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Sample Problem 24.1 Fats and Digestion
Answer each of the following for the digestion of triacylglycerols:
a. Where does it take place?
b. What enzyme is involved?
c. What are the products of digestion?
Solution
a. the small intestine
b. pancreatic lipase
c. monoacylglycerols and fatty acids
Study Check 24.1
How are the nonpolar triacylglycerols emulsified so that they can be digested?
Answer
The triacylglycerols entering the small intestine are emulsified by bile salts into smaller droplets of fat that can react with
the lipases.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Sample Problem 24.2 β Oxidation
Match each of the following (a to d) with one of the reactions (1 to 4) in the β oxidation cycle:
(1) first oxidation (2) hydration
(3) second oxidation (4) cleavage (thiolysis)
a. Water is added to a trans double bond.
b. An acetyl CoA is removed.
c. FAD is reduced to FADH2.
d. A 3-hydroxyacyl CoA is converted to 3-ketoacyl CoA.
Solution
a. (2) hydration
b. (4) cleavage (thiolysis)
d. (3) second oxidation
c. (1) first oxidation
Study Check 24.2
Determine the number of β oxidation cycles and number of acetyl CoA molecules produced for cerotic acid (C26), a fatty
acid found in beeswax and carnauba wax.
Answer
Cerotic acid (C26) requires 12 β oxidation cycles, and produces 13 acetyl CoA molecules.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Sample Problem 24.3 ATP Production from β Oxidation
How many ATP will be produced from the β oxidation of palmitic acid, a C16 saturated fatty acid?
Solution
Palmitic acid (C16) requires seven β oxidation cycles, which produce seven NADH and seven FADH2. The total number
of acetyl CoA is eight. The activation of palmitic acid decreases the total produced by two ATP.
Study Check 24.3
Compare the total ATP from the reduced coenzymes and from acetyl CoA in the β oxidation of palmitic acid.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Sample Problem 24.3 ATP Production from β Oxidation
Continued
Answer
Eight acetyl CoA units yield 80 ATP whereas seven NADH and seven FADH2 yield 28 ATP.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Sample Problem 24.4 Ketogenesis
The process called ketogenesis takes place in the liver.
a. What are some conditions that promote ketogenesis?
b. What are the names of the three compounds called ketone bodies?
c. What ketone bodies are responsible for the acidosis that occurs in ketogenesis?
Solution
a. When excess acetyl CoA cannot be processed by the citric acid cycle, acetyl CoA molecules enter the ketogenesis
pathway and form ketone bodies.
b. The ketone bodies are acetoacetate, β-hydroxybutyrate, and acetone.
c. The ketone bodies acetoacetate and β-hydroxybutyrate, which are carboxylic acids, can decrease the pH of the blood
(metabolic acidosis).
Study Check 24.4
Which ketone bodies contain each of the following functional groups?
a. ketone
b. alcohol
Answer
a. Acetone contains a ketone group.
b. β-Hydroxybutyrate contains an alcohol group.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Sample Problem 24.5 Fatty Acid Synthesis
Malonyl ACP is required for the elongation of fatty acid chains.
a. Complete the following equations for the formation of malonyl ACP from the starting material:
Acetyl CoA + HCO3− + ATP →
Malonyl CoA + HS—ACP →
b. What enzymes catalyze these reactions?
Solution
a. Acetyl CoA combines with bicarbonate to form malonyl CoA, which reacts with ACP to form malonyl ACP.
Acetyl CoA + HCO3− + ATP → malonyl CoA + ADP + Pi + H+
Malonyl CoA + HS—ACP → malonyl ACP + HS—CoA
b. The enzyme for the first reaction is acetyl CoA carboxylase. The enzyme for the second reaction is malonyl CoA
transacylase.
Study Check 24.5
If malonyl ACP is a three-carbon acyl group, why are only two carbon atoms added each time malonyl ACP is combined
with a fatty acid chain?
Answer
In each cycle of fatty acid synthesis, a two-carbon acetyl group from the three-carbon group in malonyl ACP adds to the
growing fatty acid chain and one carbon forms CO2.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Sample Problem 24.6 Digestion of Proteins
What are the sites and end products for the digestion of proteins?
Solution
The digestion of proteins begins in the stomach and is completed in the small intestine to yield amino acids.
Study Check 24.6
What is the function of HCl in the stomach?
Answer
HCl denatures proteins and activates enzymes such as pepsin.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Sample Problem 24.7 Nitrogen Balance
With a positive nitrogen balance, why are excess amino acids excreted?
Solution
Because the body cannot store nitrogen, amino acids that are not needed for the synthesis of proteins are excreted.
Study Check 24.7
Under what condition does the body have a negative nitrogen balance?
Answer
In conditions such as fasting or starvation, a diet insufficient in protein leads to a negative nitrogen balance.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Sample Problem 24.8 Transamination
Write the equation for the transamination of glutamate and oxaloacetate by drawing the condensed structural
formulas.
Solution
Study Check 24.8
What are some other possible names for the enzyme that catalyzes the reaction in Sample Problem 24.8?
Answer
glutamate aminotransferase, glutamate transaminase, aspartate aminotransferase
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Sample Problem 24.9 Transamination and Oxidative Deamination
Indicate whether each of the following represents a transamination or an oxidative deamination:
a. Glutamate is converted to α-ketoglutarate and NH4+.
b. Alanine and α-ketoglutarate react to form pyruvate and glutamate.
Solution
a. Oxidative deamination occurs when the —NH3+ group in glutamate is removed as an ammonium ion, NH4+.
b. Transamination occurs when an amino group is transferred from an amino acid to an α-keto acid such as
α-ketoglutarate.
Study Check 24.9
Is the reaction catalyzed by glutamate dehydrogenase, which requires NAD +, an example of transamination or oxidative
deamination?
Answer
oxidative deamination
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Sample Problem 24.10 Urea Cycle
Indicate the reaction in the urea cycle where each of the following compounds is a reactant:
a. aspartate
b. ornithine
c. arginine
Solution
a. Aspartate and citrulline undergo condensation in reaction 2.
b. A carbamoyl group is transferred to ornithine in reaction 1.
c. Arginine is cleaved in reaction 4.
Study Check 24.10
Name the products of each reaction in Sample Problem 24.10.
Answer
a. argininosuccinat
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
b. citrulline
c. urea and ornithine
© 2016 Pearson Education, Inc.
Sample Problem 24.11 Degradation of Amino Acids
Identify the citric acid cycle component and the number of ATP produced by each of the following amino acids:
a. proline
b. tyrosine
c. tryptophan
Solution
a. Proline is converted to the citric acid cycle intermediate α-ketoglutarate. The reactions in the remaining part of the
citric acid cycle from α-ketoglutarate to oxaloacetate produce two NADH, one GTP, and one FADH2. The two NADH
provide five ATP, one GTP provides one ATP, and one FADH2 provides 1.5 ATP, for a total of 7.5 ATP from proline.
b. Tyrosine is converted to the citric acid cycle intermediate fumarate. The remaining reactions of the citric acid cycle
from fumarate to oxaloacetate produce one NADH, which provides 2.5 ATP.
c. Tryptophan is converted to the citric acid cycle intermediate acetyl CoA. The reactions in the citric acid cycle starting
with acetyl CoA produce three NADH, one GTP, and one FADH2. The three NADH provide 7.5 ATP, one GTP
provides one ATP, and one FADH2 provides 1.5 ATP, for a total of 10 ATP from tryptophan.
Study Check 24.11
Why does the citric acid cycle component from leucine supply more ATP than the intermediate from phenylalanine?
Answer
Leucine forms the citric acid cycle intermediate acetyl CoA that enters at the beginning of the citric acid cycle to provide
10 ATP. Phenylalanine forms the citric acid cycle intermediate fumarate that enters later in the citric acid cycle to
produce one NADH, which is used to provide 2.5 ATP.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.