1.
CAT OF CHE302:BIOCHEMISTRY
Date:December9, 2015
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’tknow, 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 into glucose
Fats are usually metabolized into acetyl CoA and then further processedthrough
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. Acting catalytically
The citric acid cycle itself, which is composedofenzymatically catalyzed steps,
can be thought of essentially as the productof a supramolecular enzyme. Explain.
/2 marks
3. After lipolysis

2.
Write a balanced equation for the conversion of glycerol into pyruvate. Which
enzymes are required in addition to those of the glycolytic pathway? /3 marks
4. Tracing carbons
Consider a cell extract that actively synthesizes palmitate. Supposethat a fatty acid
synthase in this preparation forms one molecule of palmitate in about 5 minutes. A
large amount of malonyl CoA labeled with 14C in each carbon of its malonyl unit
is suddenly added to this system, and fatty acid synthesis is stopped a minute later
by altering the pH. The fatty acids in the supernatant are analyzed for radioactivity.
Which carbon atom of the palmitate formed by this system is more radioactive C-1
or C-14? /2 marks
5. Ill-advised diet
Supposethat, 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 doseof odd-chain fatty
acids. Does your friend have your best interests at heart? Explain. /1 mark
6. Distinctive sugars
The intravenous infusion of fructoseinto 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) Fructosehas been used in place of glucose for intravenous feeding. Why is this
use of fructose unwise? /1 mark
7. Metabolic mutants
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 phosphatasedomain 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-bisphosphatein pyruvate kinase. /1
mark

3.
8. Completing the cycle
Four high-transfer-potential phosphorylgroups are consumed in synthesizing urea.
In this reaction, aspartate is converted into fumarate. Supposethat fumarate is
converted back into aspartate. What is the resulting stoichiometry of urea
synthesis? How many hightransfer- potential phosphorylgroups are spent? /2
marks
9. Ammonia toxicity
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. Fuel choice
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