1.
MARKING SCHEME FOR CHE302: BIOCHEMISTRY
Exam Date: December 30, 2015
Max: 50 Marks
1. Compare and contrast fatty acid oxidation and synthesis with respect to: /6 marks
(a) site of the process.
(b) acyl carrier.
(c) reductants and oxidants.
(d) stereochemistry of the intemediates.
(e) direction of synthesis or degradation.
(f) organization of the enzyme system.
Answer
(a) Oxidation in mitochondria; synthesis in the cytosol.
(b) Acetyl CoA in oxidation; acyl carrier protein for synthesis.
(c) FAD and NAD+
in oxidation; NADPH for synthesis.
(d) l isomer of 3-hydroxyacyl CoA in oxidation; d isomer in synthesis.
(e) From carboxyl to methyl in oxidation; from methyl to carboxyl in synthesis.
(f) The enzymes of fatty acid synthesis, but not those of oxidation, are organized in a multienzyme
complex.
2. What is the role of decarboxylation in fatty acid synthesis? Name another key reaction in a
metabolic pathway that employs this mechanistic motif. /2 marks
Answer
Decarboxylation drives the condensation of malonyl ACP and acetyl ACP. In contrast, the
condensation of two molecules of acetyl ACP is energetically unfavorable. In gluconeogenesis,
decarboxylation drives the formation of phosphoenolpyruvate from oxaloacetate.
3. Peroxisomes have an alternative pathway for oxidizing polyunsaturated fatty acids. They contain
a hydratase that converts d-3-hydroxyacyl CoA into trans-D 2-enoyl CoA. How can this enzyme be
used to oxidize CoAs containing a cis double bond at an even-numbered carbon atom (e.g., the cis-
D 12 double bond of linoleate)? /3 marks
Answer
In the fifth round of β-oxidation, cis-D 2-enoyl CoA is formed. Dehydration by the classic
hydratase yields d-3-hydroxyacyl CoA, the wrong isomer for the next enzyme in β-oxidation. This
dead end is circumvented by a second hydratase that removes water to give trans-D 2-enoyl CoA.
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2.
The addition of water by the classic hydratase then yields l-3- hydroxyacyl CoA, the appropriate
isomer. Thus, hydratases of opposite stereospecificities serve to epimerize (invert the configuration
of) the 3-hydroxyl group of the acyl CoA intermediate.
4. An animal is fed stearic acid that is radioactively labeled with 14
C carbon. A liver biopsy reveals
the presence of 14
C-labeled glycogen. How is this possible in light of the fact that animals cannot
convert fats into carbohydrates? /2 marks
Answer
A labeled fat can enter the citric acid cycle as acetyl CoA and yield labeled oxaloacetate, but only
after two carbon atoms have been lost as CO2. Consequently, even though oxaloacetate may be
labeled, there can be no net synthesis in the amount of oxaloacetate and hence no net synthesis of
glucose or glycogen.
5. Suggest an explanation for the fact that the amount of glycogen in type I glycogen-storage
disease (von Gierke disease) is increased. /2 marks
Answer
The high level of glucose 6-phosphate in von Gierke disease, resulting from the absence of glucose
6-phosphatase or the transporter, shifts the allosteric equilibrium of phosphorylated glycogen
synthase toward the active form.
6. Patients in shock will often suffer from lactic acidosis due to a deficiency of O2. Why does a lack
of O2 lead to lactic acid accumulation? One treatment for shock is to administer dichloroacetate,
which inhibits the kinase associated with the pyruvate dehydrogenase complex. What is the
biochemical rationale for this treatment? /2 marks
Answer
A decrease in the amount of O2 will necessitate an increase in anaerobic glycolysis for energy
production, leading to the generation of a large amount of lactic acid. Under conditions of shock,
the kinase inhibitor is administered to ensure that pyruvate dehydrogenase is operating maximally.
7. Name the α-ketoacid that is formed by transamination of each of the following amino acids:
/6 marks
(a) Alanine
(b) Aspartate
(c) Glutamate
(d) Leucine
(e) Phenylalanine
(f) Tyrosine

3.
Answer
(a) Pyruvate; (b) oxaloacetate; (c) α-ketoglutarate; (d) α-ketoisocaproate; (e) phenylpyruvate; (f)
hydroxyphenylpyruvate.
8. What is the yield of ATP when each of the following substrates is completely oxidized to CO2 by
a mammalian cell homogenate? Assume that glycolysis, the citric acid cycle, and oxidative
phosphorylation are fully active. /6 marks
(a) Pyruvate
(b) Lactate
(c) Fructose 1,6-bisphosphate
(d) Phosphoenolpyruvate
(e) Galactose
(f) Dihydroxyacetone phosphate
Answer
(a) 12.5; (b) 14; (c) 32; (d) 13.5; (e) 30; (f) 16.
9. Glucose labeled with 14
C at C-1 is incubated with the glycolytic enzymes and necessary
cofactors.
(a) What is the distribution of 14
C in the pyruvate that is formed? (Assume that the interconversion
of glyceraldehyde 3-phosphate and dihydroxyacetone phosphate is very rapid compared with the
subsequent step.)
(b) If the specific activity of the glucose substrate is 10 mCi mM-1, what is the specific activity of
the pyruvate that is formed? /2 marks
Answer
(a) The label is in the methyl carbon atom of pyruvate.
(b) 5 mCi/mM. The specific activity is halved because the number of moles of product (pyruvate) is
twice that of the labeled substrate (glucose).
10. Which of the 20 amino acids can be synthesized directly from a common metabolic
intermediate by a transamination reaction? /3 marks
Answer
Alanine from pyruvate; aspartate from oxaloacetate; glutamate from α-ketoglutarate.
11. What are the likely consequences of a genetic disorder rendering fructose 1,6-bisphosphatase in
liver less sensitive to regulation by fructose 2,6-bisphosphate? /2 marks
Answer

4.
Fructose 2,6-bisphosphate, present at high concentration when glucose is abundant, normally
inhibits gluconeogenesis by blocking fructose 1,6-bisphosphatase. In this genetic disorder, the
phosphatase is active irrespective of the glucose level. Hence, substrate cycling is increased. The
level of fructose 1,6-bisphosphate is consequently lower than normal. Less pyruvate is formed and
thus less ATP is generated.
12. What is the direction of each of the following reactions when the reactants are initially present
in equimolar amounts? Use the data given in the table below. /4 marks

5.
Answer
Reactions in parts a and c, to the left; reactions in parts b and d, to the right.
13. What information do the ∆G° data given in Table 13.6 provide about the relative rates of
hydrolysis of pyrophosphate and acetyl phosphate? /1 marks
Answer
None whatsoever.
14. What is the structural feature common to ATP, FAD, NAD+, and CoA? /1 marks
Answer
An ADP unit.
15. The following graph shows how the ∆G for the hydrolysis of ATP varies as a function of the
Mg2+
concentration (pMg = log 1/[Mg2+
]). /2 marks
(a) How does decreasing [Mg2+]
affect the ∆G of hydrolysis for ATP?
(b) How can you explain this effect?
Answer
(a) As the Mg2+
concentration falls, the ∆G of hydrolysis rises. Note that pMg is a logarithmic plot,
and so each number on the x-axis represents a 10-fold change in [Mg2+
].

6.
(b) Mg2+
would bind to the phosphates of ATP and help to mitigate charge repulsion. As the [Mg2+
]
falls, charge stabilization of ATP would be less, leading to greater charge repulsion and an increase
in ∆G on hydrolysis.
16. The two basic mechanisms for the elongation of biomolecules are represented in the adjoining
illustration. In type 1, the activating group (X) is released from the growing chain. In type 2, the
activating group is released from the incoming unit as it is added to the growing chain. Indicate
whether each of the following biosynthesis is by means of a type 1 or a type 2 mechanism: /6
marks
(a) Glycogen synthesis
(b) Fatty acid synthesis
(c) C5 C10 C15 in cholesterol synthesis
(d) DNA synthesis
(e) RNA synthesis
(f) Protein synthesis
Answer
(a, d, and e) Type 2; (b, c, and f) type 1.
Examiner: Cleophas Rwemera Moderator: Dr. Mubamba Theodore

7.
(b) Mg2+
would bind to the phosphates of ATP and help to mitigate charge repulsion. As the [Mg2+
]
falls, charge stabilization of ATP would be less, leading to greater charge repulsion and an increase
in ∆G on hydrolysis.
16. The two basic mechanisms for the elongation of biomolecules are represented in the adjoining
illustration. In type 1, the activating group (X) is released from the growing chain. In type 2, the
activating group is released from the incoming unit as it is added to the growing chain. Indicate
whether each of the following biosynthesis is by means of a type 1 or a type 2 mechanism: /6
marks
(a) Glycogen synthesis
(b) Fatty acid synthesis
(c) C5 C10 C15 in cholesterol synthesis
(d) DNA synthesis
(e) RNA synthesis
(f) Protein synthesis
Answer
(a, d, and e) Type 2; (b, c, and f) type 1.
Examiner: Cleophas Rwemera Moderator: Dr. Mubamba Theodore