1.
CAT OF CHE302: BIOCHEMISTRY
Date: January 7, 2016
Max: 25 Marks
Duration: 2 hours
Instructions
You have 100 minutes for this exam.
Exams written in pencil or erasable ink will not be re-graded under any circumstances.
Explanations should be concise and clear.
You do not need a calculator for this exam, and no other study aids or materials are permitted.
Generous partial credit will be given, i.e., if you don’t know, guess.
Honor Pledge: At the end of the exam time, please write out the following sentence and sign it, or
talk to me about it:
“I pledge on my honor that I have not given or received any unauthorized assistance on this
examination.”
Questions
1. Fats are usually metabolized into acetyl CoA and then further processed through the citric acid
cycle. In the Fatty acid Catabolism, we learned that glucose could be synthesized from oxaloacetate,
a citric acid cycle intermediate. Why, then, after a long bout of exercise depletes our carbohydrate
stores, do we need to replenish those stores by eating carbohydrates? Why do we not simply replace
them by converting fats into carbohydrates? /2 marks
2. The citric acid cycle itself, which is composed of enzymatically catalyzed steps, can be thought
of essentially as the product of a supramolecular enzyme. Explain. /2 marks
3. (a) Write a balanced equation for the conversion of aspartate into glucose through the
intermediate oxaloacetate. Which coenzymes participate in this transformation? (b) Write a
balanced equation for the conversion of aspartate into oxaloacetate through the intermediate
fumarate. /3 marks
4. How might increased synthesis of aspartate and glutamate affect energy production in a cell?
How would the cell respond to such an effect? /2 marks
EMAIL: deanfs@kie.ac.rw P.O. Box: 5039 Kigali WEBSITE: www.ur.ac.rw
COLLEGE OF EDUCATION
School of Education

2.
5. Suppose that, for some bizarre reason, you decided to exist on a diet of whale and seal blubber,
exclusively.
(a) How would lack of carbohydrates affect your ability to utilize fats? /1 mark
(b) What would your breath smell like? /1 mark
(c) One of your best friends, after trying unsuccessfully to convince you to abandon this diet, makes
you promise to consume a healthy dose of odd-chain fatty acids. Does your friend have your best
interests at heart? Explain. /1 mark
6. The intravenous infusion of fructose into healthy volunteers leads to a two- to fivefold increase in
the level of lactate in the blood, a far greater increase than that observed after the infusion of the
same amount of glucose.
(a) Why is glycolysis more rapid after the infusion of fructose? /1 mark
(b) Fructose has been used in place of glucose for intravenous feeding. Why is this use of fructose
unwise? /1 mark
7. Predict the effect of each of the following mutations on the pace of glycolysis in liver cells:
(a) Loss of the allosteric site for ATP in phosphofructokinase. /1 mark
(b) Loss of the binding site for citrate in phosphofructokinase. /1 mark
(c) Loss of the phosphatase domain of the bifunctional enzyme that controls the level of fructose
2,6-bisphosphate. /1 mark
(d) Loss of the binding site for fructose 1,6-bisphosphate in pyruvate kinase. /1 mark
8. A number of genetic deficiencies in acyl CoA dehydrogenases have been described. This
deficiency presents early in life after a period of fasting. Symptoms include vomiting, lethargy, and
sometimes coma. Not only are blood levels of glucose low (hypoglycemia), but starvation-induced
ketosis is absent. Provide a biochemical explanation for these last two observations./2 marks
9. Glutamate is an important neurotransmitter whose levels must be carefully regulated in the brain.
Explain how a high concentration of ammonia might disrupt this regulation. How might a high
concentration of ammonia alter the citric acid cycle? /2 marks
10. Within a few days after a fast begins, nitrogen excretion accelerates to a higher-than-normal
level. After a few weeks, the rate of nitrogen excretion falls to a lower level and continues at this
low rate. However, after the fat stores have been depleted, nitrogen excretion rises to a high level.
(a) What events trigger the initial surge of nitrogen excretion? /1 mark
(b) Why does nitrogen excretion fall after several weeks of fasting? /1 mark
(c) Explain the increase in nitrogen excretion when the lipid stores have been depleted. /1 mark
Examiner: Cleophas Rwemera Moderator: Dr. Mubamba Theodore