Download NUCLEOTIDE metabolism class of 2016

CASE: BUSINESSMAN WITH PAIN IN BIG TOE
• History Of Presenting Illness: Started to have severe pain
in his big toe in the early morning . Binge eating and
drinking (beer) at a dinner party last night.
• Past history: Similar attacks in the past. Kidney stones
removed 2 years back.
• Social history: Upper middle class, drinks beer every night
• Musculoskeletal: The big toe red, hot and swollen; painful
to active and passive movement; tophaceous deposits in
left ear and olecranon bursitis.
• Investigations: WBCs: 12000, Serum Uric Acid: 8.0 mg/dL
• X-ray: Punched-out erosions in right big toe at
metatarsophalangeal joint, producing “overhanging’’
spicules
LEARNING OBJECTIVES
Describe the metabolic basis of gouty arthritis including
• the key reactions of de novo and salvage pathway in the
biosynthesis of purine ribo- and deoxyribonucleotides (Focus
on the sources of the individual atoms in the purine rings and
the key regulatory steps)
• the key steps in the degradation of purines
• causes of primary and secondary hyperuricemia
Also familiarize yourselves with
• the basis of other disorders associated with the nucleotide
metabolism (Lesch-Nyhan syndrome, adenosine deaminase
deficiency)
• why/how nucleotide analogs and inhibitors of nucleotide
synthesis are used in various drug therapies
Q.
Nucleotides serve all of the following roles
EXCEPT
A.
B.
C.
D.
E.
monomeric units of nucleic acids
physiological mediators
sources of chemical energy
structural components of membranes
structural components of coenzymes
FUNCTIONS OF NUCLEOTIDES
• Polymerize to make DNA and RNA
• Energy currency of the cell e.g. ATP, GTP
• Act as carriers of active intermediates in various
metabolic pathways e.g. UDP-glucose in
glycogen synthesis, SAM
• Component of coenzymes e.g. FAD, NADH,
NADPH
• Act as 2nd messengers e.g. cAMP and cGMP
• Allosteric regulation of various metabolic
pathways e.g. ATP inhibits PFK-1
NITROGENOUS BASES
A
B
Q.
Which one of the following is not a purine?
A.
B.
C.
D.
E.
Adenine
Guanine
Cytosine
Xanthine
Hypoxanthine
THE PENTOSE SUGARS
A
B
NUCLEOTIDE SYNTHESIS:
DE NOVO AND SALVAGE PATHWAYS
De novo pathway
Synthesis from low molecular weight
precursors
Salvage pathway
Synthesis from nucleosides or bases that
become available through the diet or from
degradation of nucleic acids
Q.
Which of the following do NOT serve as a
source of C in purine ring synthesis?
A.CO2
B.Glycine
C.Aspartate
D.Glutamine
E.Tetrahydrofolate
DE NOVO PURINE SYNTHESIS
Source of Nitrogen
• 2 from Glutamine
• 1 from Aspartate
• 1 from Glycine
Source of Carbon
•
2 from Glycine
• 2 from N10-Formyl tetrahydrofolate
• 1 from CO2
DE NOVO PURINE SYNTHESIS
PURINE SYNTHESIS: PRPP SYNTHESIS
PURINES ARE NOT MADE AS FREE BASES – BUT
AS NUCLEOTIDES
5’-PRPP
GLUTAMINE
1
PPi DISPLACEMENT BY NH2
GLYCINE + ATP
2
ADDITION OF GLYCINE
N10-FORMYL THF
3
FORMYL GROUP TRANSFER
4
NH2 GROUP TRANSFER
ATP
5
RING CLOSURE
CO2
6
COO- ADDITION
7
ASPARTATE ADDITION
8
FUMARATE LOSS
9
FORMYL GROUP TRANSFER
GLUTAMINE + ATP
ASPARTATE + ATP
N10-FORMYL THF
10
RING CLOSURE
INOSINE 5’- MONOPHOSPHATE (IMP)
INOSINE 5’- MONOPHOSPHATE (IMP)
Adenylosuccinate
synthetase
GTP + Asp
GDP + Pi
Adenylosuccinate
Adenylosuccinase
Fumarate
NAD+
IMP
dehydrogenase
NADH
Xanthine monophosphate
(XMP)
ATP + Gln
GMP
synthetase
AMP + Glu
Adenosine
monophosphate (AMP)
Guanosine
monophosphate (GMP)
CONVERSION OF NMP TO NDP & NTP
Base specific nucleoside monophosphate kinases
AMP + ATP
Adenylate kinase
2ADP
GMP + ATP
Guanylate kinase
GDP + ADP
Nucleoside diphosphate kinases
GDP + ATP
GTP + ADP
CDP + ATP
CTP + ADP
Q.
Which one of the following is NOT a regulatory
enzyme of nucleotide metabolism?
A.
B.
C.
D.
E.
PRPP Synthetase
GMP synthetase
IMP dehydrogenase
Gln:PRPP amidotransferase
Adenylosuccinate synthetase
REGULATION OF PURINE SYNTHESIS
Ribose 5 Phosphate
PRPP synthetase
PRPP
Amido tranferase
5-Phosphoribosylamine
Adenylosuccinate
synthetase
Adenylosuccinate
IMP
IMP
dehydrogenase
XMP
AMP
GMP
ADP
GDP
ATP
GTP
ENERGY REQUIREMENTS FOR PURINE
SYNTHESIS
a) Ribose-5-PO4
IMP
TOTAL = 6 high energy bonds
4 ATP, 1 PPi (2 high energy bonds)
1 GTP
b) IMP
AMP
TOTAL = 7 high energy bonds
1 ATP
c) IMP
GMP
TOTAL = 7 high energy bonds
PURINE NUCLEOTIDE SYNTHESIS:
THE SALVAGE PATHWAY
Hypoxanthine
Guanine
Hypoxanthine-guanine
phosphoribosyltransferase
PRPP
PPi
Hypoxanthine-guanine
phosphoribosyltransferase
PRPP
Adenine
Adenosine
GMP
PPi
Adenine
phosphoribosyltransferase
PRPP
PPi
Adenosine kinase
ATP
IMP
ADP
AMP
AMP
RIBONUCLEOTIDES 
DEOXYRIBONUCLEOTIDES
ADP
GDP
CDP
UDP
Ribonucleotide reductase
dADP
dGDP
dCDP
dUDP
RIBONUCLEOTIDES 
DEOXYRIBONUCLEOTIDES
NDP
Thioredoxin
NADP+
Thioredoxin
reductase
NADPH
SH
SH
Ribonucleotide
reductase
Thioredoxin
S
S
dNDP
Q.
The regulation if ribonucleotide reductase is complex!
Assume an enzymic deficiency that leads to an
increase in the cellular levels of dATP which one of
the following is most likely to occur?
A.The formation of dADP will be favored
B.The cellular levels of AMP will be reduced
C.Elevated levels of all dNTPs will occur
D.The formation of all dNTPs will be reduced
E.Reduced thioredoxin will not be formed
RIBONUCLEOTIDE REDUCTASE
• CATALYTIC SITE
– Converts NDPs to dNDPs
• ACTIVITY SITE
– Activation by ATP
– Inhibition by dATP
• SUBSTRATE SPECIFICITY SITE
– Binding by ATP = CDP & UDP→ dCDP & dUDP
– Binding by dTTP = GDP → dGDP
– Binding by dGTP = ADP → dADP
DEGRADATION OF PURINE
NUCLEOTIDES
Q.
Uric acid is
A.
B.
C.
D.
E.
a degradation product of cytidine
deficient in the condition known as gout
a competitive inhibitor of xanthine oxidase
formed from xanthine in the presence of O2
oxidized, in humans, before it is excreted in
urine
DEGRADATION OF PURINE
NUCLEOTIDES
AMP
deaminase
AMP
IMP
Pi 5’ Nucleotidase
Adenosine
Adenosine
deaminase
GMP
Pi 5’ Nucleotidase
Inosine
Pi
Guanosine
Ribose
Ribose
5’ Phosphorylase
Guanine
Hypoxanthine
Uric Acid
(excreted)
Xanthine
oxidase
Xanthine
Xanthine
oxidase
Guanase
NH3
DISEASES ASSOCIATED WITH
DEFECTS IN PURINE METABOLISM
•
•
•
•
•
HYPERURICEMIA
GOUT
LESCH-NYHAN SYNDROME
KIDNEY STONES
SEVERE COMBINED
IMMUNODEFECIENCY (SCID)
HYPERURICEMIA
Characterized by plasma urate (uric acid)
level greater than 7.0 mg/dL
Normal plasma levels
Females = 2.4 - 6 mg/dL
Males
= 3.4 - 7 mg/dL
HYPERURICEMIA
•Primary Hyperuricemia: an innate
defect in purine metabolism and/or uric
acid excretion
•Secondary Hyperuricemia: increased
availability of purines due to medications/
medical conditions or through diet.
GOUT
Gout is caused by
precipitation of sodium urate
crystals in the joints resulting
in inflammation and pain.
Progression of Hyperuricemia to Gout
Stage 1: Asymptomatic hyperuricemia. At a serum
urate concentration greater than 6.8 mg/dL, urate crystals
may start to deposit in the joints. No evidence that
treatment is required.
Stages 2 : Acute gout. If sufficient urate deposits develop
around joints, and if the local environment or some
trauma triggers the release of crystals into the joint space,
an inflammatory response occurs. These flares can be
self-resolving but are likely to recur.
Stage 3: Intercritical periods. These are the intervals
between attacks. During these periods, crystals may still
be present at a low level in the synovial tissue and fluid,
resulting in future attacks.
Stage 4: Advanced gout. If crystal deposits continue to
accumulate, patients may develop chronically stiff, swollen
joints and tophi. This advanced stage of gout is relatively
uncommon generally avoidable with therapy.
Clinical features – Swollen MTP joint
and ankle
Clinical features - tophaceous
deposits in left ear
Clinical features – Olecranon bursitis
Clinical features – punched out
erosions with overhanging spicules
GOUT - DIAGNOSIS
Definitive diagnosis is made by the presence of urate
crystals in the synovial fluid of the joint – crystals are
needle-shaped and negatively birefringent
GOUT - Causes
• Underexcretion of uric acid
• Diet rich in purines/alcohol; deficient in dairy
products
• Increased purine degradation
• Increased PRPP Synthetase activity
overproduction of PRPP = increased purine synthesis =
increased purine degradation = increased uric acid
production
• Decreased/partial HGPRT activity
1) Deficiency of HGPRT = increased HX and G
2) Deficiency of HGPRT = accumulation of PRPP =
increased purine synthesis = increased uric acid
levels
3) Deficiency of HGPRT = decreased IMP and
GMP = decreased inhibitors for purine synthesis
GOUT - Treatment
• Colchicine –reduces inflammation
• Uricosuric agents – increase renal excretion of uric
acid (probenecid)
• Allopurinol – inhibits uric acid synthesis
• Low purine diet - Foods that are high in purine
include:
– Red meat and organ meats (eg. liver)
– Yeasts and yeast extracts (eg. beer and alcoholic
beverages)
– Asparagus, spinach, beans, peas, lentils, oatmeal,
cauliflower and mushrooms
• Avoid caffeine and alcohol
• Keep hydrated
KIDNEY STONES
When uric acid is present
in high concentrations in
the blood, it may
precipitate as a salt in the
kidneys. The salt can form
stones, which can in turn
cause pain, infection, and
kidney damage.
LESCH-NYHAN SYNDROME
• X-Linked recessive disorder
• Complete HGPRT deficiency
• Characterized by mental deficiency, aggression,
self-destructive behavior, characterized by lip and
finger biting.
• Since high urate levels are present in the blood,
individuals with this condition are also prone to
gout and kidney stones
Q.
Lesch-Nyhan disease is due to the inability to
catalyze which one of the following reactions?
A.
B.
C.
D.
E.
AMP to ATP
Adenine to AMP
Guanine to GMP
Adenosine to AMP
Hypoxanthine to xanthine
SEVERE COMBINED
IMMUNODEFICIENCY (SCID)
• Adenosine deaminase deficiency
• Accumulation of dATP = inhibition of
ribonucleotide reductase =B and T cells unable
to divide
Q.
Methotrexate affects nucleotide synthesis by
inhibiting
A.IMP dehydrogenase
B.dihydrofolate reductase
C.dihydropteroate synthase
D.glutamine:PRPP amidotransferase
E.PRPP synthase