Biochemistry 102 PROBLEM SET #9 Paul Price
1. Glucogenic Substrates. A common procedure for determining the effectiveness of compounds as
precursors of glucose in mammals is to fast the animal until the liver glycogen stores are depleted and
then administer the substrate in question. A substrate that leads to a net increase in liver glycogen is
termed glucogenic because it must first be converted to glucose-6-phosphate. Which of the following
substances are glucogenic?
(a) succinate; (b) Glycerol; (c) Acetyl CoA; (d) pyruvate; (e) CH3CH 2H (C2) CoA; (f) Aspartate
2. Pathway of atoms in gluconeogenesi s. A liver extract capable of ca rrying out all of the reactions of
gluconeogenesis is briefly incubate d in separate experiments with the following precursors. Where
will the14C label be located in the product glucose? (a) 14C-labeled bicarbonate (H CO3 ); (b)
pyruvlaaeeweith C in the carboxyl carbon.
3. Although palmitate cannot be used to make glucose, 14C label in palmitate will nevertheless be
incorporated into glucose. Explain this appa rent paradox. Which carbon atoms of glucose will
become labeled if palmitate is labeled with14C in carbon 16? in carbon 15?
4. A congenital defect in the liver enzyme fructose 1,6-bisphosphatase results in abnormally high levels
of lactate in blood plasma. Explain.
5. Blood Lactate Levels During Vigorous
Exercise. The concentration of lactate in
blood plasma before,during, and after a 400
m sprint are shown.
(a)What causes the rapid rise in lactate
(b)What causes the decline in lactate
concentration after completion of the run?
Why does the decline occur more slowly
than the increase?
(c) Why is the concentra tion of lactate not zero
during the resting state?
What is the cost of storing gluc ose as glycogen? Write the sequen ce of the steps and the net reaction
required to calculate the cost in number of ATPs of converti ng cytoplasmic glucose 6-phosphate into
glycogen and back into glucose 6-phosphate. What fraction of the maximum number of ATPs that
are available from complete catabolism of glucose 6-phosphate does this cost represent?
7. At what point Is Glycogen S ynthesis Regulated? Explain how the two following observations
identify the point of regulation in the synthesis of glycogen in skeletal muscle.
(a) The measured activity of glycogen synthase in resting muscle, expressed in micromoles of
UDP-glucose used per gram per minute, is lowe r than the activity of phosphoglucomutase or UDP-
glucose pyrophosphorylase, each measured in terms of micromoles of substrate transformed per gram
(b) Stimulation of glycogen synt hesis leads to a small decrease in the concentrations of glucose-6-
phosphate and glucose-1-phosphate, a large decrease in the concentration of UDP-glucose, but a
substantial increase in the concentration of UDP.
8. Explain the effects of each of the following on gl uconeogenesis and glycolysis in a liver cell: (a)
increasing the concentration of AMP, (b) decreasing the concentration of acetyl CoA, (c) increasing
the concentration of F2,6BP, (d) phosphorylation of PFK-2, (e) increasing the concentration of citrate,
(f) phosphorylation of fructose 2,6-bisphosphatase. 9. The increased level of blood lact ate generated by muscle cells duri ng vigorous exercise is converted
back to glucose and glycogen in the liver. How many ATP equivale nts and how many NADH
molecules must be supplied by the liver cell to conve rt 2 lactate molecules to glucose? to convert 2
lactate molecules to a glucose subunit of glycogen?
10. Avidin, a 70-kd protein in egg white, has a very high affinity for biotin. In fact, it is a highly specific
inhibitor of biotin enzymes. Which of the following conversions would be blocked by the addition of
avidin to a cell homogenate?
(a) Glucose → pyruvate (d) Glucose→ ribose 5 phosphate
(b) Pyruvate → glucose (e) Pyruvate→ oxaloacetate
(c) Oxaloacetate → glucose
11. A sample of liver tissue was obtained post-mort em from the body of a pa tient believed to be
genetically deficient in one of the enzymes of ca rbohydrate metabolism. A ho mogenate of the liver
sample had the following characteristics: (1) it degr aded glycogen to glucose 6-phosphate, (2) it was
unable to make glycogen from any sugar or to ut ilize galactose as an energy source, and (3) it
synthesized glucose 6-phos phate from lactate. Which of the fo llowing three enzymes is deficient?
Give reasons for your choice.
(a) Glycogen phosphorylase (c) UDP-glucose pyrophosphorylase
ANSWERS TO PROBLEM SET #9
1. (a), (b), (d), and (f) are glucogenic; (c) and (e) are not.
2. (a) In the pyruvate carboxylase reaction,CO 2s added to pyruvate to form labeled oxaloacetate, but
phosphoenolpyruvate carboxykinase removes the same 14CO in the next step. 1C is therefore
not incorporated into glucose.
(b) 1C label will be in carbon atoms 3 and 4 of the product glucose.
3. Fatty acids cannot be converted to glucose beca use the acetyl CoA units generated by FAO do not,
when they enter the TCA cycle, increase the to tal concentration of TCA cycle intermediates.
Gluconeogenesis can only be supported by com pounds, such as most amino acids, which are
catabolized to pyruvate or TCA cycle intermediates.14C label in fatty acids does, however, enter
glucose even though fatty acids do not support net gl ucose synthesis. This occurs because tCe
labeled acetate that enters citrate in the citrat e synthase reaction is retained in malabeled
palmitate yields CH3labeled acetyl CoA, and subsequently yields malate labeled in carbons 2 and 3.
These become carbons 1, 2, 5 and 6 of glucose. C15labeled palmitate yields acetyl CoA labeled in
the carboxyl carbon, and subsequently yields malate labeled in carbons 1 and 4. Carbon 4 is lost as
14CO ; carbon 1 becomes carbons 3 and 4 of glucose.
4. The major pathway for the removal of la