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BIOC 801 - Dr. Tischler
Lecture 20 – February 10, 2006
METABOLISM: NUCLEOTIDE SYNTHESIS & DISORDERS
CLINICAL PREMISE
S.G., a 45-year-old Caucasian male, presented to your office complaining
of foot pain. The pain began approximately one week ago when he noticed
one morning that his right big toe was swollen and painful to touch. He
attributed the pain to “stubbing” his toe two days earlier on a coffee table.
He initially took aspirin and Tylenol with some minimal improvement in the
pain, but over the past week the pain has increased and now the big toe is
red. Further history reveals that S.G. is an accountant, has had a weight
problem “most of his life”, doesn’t exercise and is a wine connoisseur.
Physical examination reveals an obese, middle-aged white male. His vital
signs are normal with the exception of elevated BP. His right toe is swollen
and the skin over the joint is inflamed and tender to touch. His left big toe
is swollen but is not inflamed.
A complete blood count is normal. His blood chemistry values were normal
except for uric acid of 14.5 mg/dl (nl: 3-9 mg/dL). Urinalysis reveals a
crystalluria. Analysis of RBC enzymes relevant to uric acid metabolism
shows normal activity and normal regulation of the metabolic pathway.
ROLES OF NUCLEOTIDES
building blocks for DNA and RNA
“second messengers” in signal transduction cascades
energy “currency” of the cell
Glucose-6-P
pentose phosphate
pathway
ATP
Ribose-5-phosphate
inosine
monophosphate
(IMP)
AMP
5-phosphoribosylpyrophosphate (PRPP)
PRPP synthetase
Gln
PRPP
Amino acids:
amidotransferase
Glu
Gly + Gln + Asp
5-phospho--ribosylamine
Cofactors:
N10-formyl THF
Figure 1. Synthesis of inosine monophosphate (IMP)
NAD+
aspartate + GTP
IMP
GDP + Pi
NADH
Gln + ATP
fumarate
Glu+AMP+PPi
guanosine monophosphate
(GMP)
adenosine monophosphate
(AMP)
GDP
ADP
GTP
ATP
Figure 2. Formation of AMP and GMP from IMP
PRPP synthetase
PRPP amidotransferase
IMP
allosteric inhibition
GMP
AMP
GDP
ADP
Figure 3. Allosteric inhibition of purine biosynthesis;
also ATP stimulates stimulates formation of AMP.
GMP, GDP or GTP
AMP, ADP or ATP
Adenosine
Guanosine
NH3
adenosine deaminase
Inosine
Ribose-1-P
purine nucleoside
phosphorylase
+PRPP Guanine
HGPRT
Hypoxanthine
Xanthine
xanthine
oxidase
GMP
Ribose-1-P
xanthine
oxidase
+PRPP
HGPRT
IMP
Uric acid
Figure 4. Degradation of purines to uric acid and salvage of purine bases via
hypoxanthine-guanine phosphoribosyl transferase (HGPRT).
Glucose-6-P
pentose phosphate
pathway
ATP
Ribose-5-phosphate
inosine
monophosphate
(IMP)
AMP
X1a
5-phosphoribosylpyrophosphate (PRPP)
PRPP synthetase
Gln
PRPP
Amino acids:
amidotransferase
Glu
Gly + Gln + Asp
5-phospho--ribosylamine
Cofactors:
N10-formyl THF
Figure 1. Hyperuricemia: Gout: X1a = PRPP synthetase defects associated with a
superactive enzyme characterized by an increased Vmax or an enzyme with a
reduced Km for ribose-5-P.
X1b
PRPP synthetase
PRPP amidotransferase
IMP
GMP
AMP
GDP
ADP
Figure 3. Gout: X1b = PRPP synthetase defect associated with
resistance to feedback inhibition.
GMP, GDP or GTP
AMP, ADP or ATP
Adenosine
Guanosine
NH3
adenosine deaminase
Inosine
Ribose-1-P
purine nucleoside
phosphorylase
+PRPP Guanine
HGPRT
GMP
Hypoxanthine
Xanthine
X2
Ribose-1-P
Inhibited by
allopurinol
xanthine
oxidase
xanthine
oxidase
+PRPP
HGPRT
X2
IMP
Uric acid
Figure 4. X2 = moderate defect (>50% activity) leading to gout; severe defect
(very low activity) leads to Lesch-Nyhan syndrome.
GOUT AND LESCH-NYHAN SYNDROME
Gout = hyperuricemia due to a variety of causes
Lesch-Nyhan syndrome = excessive hyperuricemia; leads to self-mutilation
Urate crystals appearing in a diaper – often found in synovial fluid of joints
Incan Mask Depicting An Individual Presumably
with Lesch-Nyhan as Evidenced by the Self-Mutilation
 ATP, PRPP
carbamoyl phosphate
synthetase II (gln)
Glutamine +
2ATP + CO2
-
UDP
UTP
Glu + Pi
+ ADP
CTP
Glu + Pi
+ 2 ADP
Gln +
ATP
+2 ADP
UTP
carbamoyl
phosphate
aspartate
transcarbamoylase
carbamoylaspartate
aspartate
+2 ATP
orotate
orotate phosphoribosyl
transferase
UMP
OMP
orotidylic acid
decarboxylase
Figure 5. Biosynthesis of the pyrimidine nucleotides UTP and CTP.
+PRPP
ADP
GDP
UDP
CDP
ribonucleotide reductase
reduced
thioredoxin
dADP
dGDP
dUDP
dCDP
oxidized
thioredoxin
dUMP
thymidylate
synthase
N5,N10-methylene THF
dihydrofolate
TMP
NADP +
thioredoxin
reductase
+NADPH + H+
Figure 6. Biosynthesis of deoxyribonucleotides by ribonucleotide reductase
and of thymidine monophosphate (TMP) by thymidylate synthase.
PRPP
N5,N10-Methenyl
THF
Purines: AMP + GMP
N10-Formyl THF
N5,N10Methylene
THF
Glycine
Suicide inhibitor
F4 dUMP
5-F-uracil
X
thymidylate
synthase
N5-Methyl THF
TMP
Dihydrofolate (DHF)
OH-B12
folate
trap
dUMP
Serine
X3
X5
THF
DHF reductase
MTX
Methyl B12
Figure 7. Metabolism of dietary folic acid to its various cofactor forms.
DIETARY
FOLIC ACID
X34 = site at which cobalamin (B12) deficiency causes
X 5= fluorodeoxyuridylate (F-dUMP), synthesized from 5-flurouracil, acts as
folate
to be
as N5-methyl(MTX)
THF acts as a chemotherapeutic agent;
X = site
attrapped
which agent
methotrexate
chemotherapeutic