Download Chapter 21 Lipid Biosynthesis

Chapter 21 Lipid Biosynthesis
Multiple Choice Questions
1. Biosynthesis of fatty acids and eicosanoids
Ans: E
Which of the following is not required in the synthesis of fatty acids?
A)
B)
C)
D)
E)
Acetyl-CoA
Biotin
–
HCO3 (CO2)
Malonyl-CoA
NADH
2. Biosynthesis of fatty acids and eicosanoids
Ans: D
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If malonyl-CoA is synthesized from CO2 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)
B)
C)
D)
E)
every C.
every even-numbered C-atom.
every odd-numbered C-atom.
no part of the molecule.
only the omega-carbon atom (farthest carbon from C-1).
3. Biosynthesis of fatty acids and eicosanoids
Ans: E
Which one of the following statements best applies to synthesis of fatty acids in E. coli extracts?
A)
B)
C)
D)
E)
Acyl intermediates are thioesters of a low molecular weight protein called acyl carrier protein.
–
CO2 or HCO3 is essential.
Reducing equivalents are provided by NADPH
The ultimate source of all the carbon atoms in the fatty acid product is acetyl-CoA.
All of the above are true.
4. Biosynthesis of fatty acids and eicosanoids
Ans: D
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.
Chapter 21 Lipid Biosynthesis
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.
5. Biosynthesis of fatty acids and eicosanoids
Ans: E
Which of the following is not true of the fatty acid synthase and the fatty acid -oxidation systems?
A)
B)
C)
D)
E)
A derivative of the vitamin pantothenic acid is involved.
Acyl-CoA derivatives are intermediates.
Double bonds are oxidized or reduced by pyridine nucleotide coenzymes.
The processes occur in different cellular compartments.
The processes occur in the mitochondrial matrix.
6. Biosynthesis of fatty acids and eicosanoids
Ans: D
The rate-limiting step in fatty acid synthesis is:
A)
B)
C)
D)
E)
condensation of acetyl-CoA and malonyl-CoA.
formation of acetyl-CoA from acetate.
formation of malonyl-CoA from malonate and coenzyme A.
the reaction catalyzed by acetyl-CoA carboxylase.
the reduction of the acetoacetyl group to a -hydroxybutyryl group.
7. Biosynthesis of fatty acids and eicosanoids
Ans: A
Which of these can be synthesized by plants but not by humans?
A)
B)
C)
D)
E)
9,12
Linoleate [18:2( )]
Palmitate (16:0)
Phosphatidylcholine
Pyruvate
Stearate (18:0)
8. Biosynthesis of triacylglycerols
Ans: B
The biosynthesis of triacylglycerols from acetate occurs mainly in:
A)
B)
C)
D)
E)
animals but not in plants.
humans after ingestion of excess carbohydrate.
humans with low carbohydrate intake.
plants but not in animals.
none of the above.
Chapter 21 Lipid Biosynthesis
9. Biosynthesis of triacylglycerols
2 Ans: D
The synthesis of both glycerophospholipids and triacylglycerols involves:
A)
B)
C)
D)
E)
CDP-choline.
CDP-diacylglycerol.
phosphatidate phosphatase.
phosphatidic acid.
phosphoethanolamine.
10. Biosynthesis of triacylglycerols
Ans: B
Which of these statements about triacylglycerol synthesis is correct?
A)
B)
C)
D)
E)
Humans can store more energy in glycogen than in triacylglycerols.
Insulin stimulates conversion of dietary carbohydrate into triacylglycerols.
It is not a hormone-sensitive process.
Mammals are unable to convert carbohydrates into triacylglycerols.
Phosphatidate is not on the pathway of triacylglycerol synthesis.
Short Answer Questions
11. Biosynthesis of fatty acids and eicosanoids
In the conversion shown below (which occurs during fatty acid synthesis), if the compound on the left
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were labeled with C in its middle carbon (*), where would the label be in the compound on the
right? Circle the atoms that would be labeled. (Not all reactants are shown.)
Ans: The compound on the right, butyryl-ACP, would be labeled in C-2 (C-1 is the acyl carbon).
12. Biosynthesis of fatty acids and eicosanoids
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.
+
+
Ans: -ketoacyl-ACP + NADPH + H hydroxyacyl-ACP + NADP
13. Biosynthesis of fatty acids and eicosanoids
Fatty acid synthesis and fatty acid breakdown occur by similar pathways. Describe, very briefly, four
ways in which the synthetic and breakdown pathways differ.
Ans: Fatty acid synthesis (in any order) (1) employs NADPH as reducing agent; (2) involves an acyl
group bound to a protein, ACP; (3) takes place in the cytosol of animals; (4) involves the
condensation of malonyl- and acetyl-groups; (5) involves the formation of the D--hydroxyacyl
Chapter 21 Lipid Biosynthesis
derivative. Fatty acid breakdown (1) employs NAD+ as electron acceptor; (2) involves acyl groups
bound to coenzyme A; (3) occurs in the mitochondrial matrix; (4) does not involve malonylderivatives; (5) involves the L-stereoisomer of the -hydroxyacyl derivative.
14. Biosynthesis of fatty acids and eicosanoids
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.)
Activating group
Electron carrier coenzyme(s)
Basic units added or removed
Cellular location of process
Synthesis
 oxidation
——————————————————
_______________
______________
_______________
______________
_______________
______________
_______________
______________
Ans:
Synthesis
 oxidation
————————————————————
Activating group
Electron carrier coenzyme(s)
Basic units added or removed
Cellular location of process
acyl carrier protein
NADPH
malonyl- and acetylcytosol in animals,
chloroplast in plants
CoA—SH
NAD+
acetylmitochondrial matrix
15. Biosynthesis of fatty acids and eicosanoids
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.
2
Ans: -hydroxybutyryl-ACP is first dehydrated, yielding trans- -butenoyl-ACP, which is then
reduced to butyryl-ACP, with NADPH as the reducing agent. (See Fig. 21-6, p. 810.)
16. Biosynthesis of fatty acids and eicosanoids
Pages: 815-816
Difficulty: 2
Explain briefly why we require fats in our diets.
Ans: Dietary fats provide the linoleate and linolenate that we need (for eicosanoid synthesis) but
cannot synthesize.