Biochemistry 102 PROBLEM SET #7 Paul Price 2005
Fatty Acid Biosynthesis/Ketone Bodies/Cholesterol
1. Energeticof β-Ketoacyl-ACP Synthase. In the condensation reaction catalyzed by β-ketoacyl-
ACP synthase, a four-carbon unit is synthesized by the combination of a two-carbon unit and a
three-carbon unit, with the release of CO . What is the thermodynamic advantage of this process
over one that simply combines two two-carbon units?
2. Oxygen Requirement for Desaturases. The biosynthesis of palmitoleate (Fig. 20-14), a common
unsaturated fatty acid with a cis double bond in the ∆ position, uses palmitate as a precursor. Can
this be carried out under strictly anaerobic conditions? Explain.
3. Energy Cost of Triacylglycerol Synthesis. Use a net equation for the biosynthesis of
tripalmitoylglycerol (tripalmitin) from glycerol and palmitate to show how many ATPs are required
per molecule of tripalmitin formed.
4. Why is it important that liver cells lack 3-keto acyl-CoA transferase (the enzyme that transfers CoA
from succinyl CoA to acetoacetate)?
5. The tricarboxylate transport system supplies cytosolic acetyl CoA for palmitate synthesis. What
percentage of the NADPH required for palmitate synthesis is thereby provided?
6. Synthesis of Isopentenyl Pyrophosphate. If 2-[ 14C]acetyl-CoA is added to a rat liver homogenate
that is synthesizing cholesterol, where will the14C label appear in ∆ -isopentenyl pyrophosphate,
the activated form of an isoprene unit?
7. HMG-CoA in Ketone Body Synthesis. The rate- limiting step in the early stages of cholesterol
biosynthesis is the conversion of β-hydroxy-β-methylglutaryl-CoA to mevalonate, catalyzed by
HMG-CoA reductase. The liver of a fasting anim al has decreased reductase activity. When the
flow through this reaction is reduced, what is th e effect on the formation of ketone bodies from
acetyl-CoA? How does this help to explain increased ketosis during fasting?
8. Which carbon atoms in palmitate will be labeled most rapidly when a cell extract is given the
following labeled substrates. Unless otherwise stat ed, assume all malonyl CoA is generated from
the (possibly) labeled acetyl CoA.
(a) 14CH (CO) SCoA + excess unlabeled malonyl CoA
(b) CH -3 CO) SCoA + excess unlabeled malonyl CoA
(c) CH (3O)SCoA (unlabeled) + excess O C CH2(CO)SCo2
(d) CH (3O) SCoA + excess O 2 CCH (2O)SCoA
(e) CH (3O)SCoA + excess O CCH 2( CO)2CoA (f) 14CO 2
9. How many deuterium atoms will be incorporated into each palmitate starting with:
(a) CD (CO)SCoA only
(b) CD (3O)SCoA + excess unlabeled malonyl CoA
(c) CH (3O)SCoA + excess O CCD 2CO)SC2A
10. A sample of 3- 14C-alanine is injected into a rat. After 1 h the animal is sacrificed, the liver
removed, and14he lipids extracted. The isolated palmitate contains C. Explain. What is the
location of C in the isolated palmitate? Can alanine be used as a precursor for the net synthesis of
11. Identify the committed steps for fatty acid biosynt hesis and fatty acid oxidation, and describe the
roles of allosteric effectors and covalent regulation in achieving the following objectives:
(a) To ensure that FAB and FAO do not occur simultaneously in a cell.
(b) To reduce the rate of FAB when the end product of the fatty acid synthase reaction, palmitate,
is present in the cytosol of the cell. ANSWERS TO PROBLEM SET #7
1. The thermodynamic advantage of forming the 4 carbon unit by condensation of a 3 carbon unit and
a 2 carbon unit with the release o2 CO that carbon-carbon bond formation is driven by the
hydrolysis of ATP. If we let R-SH stand for CoA-SH, E -SH, and ACP-SH, we can simplify the
FAB reactions as follows:
CH (CO)SR + CO + ATP → O C-CH (CO)SR + ADP +P