Chapter 21 Cannot Further Metabolized Oxidation However Separate

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Chapter 21

Lipid Metabolism

SUMMARY

Section 21.2

 Fatty acids are activated and transported to the mitochondrial matrix for further

catabolism.

Section 21.3

 The complete oxidation of fatty acids by the citric acid cycle and the electron–

transport chain releases large amounts of energy.

Section 21.4

 Fatty acids with uneven numbers of carbon atoms produce propionyl-CoA in the

Section 21.5

 If an organism has an excess of acetyl-CoA, it produces substances related to

acetone, thus the name ―ketone bodies.‖

Section 21.6

 Acetyl-CoA is transported to the cytosol and converted to malonyl-CoA. Chain

Section 21.7

 Most compound lipids such as triacylglycerols, phosphoacylglycerols, and

sphingolipids, have fatty acids as precursors.

2 Chapter 21

Section 21.8

 The biosynthesis of cholesterol proceeds by the condensation of five-carbon

isoprenoid units.

Section 21.9

 Accumulation of fats in tissues, especially as fat cells (adipocytes) gives rise to

overweight and obesity, which, in turn, lead to drastic health problems such as

diabetes, heart attacks, and strokes. In addition obesity can predispose to some

kinds of cancer

 The neurons that tend to inhibit eating produce melanocortins, another class of

peptide hormones. The neurons that suppress appetite have receptors for

melanocortin and one of several kinds of receptors for NPY, as well as for other

hormones such as insulin or leptin. The neurons that stimulate appetite have

various kinds of NPY receptors, as well as receptors for insulin and other

hormones

LECTURE NOTES

This chapter deals with both lipid catabolism and anabolism, allowing for direct

comparison of the two processes. The information will likely be new to students;

Lipid Metabolism 3

LECTURE OUTLINE

I. Catabolism of lipids

A. Lipases and phospholipases

B. Role of coenzyme A

C. Role of carnitine

D. β-oxidation

1. Oxidation yielding FADH2

II. Energy yield from the oxidation of fatty acids

III. Catabolism of unsaturated fatty acids and odd-carbon fatty acids

A. Odd-numbered fatty acids yield priopionyl-CoA

2. cis/trans isomerization

IV. Ketone bodies

V. Fatty acid biosynthesis

A. Sources of acetyl-CoA

B. Use of NADPH vs. NADH

C. Production of malonyl-CoA

D. Fatty acid synthase

VI. Synthesis of acylglycerols and compound lipids

A. Triacylglycerols

B. Phosphoacylglycerols

C. Sphingolipids

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ANSWERS TO PROBLEMS

21.1 Lipids Are Involved in the Generation and Storage of Energy

(a) For mobile organisms—such as a migrating hummingbird—weight can be a

critical factor, and packing the most energy into the least weight is decidedly

advantageous. A 2.5-g hummingbird needs to add about 2 g of fat for migration

energy, which would increase body weight by 80%. The equivalent amount of

21.2 Catabolism of Lipids

2. Phospholipase A1 hydrolyzes the ester bond to carbon-1 of the glycerol

backbone; phospholipase A2 hydrolyzes the ester bond to carbon-2 of the

backbone.

3. A hormone signal activates adenylate cyclase, which makes cAMP. This

activates protein kinases, which phosphorylate the lipases, thereby activating

them.

4. Acyl-CoAs are high-energy compounds. An acyl-CoA has sufficient energy to

The two carbons shown in boldface type are the ones that will have the double

Lipid Metabolism 5

21.3 The Energy Yield from the Oxidation of Fatty Acids

10. One obtains 6.7 ATP per carbon and 0.42 ATP per gram for stearic acid versus 5

ATP per carbon and 0.17 ATP per gram for glucose. More energy is available

from stearic acid than from glucose.

11. The processing of the acetyl-CoA through the citric acid cycle and the electron

13. The humps of camels contain lipids that can be degraded as a source of

metabolic water, rather than water as such.

21.4 Catabolism of Unsaturated Fatty Acids and Odd-Carbon Fatty Acids

14. For an odd-chain fatty acid, -oxidation proceeds normally until the last round.

When five carbons are left, that round of -oxidation releases one acetyl-CoA

and one propionyl-CoA. Propionyl-CoA cannot be further metabolized by –

CoA isomerase.

17. For a polyunsaturated fatty acid, two additional enzymes are needed, the enoyl-

CoA isomerase and 2,4-dienoyl-CoA reductase.

18. From seven cycles of -oxidation: 7 acetyl-CoA, 1 propionyl-CoA, 7 FADH2, 7

NADH. From the processing of 7 acetyl-CoA in the citric acid cycle: 7 FADH2, 21

NADH, and 7 GTP. From the processing of the propionyl-CoA: –1 ATP for

conversion to succinyl-CoA, –1 GTP from the citric acid cycle, and 1 NADH and 1

FADH2 from the citric acid cycle. From reoxidation of all FADH2 and NADH: 22.5

6 Chapter 21

21. It would take seven cycles of -oxidation to release 14 carbons as acetyl-CoA,

with the last three being released as propionyl-CoA.

22. Fats cannot produce a net yield of glucose because they must enter the citric

acid cycle as the two-carbon unit acetyl-CoA. In the first few steps, two carbons

21.5 Ketone Bodies

23. Ketones are produced when there is an imbalance in lipid catabolism, compared

with carbohydrate catabolism. If fatty acids are being -oxidized to produce

acetyl-CoA, but there is insufficient oxaloacetate because it is being drawn off for

gluconeogenesis, the acetyl-CoA molecules combine to form ketone bodies.

24. Two acetyl-CoA molecules combine to form acetoacetyl-CoA. This can then

diet or drinking more water to flush the system more thoroughly.

21.6 Fatty-Acid Biosynthesis

28. The two pathways have in common the involvement of acetyl-CoA and

thioesters, and each round of breakdown or synthesis involves two-carbon units.

The differences are many: malonyl-CoA is involved in biosynthesis, not in

29. Step 1: biotin is carboxylated using bicarbonate ion (HCO3–) as the source of the

carboxyl group. Step 2: the carboxylated biotin is brought into proximity with

enzyme-bound acetyl-CoA by a biotin carrier protein. Step 3: the carboxyl group

is transferred to acetyl-CoA, forming malonyl-CoA.

Lipid Metabolism 7

32. In -oxidation, FAD is the coenzyme for the first oxidation reaction, while NAD+ is

the coenzyme for the second. In fatty-acid synthesis, NADPH is the coenzyme

a protein.

34. ACP is a molecule that earmarks acyl groups for fatty-acid synthesis. It can be

managed separately from acyl-CoA groups. Also, the ACP attaches to the acyl

groups like a ―swinging arm‖ that tethers it to the fatty-acid synthase complex.

37. Acetyl groups condense with oxaloacetate to form citrate, which can cross the

mitochondrial membrane. Acetyl groups are regenerated in the cytosol by the

reverse reaction.

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39. Energy is needed to condense an acetyl group to the growing fatty acid. In

theory, such could be done with acetyl-CoA, using ATP. In practice, the ATP is

used to convert acetyl-CoA to malonyl-CoA; the condensation of the acetyl

40.

(a) The lipoate ―swinging arm‖ of the pyruvate dehydrogenase complex.

21.7 Synthesis of Acylglycerols and Compound Lipids

41. The glycerol comes from degradation of other acylglycerols or from glycerol-3-

phosphate derived from glycolysis.

21.8 Cholesterol Biosynthesis

44. In steroid biosynthesis, three acetyl-CoA molecules condense to form the six-

carbon mevalonate, which then gives rise to a five-carbon isoprenoid unit. A

second and then a third isoprenoid unit condense, giving rise to a 10-carbon and

biosynthetic origin.

48. One oxygen atom from O2 is needed to form the epoxide. The NADPH is needed

to reduce the other oxygen atom to water.

Lipid Metabolism 9

21.9 Hormonal Control of Appetite

51. Neuropeptide Y operates in the central nervous system. Its function is to start a

chain of events that stimulates appetite.

52. Melanocortins stimulate a chain of events that suppress appetite.