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Chapter 29: The Organic Chemistry of Metabolic Pathways

Every problem in the chapter beside the book’s answer, a chapter at a time as you scroll.

Chapter 29: The Organic Chemistry of Metabolic Pathways

16 problems · 4 figures

29-1 §29.1 source ↗

One of the steps in fat metabolism is the reaction of glycerol (1,2,3-propanetriol) with ATP to yield glycerol 1-phosphate. Write the reaction, and draw the structure of glycerol 1-phosphate.

Answer

HOCH2CH(OH)CH2OH + ATP ​→ ​HOCH2CH(OH)CH2OPO32− + ADP

29-2 §29.3 source ↗

Write the equations for the remaining passages of the β-oxidation pathway following those shown in Figure 29.5.
Figure 29.5 in the book ↗
Catabolism of the 14-carbon myristic acid by the β-oxidation pathway yields seven molecules of acetyl CoA after six passages.

Answer

Caprylyl CoA ​→ ​Hexanoyl CoA ​→ ​ Butyryl CoA ​→ ​2 Acetyl CoA

29-3 §29.3 source ↗

How many molecules of acetyl CoA are produced by catabolism of the following fatty acids, and how many passages of the β-oxidation pathway are needed?
(a)
Palmitic acid, CH3(CH2)14CO2H
(b)
Arachidic acid, CH3(CH2)18CO2H

Answer (2 parts)

(a)
8 acetyl CoA; 7 passages
(b)
10 acetyl CoA; 9 passages

29-4 §29.4 source ↗

Write a mechanism for the dehydration reaction of β-hydroxybutyryl ACP to yield crotonyl ACP in step 7 of fatty-acid synthesis.

Answer

The dehydration is an E1cB reaction.

29-5 §29.4 source ↗

Evidence for the role of acetate in fatty-acid biosynthesis comes from isotope-labeling experiments. If acetate labeled with 13C in the methyl group (13CH3CO2H) were incorporated into fatty acids, at what positions in the fatty-acid chain would you expect the 13C label to appear?

Answer

At C2, C4, C6, C8, and so forth

29-6 §29.4 source ↗

Does the reduction of acetoacetyl ACP in step 6 occur on the Re face or the Si face of the molecule?

Answer

The Si face

29-7 §29.5 source ↗

Identify the two steps of glycolysis in which ATP is produced.

Answer

Steps 7 and 10

29-8 §29.5 source ↗

Look at the entire glycolysis pathway, and make a list of the kinds of organic reactions that take place—nucleophilic acyl substitutions, aldol reactions, E1cB reactions, and so forth.

Answer

Steps 1, 3: Phosphate transfers; steps 2, 5, 8: isomerizations; step 4: retro-aldol reaction; step 5: oxidation and nucleophilic acyl substitution; steps 7, 10: phosphate transfers; step 9: E1cB dehydration

29-9 §29.6 source ↗

Which carbon atoms in glucose end up as –CH3 carbons in acetyl CoA? Which carbons end up as CO2?

Answer

C1 and C6 of glucose become –CH3 groups; C3 and C4 become CO2.

29-10 §29.7 source ↗

Which of the substances in the citric acid cycle are tricarboxylic acids, thus giving the cycle its alternative name?

Answer

Citrate and isocitrate

29-11 §29.7 source ↗

Write mechanisms for step 2 of the citric acid cycle, the dehydration of citrate and the addition of water to aconitate.

Answer

E1cB elimination of water, followed by conjugate addition

29-12 §29.7 source ↗

Is the pro-R or pro-S hydrogen removed from citrate during the dehydration in step 2 of the citric acid cycle? Does the elimination reaction occur with syn or anti geometry?

Answer

pro-R; anti geometry

29-13 §29.8 source ↗

Write a mechanism for step 6 of gluconeogenesis, the reduction of 3-phosphoglyceryl phosphate with NADH/H+ to yield glyceraldehyde 3-phosphate.

Answer

The reaction occurs by two sequential nucleophilic acyl substitutions, the first by a cysteine residue in the enzyme, with phosphate as leaving group, and the second by hydride donation from NADH, with the cysteine residue as leaving group.

29-14 §29.9 source ↗

Write all the steps in the transamination reaction of PMP with α-ketoglutarate plus a lysine residue in the enzyme to give the PLP–enzyme imine plus glutamate.

Answer

Initial imine formation between PMP and α-ketoglutarate is followed by double-bond rearrangement to an isomeric imine and hydrolysis.

29-15 §29.9 source ↗

What α-keto acid is formed on transamination of leucine?

Answer

(CH3)2CHCH2COCO2−

29-16 §29.9 source ↗

From what amino acid is the following α-keto acid derived?

Answer

Asparagine

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Problems and answers are adapted from Organic Chemistry: A Tenth Edition by John McMurry, OpenStax, and are used under the CC BY-NC-SA 4.0 licence. Access for free at https://openstax.org/books/organic-chemistry/pages/1-why-this-chapter. Illustrations: Copyright Rice University, OpenStax, under CC BY-NC-SA 4.0 license. This adaptation is released under the same licence, is non-commercial, and is neither affiliated with nor endorsed by OpenStax or Rice University.