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AN APPLE A DAY KEEPS
THE DOCTOR IN PAY
BCHM2972 Lecture 10 2006
Presentation and history:
• A 10-year old girl, Jane, has a long history of problems
after eating fruit or any foods containing sugar.
• The symptoms include vomiting, stomach cramps,
trembling, sweating, dizziness, ~ convulsions.
• 'sickly' since weaned from breast milk to infant formula
and solids.
• OK foods?  trial and error.
– glucose OK but she does not enjoy the taste.
– cow's milk OK
• physical development
• Difficult to maintain sugar free diet.
• teeth show no caries!
• Jane has presented at hospital tonight with
violent convulsions which were preceded
by stomach cramps and sweating.
• After soft drink and lollies at a friend's
slumber party (peer pressure!)
• Refer to the handout of Test results
What is the likely cause of the sweats,
convulsions, etc?
A
B
C
D
Hypoglycemia
Hyperglycemia
Hypoinsulinemia
Hyperfructosemia
Which ‘sugars’ is Jane reacting to?
A. glucose
B. sucrose
C. galactose
D. fructose
E. lactose
Which ‘sugars’ is Jane reacting to?

A. glucose
B. sucrose
C. galactose
D. fructose
E. lactose

•
•
•
•
•

sucrose
invertase
We know
foods with glucose OK
cows milk OK
FRUIT a problem
lollies and soft drinks too
monosaccharides
fructose + glucose
in small intestine
absorbed to blood
so…a problem with fructose
inborn error of metabolism?
• long history
• always ‘sickly since
weaning’
• Why not since birth?
• no fructose in breast
milk!
• Aldolase B not
expressed in infants
which do you expect to be abnormal?
fructose tolerance test:
infuse 200mg/kg fructose intravenously
measure blood levels of:
–fructose
–glucose
–phosphorus
–magnesium
–uric acid
confirmed blood test levels
are in response to fructose
What the…?
• Jane has a problem eating fructose,
but fructose clearance is normal !!
• Why is glucose   ( symptoms) ?
• ……and why  P and  Mg?
• and uric acid and bilirubin??
• don’t these mean liver damage?
• It appears fructose is very efficiently
taken up and trapped in tissues
• it's the metabolism of the fructose in
the tissues that's the problem
So…..let’s review “sugar”
metabolism
nucleophile (donates e-)
1st step in glycolysis
Breakage of the Phosphoanhydride Bond in ATP
Glucose
P ATP
PHOSPHORYLATION
hexokinase
ISOMERISATION
isomerase
Fructose 6 P 6
ATP
phospho
fructo
kinase
PHOSPHORYLATION
‘CLEAVAGE’
Aldolase A
P
DHAP
~30 ATP
per glucose
GA 3 P P
CITRIC
Pyr deHydr
pyruvate
ACID
CYCLE
electron
transport
and
OxPhos
ATP
Glucose
P ATP
Fructose
hexokinase
ATP
hexokinase
isomerase
 affinity for fructose
Fructose 6 P 6
phospho
fructo
kinase
ATP
in muscle,
fat cells
Aldolase A
P
DHAP
GA 3 P P
CITRIC
ACID
pyruvate
CYCLE
electron
transport
and
OxPhos
ATP
Glucose
ATP
Fructose
ATP
hexokinase
in liver
fructokinase
hexokinase has affinity for glucose but
Fructose 6 P
affinity for fructose. lots of glucose in
6 liver so instead of hexokinase,
fructokinase adds P to trap fructose in
cells
1 Fructose 1 P
ATP
Aldolase B
ATP
P
DHAP
Glyceraldehyde
(GA)
GA 3 P P
CITRIC
ACID
pyruvate
CYCLE
electron
transport
and
OxPhos
DHAP
ATP
P
What is the result of the first step in
fructose metabolism?
A.
B.
C.
D.
E.
phosphorylation of fructose
ATP used up to ph’late fructose
cellular energy is reduced
phosphate ‘trapped’ in fructose 1 P
all of the above
What is the result of a mutation in
fructokinase?
A. can’t ph’late fructose  fructose- 1P
B. reduced clearance of fructose from
blood
C. blood fructose remains high
D. ATP not used to p’late fructose
E. all of the above
A. Yes
B. No
fructose (mg/dL)
Can Jane’s liver trap fructose in the
tissue (clearing it from blood)?
12
10
8
6
4
2
0
-2
0
30
60
time (min)
90
120
Does Jane have a mutation
in fructokinase?
A. Yes
B. No
Her blood fructose does not rise rapidly after the
fructose load because fructokinase is very good a
mopping up and trapping fructose in the liver. And/or
hexokinase in other tissues is very good at mopping
up fructose into F6P
Which enzyme catalyses the second
step of fructose catabolism?
A.
B.
C.
D.
E.
hexokinase
fructokinase
phosphofructokinase
aldolase A
aldolase B
What is the result of a mutation in
aldolase B?
A.
B.
C.
D.
E.
fructose -1P builds up
ph’lation of F-1P uses up ATP
cellular energy is reduced
phosphate ‘trapped’ in fructose- 1 P
all of the above
Fructose
Glucose
ATP
fructokinase
ATP
glycolysis
ATP
pyruvate

1
Fructose 1 P
Aldolase B
30 ATP
How do we explain Jane’s results
by a mutation in aldolase B?
Why the  Pblood?
A.
B.
C.
D.
E.
P trapped in fructose 1-P
P trapped in fructose 6-P
P trapped in fructose 1, 6 -biP
P trapped in glyceraldehyde 3P
all of the above
How do we explain Jane’s results
by a mutation in aldolase B?
Why the  Pblood?
A.
B.
C.
D.
E.
P trapped in fructose 1-P
P trapped in fructose 6-P
P trapped in fructose 1, 6 -biP
P trapped in glyceraldehyde 3P
all of the above
What are the consequences of
 Pblood?
• ADP + P  ATP
low cellular energy (ATP)
cell damage /death
 release of liver enzymes and bilirubin
Why the high Mg?
• ATP in complex with Mg2+
• ATP  Mg2+ released to blood
magnesium (mg/dL)
0.5
0.4
0.3
0.2
0.1
0
0
30
60
time (min)
90
120
ATP, Mg2+, and +ve residues at an enzyme active site
Since all the negative charges in ATP are neutralized,
ATP is readily approached by nucleophiles
substrate level phosphorylation to
compensate for ATP
Glucose
P ATP
hexokinase
ATP
isomerase
Fructose 6 P 6
phospho
fructo
kinase
ATP
Aldolase A
P
DHAP
AMP
ADP
Fructose
ADP
+
 PFK   rate gylcolysis
(important regulatory point)
GA 3 P P
CITRIC
ACID
pyruvate
CYCLE
electron
transport
and
OxPhos
ATP
NAD+
glucose
PFK
glycolysis
CITRIC
PDH
pyruvate
AcCoA
ACID
CYCLE
NADH
normally……
electron
transport
and
OxPhos
ATP
NAD+
AMP
+
glucose
PFK
glycolysis
PDH
AcCoA
pyruvate
CITRIC
ACID
CYCLE
electron
transport
and
OxPhos
NADH
In Jane’s liver, pyruvate is produced too fast to enter
mitochondria for TCA (PDH is like the ‘plughole’)
NAD not regenerated
ATP
cell must regenerate NAD+ to keep
glycolysis running
NAD+
glucose
glycolysis
pyruvate
lactate
ATP
NADH
desperate attempt to make ATP by glycolysis (2 per glucose)
What processes normally lead to
increased blood lactate?
A Decreased liver gluconeogenesis
B Increased liver glycolysis (This happens in
Jane via stimulation of PFK by AMP)
C Increased muscle glycolysis
D Decreased PDH activity (would  entry of
substrates to Citric Acid Cycle)
E All of the above
all increase pyrlactate to regenerate NAD
Fructose
glycogen
phosphorylase
fructokinase
ATP
Fructose P
1P
Glucose
fructose 1,6
bisphosphatase
Fructose 1,6 BP
Aldolase B
Why is glucose sooooo low?
in addition to  glycolysis….
What is  [uric acid*] blood
indicative of?
A  cellular 'energy charge'
B  rate of deamination of adenosine
C  in ATP
D  in AMP
E All of the above
*not to be confused with urea! (product of amino acid catabolism)
H
H
H
deamination
P
P
AMP
inosine MP
uric acid
uric acid is a product of
purine base degradation
What problems can fructose cause
for ‘normal’ people?
• does NOT stimulate insulin or leptin
– regulators of energy intake and body adiposity
• bypasses phosphofructokinase
– determines the rate of glycolysis (pyruvate)
• So…..  fructose   acetyl CoA
Lipogenesis
 liver TG
insulin sensitivity
 VLDL
Hepatic fructose metabolism:
A highly lipogenic pathway.
http://www.nutritionandmetabolism.com/content/figures/1743-7075-2-5-2-l.jpg
In Summary…
You should make a summary diagram yourselves!
Show main control points, interconnections, and show
how the non-functioning enzyme
lab results