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Chapter 21 Lipid Biosynthesis
Multiple Choice Questions
1. Which of the following is not required in the synthesis of fatty acids?
A) Acetyl-CoA
B) Biotin
C) HCO3
– (CO2)
D) Malonyl-CoA
E) NADH
2. Which of the following is not true of the reaction producing malonyl-CoA during fatty acid synthesis?
A) It is stimulated by citrate.
B) It requires acyl carrier protein (ACP).
C) It requires CO2 (or bicarbonate).
D) One mole of ATP is converted to ADP + Pi for each malonyl-CoA synthesized.
E) The cofactor is biotin.
3. If malonyl-CoA is synthesized from 14CO2 and unlabeled acetyl-CoA, and the labeled malonate is
then used for fatty acid synthesis, the final product (fatty acid) will have radioactive carbon in:
A) every C.
B) every even-numbered C-atom.
C) every odd-numbered C-atom.
D) no part of the molecule.
E) only the omega-carbon atom (farthest carbon from C-1).
4. Which one of the following statements best applies to synthesis of fatty acids in E. coli extracts?
A) Acyl intermediates are thioesters of a low molecular weight protein called acyl carrier protein.
B) CO2 or HCO3
– is essential.
C) Reducing equivalents are provided by NADPH.
D) The ultimate source of all the carbon atoms in the fatty acid product is acetyl-CoA.
E) All of the above
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248
5. In comparing fatty acid biosynthesis with
oxidation of fatty acids, which of the following
statements is incorrect?
A) A thioester derivative of crotonic acid (trans-2-butenoic acid) is an intermediate in the synthetic
path, but not in the degradative path.
B) A thioester derivative of D-
-hydroxybutyrate is an intermediate in the synthetic path, not in the
degradative path.
C) Fatty acid biosynthesis uses NADPH exclusively, whereas
oxidation uses NAD+ exclusively.
D) Fatty acid degradation is catalyzed by cytosolic enzymes; fatty acid synthesis by mitochondrial
enzymes.
E) The condensation of two moles of acetyl-CoA in the presence of a crude extract is more rapid in
bicarbonate buffer than in phosphate buffer at the same pH; the cleavage of acetoacetyl-CoA
proceeds equally well in either buffer.
6. Which of the following is not true of the fatty acid synthase and the fatty acid
-oxidation systems?
A) A derivative of the vitamin pantothenic acid is involved.
B) Acyl-CoA derivatives are intermediates.
C) Double bonds are oxidized or reduced by pyridine nucleotide coenzymes.
D) The processes occur in different cellular compartments.
E) The processes occur in the mitochondrial matrix.
7. The rate-limiting step in fatty acid synthesis is:
A) condensation of acetyl-CoA and malonyl-CoA.
B) formation of acetyl-CoA from acetate.
C) formation of malonyl-CoA from malonate and coenzyme A.
D) the reaction catalyzed by acetyl-CoA carboxylase.
E) the reduction of the acetoacetyl group to a
-hydroxybutyryl group.
8. Which of the following is not true of the fatty acid elongation system of vertebrate cells?
A) It involves the same four-step sequence seen in the fatty acid synthase complex.
B) It is located in the smooth endoplasmic reticulum.
C) It produces stearoyl-CoA by the extension of palmitoyl-CoA.
D) It uses malonyl-CoA as a substrate.
E) The immediate precursor of the added carbons is acetyl-CoA.
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249
9. Which of the following is not true about precursors required for fatty acid synthesis in animal cells?
A) NADPH is produced in the cytosol by the pentose phosphate pathway.
B) NADPH is produced in the nucleus by malic enzyme.
C) Acetyl-CoA is transported out of the mitochondrion via the citrate shuttle.
D) CoA is not transported across the mitochondrial membrane.
E) Malonyl-CoA is formed in the cytosol.
10. Which of these can be synthesized by plants but not by humans?
A) Linoleate [18:2(9,12)]
B) Palmitate (16:0)
C) Phosphatidylcholine
D) Pyruvate
E) Stearate (18:0)
11. The enzyme system for adding double bonds to saturated fatty acids requires all of the following
except:
A) a mixed-function oxidase.
B) ATP.
C) cytochrome b5.
D) molecular oxygen (O2).
E) NADPH.
12. Which of these statements about eicosanoid synthesis is true?
A) An early step in the path to thromboxanes is blocked by ibuprofen.
B) Arachidonate is derived mainly by hydrolysis of triacylglycerols.
C) Aspirin acts by blocking the synthesis of arachidonate.
D) Plants can synthesize leukotrienes, but humans cannot.
E) Thromboxanes are produced from arachidonate via the “linear” path.
13. The biosynthesis of triacylglycerols from acetate occurs mainly in:
A) animals but not in plants.
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250
B) humans after ingestion of excess carbohydrate.
C) humans with low carbohydrate intake.
D) plants but not in animals.
E) None of the above
14. The synthesis of both glycerophospholipids and triacylglycerols involves:
A) CDP-choline.
B) CDP-diacylglycerol.
C) phosphatidate phosphatase.
D) phosphatidic acid.
E) phosphoethanolamine.
15. Which of these statements about triacylglycerol synthesis is correct?
A) Humans can store more energy in glycogen than in triacylglycerols.
B) Insulin stimulates conversion of dietary carbohydrate into triacylglycerols.
C) It is not a hormone-sensitive process.
D) Mammals are unable to convert carbohydrates into triacylglycerols.
E) Phosphatidate is not on the pathway of triacylglycerol synthesis.
16. A strategy that is not employed in the synthesis of phospholipids is:
A) condensation of CDP-alcohol with diacylglycerol.
B) condensation of CDP-diacylglycerol with alcohol.
C) condensation of CDP-diacylglycerol with CDP-alcohol.
D) exchange of free alcohol with head group alcohol of phospholipid.
E) remodeling of head group alcohols by chemical modification
17. All glycerol-containing phospholipids are synthesized from:
A) cardiolipin
B) ceramide.
C) gangliosides.
D) mevalonate.
E) phosphatidic acid.
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251
18. In E. coli the synthesis of phosphatidylethanolamine directly involves:
A) acyl carrier protein.
B) biotin.
C) CDP-choline.
D) phosphatidylglycerol.
E) serine.
19. In the synthesis of phosphatidylcholine from phosphatidylethanolamine, the methyl group donor is:
A) a tetrahydrofolate derivative.
B) choline.
C) methanol.
D) S-adenosylmethionine (adoMet).
E) serine.
20. Palmitoyl-CoA, , is a direct precursor of:
A) cholesterol.
B) malonyl-CoA.
C) mevalonate
D) sphingosine.
E) squalene.
21. CDP-diglyceride is not involved in the biosynthesis of:
A) phosphatidylcholine.
B) phosphatidylethanolamine
C) phosphatidylglycerol.
D) phosphatidylserine.
E) sphingomyelin.
22. Which of the following is true of sphingolipid synthesis?
A) All of the carbon atoms of palmitate and serine are incorporated into sphingosine.
B) CDP-sphingosine is the activated intermediate.
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252
C) CO2 is produced during the synthesis of ceramide from palmitate and serine.
D) Glucose 6-phosphate is the direct precursor of the glucose in cerebrosides.
E) Phosphatidic acid is a key intermediate in the pathway.
23. Which of the following is not an intermediate in the synthesis of lanosterol from acetyl-CoA?
A) Isopentenyl pyrophosphate
B) Malonyl-CoA
C) Mevalonate
D) Squalene
)
-Hydroxy-
-methylglutaryl-CoA (HMG-CoA)
24. Cholesterol is synthesized from:
A) acetyl-CoA.
B) choline.
C) lipoic acid.
D) malate.
E) oxalate.
25. A 30-carbon precursor of the steroid nucleus is:
A) farnesyl pyrophosphate.
B) geranyl pyrophosphate.
C) isopentenyl pyrophosphate.
D) lysolecithin.
E) squalene.
26. Which of these statements about cholesterol synthesis is true?
A) Cholesterol is the only known natural product whose biosynthesis involves isoprene units.
B) Only half of the carbon atoms of cholesterol are derived from acetate.
C) Squalene synthesis from farnesyl pyrophosphate results in the release of two moles of PPi for
each mole of squalene formed.
D) The activated intermediates in the pathway are CDP-derivatives.
E) The condensation of two five-carbon units to yield geranyl pyrophosphate occurs in a “head-to-
head” fashion.
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27. Which of the following is derived from a sterol?
A) Bile salts
B) Gangliosides
C) Geraniol
D) Phosphatidylglycerol
E) Prostaglandins
28. Chylomicrons carry in the .
A) triacylglycerols; cell
B) triacylglycerols; blood
C) cholesterols; blood
D) fatty acids; blood
E) fatty acids; cell
29. Lipoprotein particles in human blood do not contain:
A) an apolipoprotein B isoform.
B) cholesterol.
C) cholesteryl esters.
D) lecithin.
E) triglycerides.
30. Which of the following contains the highest percentage of cholesteryl esters?
A) Chylomicrons
B) VLDL
C) LDL
D) HDL
E) None of the above contain any cholesteryl esters.
31. Which of the following is not a step in the uptake of cholesterol from the bloodstream into cells?
A) LDL containing ApoB-100 is recognized by the LDL receptor.
B) LDL bound to the LDL receptor leads to endocytosis of the complex.
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254
C) The cholesteryl esters in the LDL are hydrolyzed to release free cholesterol.
D) The internalized receptor is degraded.
E) The ApoB-100 protein is degraded.
32. Which of these statements about the regulation of cholesterol synthesis is not true?
A) Cholesterol acquired in the diet has essentially no effect on the synthesis of cholesterol in the
liver.
B) Failure to regulate cholesterol synthesis predisposes humans to atherosclerosis.
C) High intracellular cholesterol stimulates formation of cholesteryl esters.
D) Insulin stimulates HMG-CoA reductase.
E) Some metabolite or derivative of cholesterol inhibits HMG-CoA reductase.
33. Which of the following does not contribute to lowering the risk of atherosclerosis?
A) Statin drugs
B) HDL
C) Reverse cholesterol transport
D) ApoA-I
E) Foam cells
34. Which of these compounds is not synthesized by a pathway that includes isoprene precursors?
A) Natural rubber
B) Plastoquinone
C) Vitamin A
D) Vitamin B12
E) Vitamin K
Short Answer Questions
35. If you start with acetyl-CoA labeled with 14C in the methyl-carbon, where does the label end up in
malonyl-CoA?
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255
36. If you perform fatty acid biosynthesis with CO2 labeled with 14C, where does the label end up in a
fatty acid?
37. The reaction sequence that leads to fatty acid synthesis includes (1) condensation, (2) first reduction
reaction, (3) dehydration, and (4) second reduction. Show the first reduction reaction, with any
required cofactors.
38. Fatty acid synthesis and fatty acid breakdown occur by similar pathways. Describe, very briefly, four
ways in which the synthetic and breakdown pathways differ.
39. The synthesis of fatty acids and their breakdown by
-oxidation occur by separate pathways.
Compare the two paths by filling in the blanks below. (Some blanks may require more than one
answer.)
Synthesis
-oxidation
——————————————————
Activating group _______________ ______________
Electron carrier coenzyme(s) _______________ ______________
Basic units added or removed _______________ ______________
Cellular location of process _______________ ______________
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256
40. Show the structure of each intermediate in the conversion of
-hydroxybutyryl-ACP to butyryl-ACP
by the fatty acid synthetase complex. Show where cofactors participate. In your first intermediate,
circle the carbon atoms that are derived from malonyl-CoA.
41. Describe the mechanism for moving acetyl-CoA produced in the mitochondrial matrix into the
cytosol for fatty acid synthesis.
42. Explain briefly why we require fats in our diets.
43. Sketch the pathway from arachidonate to thromboxanes and explain how aspirin blocks the synthesis
of thromboxanes.
44. Explain the triacylglycerol cycle and how drugs such as rosiglitazone (Avandia) promote this cycle
and help lower fatty acid levels in the bloodstream.
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257
45. Describe two basic strategies for activating precursors in the biosynthesis of phospholipids.
46. Show the two different biosynthetic pathways from phosphatidic acid to phosphatidylcholine. You
may use shorthand notation for phosphatidic acid, but name any cofactors required in the path and
show where they are involved.
47. Describe briefly the four stages in the pathway from acetyl-CoA to lanosterol.
48. Show the reaction that limits the rate of cholesterol synthesis from acetate, indicating the role of any
cofactors that participate.
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258
49. Show the pathway from acetyl-CoA to mevalonate, indicating the roles of any cofactors.
50. Show the pathway from mevalonate to dimethylallyl pyrophosphate, indicating where any cofactors
participate.
51. The synthesis of cholesterol begins with the condensation of a four-carbon unit from
_______________ with the two carbons of acetyl-CoA to form a six-carbon derivative. Give its
structure, and circle the atoms that originated in the acetyl-CoA.
52. Describe the formation of farnesyl pyrophosphate from activated isoprenyl units.
53. Show the structure of isopentenyl pyrophosphate and of dimethylallyl pyrophosphate. Connect with a
dotted line the two carbon atoms that will be joined when these two molecules condense to form the
10-carbon intermediate in cholesterol biosynthesis.
54. Shown below is a molecule of squalene, which is composed of 6 isoprene units. Draw lines to
indicate the junctions between the six isoprene units.
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55. When a 10-carbon unit and a 5-carbon unit condense to a 15-carbon intermediate in the pathway to
cholesterol, the mechanism of condensation is basically different from that of the condensation of two
15-carbon units to the 30-carbon compound squalene. Describe the two condensations in enough
chemical detail to illustrate the difference between them. (You should show relevant structures.)
56. What are plasma lipoproteins? What is their general role in mammalian metabolism?
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260
57. Describe the process by which cholesterol esters in the bloodstream enter cells.
58. Describe (briefly) two classes of genetic defects in humans that could produce an elevated blood
serum cholesterol level.
59. The synthetic compound mevinolinic acid, also called lovastatin, is a potent competitive inhibitor of
HMG-CoA reductase (hydroxymethylglutaryl-CoA reductase). Predict and explain the effect of this
drug on serum cholesterol levels in humans.
