8/18/2019 Lecture 12 Sample Problem Answers

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Biosc 1000

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Lecture 12 Sample Problems

1. Fluoroacetate (FCH2COO-) is a potent poison of the citric acid cycle although it has no effect

on the activity of isolated enzymes in vitro. Fluoroacetate readily crosses cellular and

mitochondrial membranes, and is converted into fluoroacetyl CoA by acetyl CoA synthase.

a. How does fluoroacetyl CoA interfere with the citric acid cycle?

 It substitutes for acetylCoA in the citrate synthase reaction, producing

 fluorocitrate, which inhibits aconitase. 

 b. What intermediates build up as a result of fluoroacetate inhibition?

Only acetyl CoA, since the citrate synthetase enzyme still works but produces a

non-productive intermediate.

c. Why is it that fluoroacetate can inhibit the pathway without reducing the activity of an

individual enzyme?

 Because it doesn’t affect the enzymes, it affects the coupling of the reactions. The

aconitase reaction can’t proceed not because it can’t catalyze the reaction, it

can’t bind the “substrate.” 

2. Beriberi is a nutritional deficiency disease resulting from inadequate thiamine in the diet.

Individuals lacking thiamine accumulate 2 metabolites as a consequence of eating sugar –

what are they?

 Pyruvate and ! -ketoglutarate. 

3. The majority of glucose synthesized by a ewe is used by the udder to make lactose and fats.

During the winter, when grazing is poor, milk production decreases. One remedy is to supply

 propionate (an odd chain fatty acid that can be converted to succinyl-CoA. Why does this

restore milk supply?

Succinyl CoA can be use to make oxaloacetate, which can be converted to glucose in

 gluconeogenesis.

4.  Cyanide inhibits Complex IV, blocking the reduction of O2 to water. Methylene blue

can be reduced by components of the electron transport chain and can, when reduced,reduce O2 to water. Why then, are massive doses of methylene blue an effective

antidote for cyanide poisoning?

Cyanide blocks the reduction of oxygen by cytochrome c oxidase (Complex IV). Electrons from cytochrome c cannot be transferred to oxygen in Complex

 IV. Methylene blue can be reduced by cytochrome c and then reduce oxygen

8/18/2019 Lecture 12 Sample Problem Answers

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Biosc 1000

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in turn. It’s an effective antidote because it can replace cytochrome c oxidasein the pathway and allows ATP synthesis to proceed. 

5.  A newly discovered compound called U is isolated from mitochondria. The following

observations were recorded when U was incubated with a suspension ofsubmitochondrial particles.

•  Adding NADH causes a rapid reduction of coenzyme U

•  Reduced U caused a rapid reduction of added cytochrome c

•  In the presence of antimycin (inhibits Complex III), the reduction of U by added NADH took place as rapidly as in the absence of antimycin but cytochrome c

reduction by U was blocked.

•  Addition of succinate caused a rapid reduction of U.

Assign a tentative position for coenzyme U in the electron transport chain.

U is equivalent to Complex II.

6.  Yeast can grow both anaerobically and aerobically on glucose. How do you explain

Louis Pasteur’s observation that glucose consumption decreases when yeast that have been growing anaerobically are exposed to oxygen.

The energy yield from glucose oxidized aerobically is about 15 times that of yeast grown anaerobically (30 ATP/2 ATP = 15). The yeast need less glucose to

maintain a comparable growth rate.