIC ACID
production 20 of gout PRPP synthase
overactivity, hemolytic diseases,
lymphoproliferative diseases, may others
HGPRT deficiency (Lesch Nyhan Syndrome),
exacerbated by alcohol, purine
rich diet, obesity
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Gout from increased uric acid production
1. Lost regulation of PRPP Synthase PRPP
Amidotransferase
Ribose 5-phosphate
Phosphoribosyl amine
PRPP
purines
Inhibited by IMP, AMP, and GMP
Leads to net increase in biosynthetic/degradation
pathways!!
(From slide 18)
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Gout from increased uric acid production
2. Defects in salvage pathway lead to increased
PRPP Guanine
Leads to net increase in biosynthetic/degradation
pathways!!
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GOUT Treatment
NaUrate crystals in PMLs
Allopurinol a. decrease urate b. increase
xanthine hypoxanthine c. decrease PRPP
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Tophi at helix of ear
1st MTP joint
Large tophaceous deposits surrounding joint
Initiate urate lowering therapy !!
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Clinical Significance of Purine metabolism
ID A 56 year old obese white male comes to his
family doctor. Chief Complaint My big toe
hurts like !?!!?!!! History Present Illness
Pain began during the night after an episode of
binge eating and drinking. Past Medical
History Significant for removal of kidney stones
last year. Current Health/Risk Factors He
admits to being an avid meat eater and drinks
beer every night. Physical Exam fever, right
metatarsophalangial (MTP) joint red, hot and
swollen painful to motion subcutaneous
deposits in helix of left ear Pathology
Synovial fluid aspiration shows negatively
birefringent, needle-shaped crystals.
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Lesch-Nyhan Syndrome
X-linked recessive Severe HGPRT deficiency
decreased IMPGMP
increased PRPP de novo purine
pway Hyperuremia gouty arthritis, kidney
stones, tophi Neurologic disability
spasticity, hyperreflexia Behavioral problems
cognitive dysfunction, aggression, self-injury
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SCID Severe Combined Immunodeficiency Syndrome
Autosomal recessive disorder Mutations in
ADA Infants subject to bacterial, candidiasis,
viral, protazoal infections Both T and B cells
reduced (dATP is toxic) 1995-AdV
expressing ADA was successfully employed as
gene therapy strategy
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Adenosine Deaminase (ADA) deficiency
(From slide 19)
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Bottom Line
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Summary and Take-Home Points

• Identify basic structures of purines,
  nucleosides, and
• nucleotides.
• 2. Identify key relationships between glucose
  metabolism
• and purine biosynthesis.
• 3. Knowledge of how amino acids are used in AMP
  and
• GMP biosynthesis.
• 4. Understand degradation pathways of purines and
  
• their relationship to uric acid metabolism
  and gout

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Integrative Thought Question

Ribonucleotides and Deoxyribonucleotides are
essential for all cells, and represent key
convergent points in energy metabolism.
Provide specific examples in which purine
metabolism can be linked to metabolism of 1).
Glucose 2). Lipids 3). Amino Acids 4). Ammonium