Fundamentals of General, Organic, and Biological Chemistry

John McMurry, David S. Ballantine, Carl A. Hoeger

Chapter 24

Lipid Metabolism - all with Video Answers

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Chapter Questions

Problem 1

Cholesterol (see structure in margin) and cholate (a bile acid anion, whose structure is shown on p. 765) are sterols with very similar structures. However, the roles they play in the body are different: Cholate is an emulsifier, whereas cholesterol plays an important role in membrane structure. Identify the small differences in their structures that make them well suited to their jobs in the body. Given their similar structures, can the roles of these molecules be reversed?

Ramesh Singh

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Problem 2

(pp. $728-729$ ) and explain how DHAP can enter the glycolysis pathway and be converted to pyruvate.

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Problem 3

How are long-chain fatty acids released from triacylglycerides transported through the bloodstream?

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Problem 4

In $\beta$ oxidation, (a) identify the steps that are oxidations and describe the changes that occur; (b) identify the oxidizing agents; (c) identify the reaction that is an addition;
(d) identify the reaction that is a substitution.

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Problem 5

How many molecules of acetyl-CoA are produced by catabolism of the following fatty acids, and how many $\beta$ oxidations are needed?
(a) Palmitic acid, $\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{14} \mathrm{COOH}$
(b) Lignoceric acid, $\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{22} \mathrm{COOH}$

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Problem 6

Look back at the reactions of the citric acid cycle (Figure 20.9) and identify the three reactions in that cycle that are similar to the first three reactions of the $\beta$ oxidation of a fatty acid.

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Problem 7

How much energy is released as ATP from the complete oxidation of stearic acid $\left(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{16} \mathrm{COOH}\right) ?$

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Problem 8

Which of the following classifications apply to the formation of 3 -hydroxybutyrate from acetoacetate?
(a) Condensation
(b) Hydrolysis
(c) Oxidation
(d) Reduction

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Problem 9

Consider the reactions of ketogenesis.
(a) What role does acetyl-CoA play?
(b) How many acetyl-CoA molecules are used in the production of the ketone bodies?
(c) What is the essential role of ketone bodies during prolonged starvation?

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Problem 10

Starting with acetyl-S-enzyme- 1 and malonyl-CoA, how many molecules of acetylCoA are needed to synthesize an 18 -carbon fatty acid (C18:0)? How many molecules of $\mathrm{CO}_{2}$ are released in this process?

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Problem 11

Oxygen is not a reactant in the $\beta$ oxidation of fatty acids. Can $\beta$ oxidation occur under anaerobic conditions? Explain.

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Problem 12

Identify each lipoprotein described here as either chylomicron, HDL, LDL, or VLDL.
(a) Which lipoprotein has the lowest density? Why?
(b) Which lipoprotein carries triacylglycerols from the diet?
(c) Which lipoprotein removes cholesterol from circulation?
(d) Which lipoprotein contains "bad cholesterol" from a vascular disease risk standpoint?
(e) Which lipoprotein has the highest ratio of protein to lipid?
(f) Which lipoprotein carries triacylglycerols from the liver to peripheral tissues? How are triacylglycerols used?
(g) Which lipoprotein transports cholesterol from the liver to peripheral tissues?

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Problem 13

Lipid metabolism, especially triacylglycerol anabolism and catabolism, is closely associated with carbohydrate (glucose) metabolism. Insulin and glucagon levels in blood are regulated by the glucose levels in blood. Draw lines from the appropriate phrases in column A to appropriate phrases in columns $\mathrm{B}$ and $\mathrm{C}$.

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Problem 14

One strategy used in many different biochemical pathways is an initial investment of energy early on and a large payoff in energy at the end of the pathway. How is this strategy utilized in the catabolism of fats?

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Problem 15

When oxaloacetate in liver tissue is being used for gluconeogenesis, what impact does this have on the citric acid cycle? Explain.

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Problem 16

Why is it more efficient to store energy as triacylglycerols rather than as glycogen?

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Problem 17

Explain the rationale for the production of ketone bodies during starvation.

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Problem 18

Compare the differences between $\beta$ oxidation and fatty acid synthesis (lipogenesis). Are these pathways the reverse of each other?

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Problem 19

Why do lipids make you feel full for a long time after a meal?

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Problem 20

Where does digestion of lipids occur?

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Problem 21

What is the purpose of bile acids in lipid digestion?

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Problem 22

Where are bile acids synthesized, and what is the starting molecule?

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Problem 23

Write the equation for the hydrolysis of a triacylglycerol composed of stearic acid, oleic acid, and linoleic acid by pancreatic lipase.

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Problem 24

Lipases break down triacylglycerols by catalyzing hydrolysis. What are the products of this hydrolysis?

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Problem 25

What are chylomicrons, and how are they involved in lipid metabolism?

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Problem 26

What is the origin of the triacylglycerols transported by very low-density lipoproteins?

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Problem 27

How are the fatty acids from adipose tissue transported?

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Problem 28

How is cholesterol transported around the body? When it leaves the liver, what is its destination and use?

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Problem 29

The glycerol derived from lipolysis of triacylglycerols is converted into glyceraldehyde 3 -phosphate, which then enters into step 6 of the glycolysis pathway. What further transformations are necessary to convert glyceraldehyde 3-phosphate into pyruvate?

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Problem 30

If the conversion of glycerol to glyceraldehyde 3-phosphate releases 1 molecule of ATP, how many molecules of ATP are released during the conversion of glycerol to pyruvate?

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Problem 31

How many molecules of ATP are released in the overall catabolism of glycerol to acetyl-CoA? How many molecules of ATP are released in the complete catabolism of glycerol to $\mathrm{CO}_{2}$ and $\mathrm{H}_{2} \mathrm{O} ?$ (Hint: Combine pathways of glycerol to DHAP with glycolysis from DHAP to pyruvate and pyruvate to acetyl-CoA. Remember to account for any NADH and FADH $_{2}$ produced.)

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Problem 32

How many molecules of acetyl-CoA result from catabolism of 1 molecule of glyceryl trilaurate? (Hint: See Worked Example 24.3 and don't forget glycerol.)

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Problem 33

What is an adipocyte?

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Problem 34

What is the primary function of adipose tissues, and where in the body are they located?

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Problem 35

Which tissues carry out fatty acid oxidation as their primary source of energy?

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Problem 36

Where in the cell does $\beta$ oxidation take place?

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Problem 37

What initial chemical transformation takes place on a fatty acid to activate it for catabolism?

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Problem 38

What must take place before an activated fatty acid undergoes $\beta$ oxidation?

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Problem 39

Why is the stepwise oxidation of fatty acids called $\beta$ oxidation?

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Problem 40

Why is the sequence of reactions that catabolize fatty acids described as a spiral rather than a cycle?

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Problem 41

Which coenzymes are required for $\beta$ oxidation?

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Problem 42

Are these the same coenzymes necessary for fatty acid synthesis?

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Problem 43

How many moles of ATP are produced by one cycle of $\beta$ oxidation?

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Problem 44

How many moles of ATP are produced by the complete oxidation of $1 \mathrm{~mol}$ of myristic acid?

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Problem 45

Arrange these following four molecules in increasing order of their biological energy content (per mole):
(a) Sucrose
(b) Myristic acid, $\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{12} \mathrm{COOH}$
(c) Glucose
(d) Capric acid, $\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{8} \mathrm{COOH}$

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Problem 46

Arrange these four molecules in increasing order of their biological energy content per mole:
(a) Mannose
(b) Stearic acid, $\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{16} \mathrm{COOH}$
(c) Fructose
(d) Palmitic acid, $\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{14} \mathrm{COOH}$

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Problem 47

Show the products of each step in the fatty acid oxidation of hexanoic acid.

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Problem 48

Write the equation for the final step in the catabolism of any fatty acid with an even number of carbons.

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Problem 49

How many molecules of acetyl-CoA result from complete catabolism of the following compounds?
(a) Myristic acid, $\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{12} \mathrm{COOH}$
(b) Caprylic acid, $\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right){ }_{6} \mathrm{COOH}$

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Problem 50

How many cycles of $\beta$ oxidation are necessary to completely catabolize myristic and caprylic acids?

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Problem 51

What three compounds are classified as ketone bodies? Why are they so designated? What process in the body produces them? Why do they form?

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Problem 52

What is ketosis? What condition results from prolonged ketosis? Why is it dangerous?

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Problem 53

What causes acetone to be present in the breath of someone with uncontrolled diabetes?

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Problem 54

Individuals suffering from ketoacidosis have acidic urine. What effect do you expect ketones to have on $\mathrm{pH}$ ? Why is pH lowered when ketone bodies are present?

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Problem 55

Diets that severely restrict carbohydrate intake often result in ketosis for the dieter. Explain why this occurs.

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Problem 56

Name the anabolic pathway that synthesizes fatty acids.

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Problem 57

Explain why $\beta$ oxidation cannot proceed backward to produce triacylglycerols.

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Problem 58

Name the starting material for fatty acid synthesis.

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Problem 59

Why are fatty acids generally composed of an even number of carbons?

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Problem 60

How many rounds of the lipogenesis cycle are needed to synthesize stearic acid, $\mathrm{C}_{17} \mathrm{H}_{35} \mathrm{COOH} ?$

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Problem 61

How many molecules of NADPH are needed to synthesize stearic acid, $\mathrm{C}_{17} \mathrm{H}_{35} \mathrm{COOH} ?$

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Problem 62

How does the cell keep the processes of fatty acid synthesis and degradation separated?

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Problem 63

Describe two differences in the reactions for fatty synthesis and the reactions for fatty acid degradation.

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Problem 64

Consuming too many carbohydrates causes deposition of fats in adipose tissue. How does this happen?

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Problem 65

Why is extra energy consumed as carbohydrates stored as fat and not as glycogen?

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Problem 66

Are any of the intermediates in the $\beta$ -oxidation pathway chiral? Explain.

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Problem 67

Compare fats and carbohydrates as energy sources in terms of the amount of energy released per mole, and account for the observed energy difference.

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Problem 68

Lipoproteins that transport lipids from the diet are described as exogenous. Those that transport lipids produced in metabolic pathways are described as endogenous. Which of the following lipoproteins transports exogenous lipids and which transports endogenous lipids?
(a) Low-density lipoprotein (LDL)
(b) Chylomicrons

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Problem 69

Behenic acid $(\mathrm{C} 22: 0)$ is present in peanut butter.
(a) How many molecules of acetyl-CoA are produced by $\beta$ oxidation of behenic acid?
(b) How many molecules of ATP are produced in (a)?
(c) How many molecules of $\mathrm{CO}_{2}$ are produced by complete oxidation of the acetyl-CoA produced in (a)?
(d) How many molecules of ATP are produced in (c)?
(e) How many total molecules of ATP are produced by the complete oxidation of behenic acid to $\mathrm{CO}_{2} ?$

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Problem 70

High blood-cholesterol levels are dangerous because of their correlation with atherosclerosis and consequent heart attacks and strokes. Is it possible to eliminate all cholesterol from the bloodstream by having a diet that includes no cholesterol? Is it desirable to have no cholesterol at all in your body? Explain your answer.

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Problem 71

In the synthesis of cholesterol, acetyl-CoA is converted to 2 -methyl-1,3-butadiene. Molecules of 2 -methyl- 1,3 -butadiene are then joined to give the carbon skeleton of cholesterol. Draw the condensed structure of 2 -methyl- 1,3 -butadiene. How many carbon atoms does cholesterol contain? What minimum number of 2 -methyl-1,3-butadiene molecules is required to make one molecule of cholesterol?

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Problem 72

A low-fat diet of pasta, bread, beer, and soda can easily lead to an increase in body mass. The increase is stored triacylglycerols in adipocytes. Explain the increase in mass and why the excess carbohydrate is stored as fat.

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