Biochemistry: The Molecular Basis of Life
6th Edition
ISBN: 9780190209896
Author: Trudy McKee, James R. McKee
Publisher: Oxford University Press
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Chapter 15, Problem 7Q
Summary Introduction
To review:
The reactions involved in the process where the products of pyrimidine base catabolism are degraded to succinyl CoA (coenzyme A) and acetyl CoA.
Introduction:
The degradation of pyrimidine bases includes the conversion of uracil and thymine to β-alanine and β-aminoisobutyrate, respectively. These two conversion pathways run parallel to each other. The first step involves the reduction of uracil and thymine by dihydropyrimidine dehydrogenase. The products are hydrolyzedand the open rings of β-ureidopropionate and β-ureidoisobutyrate are formed. These molecules further degrade to form succinyl CoA and acetyl CoA.
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Thiolase is homologous in structure to the condensing enzyme. On the basis of this observation, propose a mechanism for the cleavage of 3-ketoacyl CoA by CoA.
The archaebacterium Haloarcula marismortui can use acetate (which is converted to acetyl-CoA) as a precursor for carbohydrate synthesis, but it lacks isocitrate lyase and therefore cannot use the glyoxylate pathway. Instead, it converts isocitrate to α-ketoglutarate, which is then converted to the amino acid methylaspartate in two more steps. This five-carbon compound is modifi ed and broken down to release glyoxylate. Describe the three reactions that lead from isocitrate to methylaspartate.
One of the regulators of the TCA cycle is succinyl CoA. Discuss the rationale for this molecule to be used to regulate the TCA cycle [include chemical structures and chemical equations where appropriate].
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Chapter 15 Solutions
Biochemistry: The Molecular Basis of Life
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- 2-Bromopalmitoyl-CoA inhibits the oxidation of palmitoyl-CoA by isolated mitochondria but has no effect on the oxidation of palmitoylcarnitine. What is the most likely site of inhibition by 2-bromopalmitoyl-CoA?arrow_forwardFatty acid biosynthesis begins with the condensation of a two-carbon acetyl residue from acetyl-COA and oxaloacetate, while oxidation leads to the release of two carbon units, acetyl-CoA and fatty acyl-CoA. However, these two processes are not simple reversal. Compare and contrast these two pathways.arrow_forwardIsoleucine is degraded to acetyl CoA and succinyl CoA. Suggest a plausible reaction sequence, based on reactions discussed in the text, for this degradation pathwayarrow_forward
- Shemin and coworkers used acetatelabeling experiments to conclude that succinyl-CoA is a key intermediate in the biosynthesis of heme. Identify the intermediates in the conversion of acetate into succinyl-CoA.arrow_forward2-Bromopalmitoyl-CoA inhibits the oxidation of palmitoyl-CoA by isolatedmitochondria but has no effect on the oxidation of palmitoylcarnitine. What is the most likely site of inhibition by 2-bromopalmitoyl-CoA?arrow_forwardFourteen NADPH molecules are required to produce one molecules of palmitic acid from acetyl CoA. Substantiate this statement by referring to the enzymatic activities involved in reduction steps during fatty acid synthesis and the number of cycles required to produce palmitic acid from acetyl CoA. How many molecules of ATP is required for the synthesis of palmitic acid from cytosolic acetyl-CoA?arrow_forward
- Several pathways exist in different organisms for the metabolism of propionyl-CoA. Most organisms utilizethe pathway that feeds into the citric acid cycle via succinyl-CoA. Another metabolic route used by Clostridium kluyveriis a "continuation of -oxidation" and consists of the following enzymatic reactions: (a) propionyl-CoA is converted toacryloyl-CoA; (b) formation of 3-hydroxypropionyl-CoA; (c) conversion to malonyl semialdehyde CoA; (d) productionof malonyl-CoA, which is then (e) decarboxylated to produce acetyl-CoA. Outline the above steps between propionylCoA and acetyl-CoA. Show structures for the metabolites and indicate appropriate cofactors (free or bound) that may beinvolved.arrow_forwardSeveral pathways exist in different organisms for the metabolism of propionyl-CoA. Most organisms utilizethe pathway that feeds into the citric acid cycle via succinyl-CoA. Another metabolic route used by Clostridium kluyveriis a "continuation of -oxidation" and consists of the following enzymatic reactions: (a) propionyl-CoA is converted toacryloyl-CoA; (b) formation of 3-hydroxypropionyl-CoA; (c) conversion to malonyl semialdehyde CoA; (d) productionof malonyl-CoA, which is then (e) decarboxylated to produce acetyl-CoA. Outline the above steps between propionylCoA and acetyl-CoA. Show structures for the metabolites and indicate appropriate cofactors (free or bound) that may beinvolved. b) Suggest a mechanism for enzyme reaction (e).arrow_forwardMost fatty acids have an even number of carbons and, therefore, are completely metabolized to acetyl-CoA. A fatty acid with an odd number of carbonsis metabolized to acetyl-CoA and one equivalent of propionyl-CoA. The following two reactions convert propionyl-CoA into succinyl-CoA, a citric acidcycle intermediate, so it can be further metabolized. Each of the reactions requires a coenzyme. Identify the coenzyme for each step. From what vitaminsare the coenzymes derived? (Hint: see Chapter 23.) COO− COO−CH2CH2methylmalonyl-CoA succinyl-CoA CH3CH2 SCoAarrow_forward
- The mechanism for the conversion from alpha kytoglutarate into succinyl CoA by alpha-ketoglutare dehydrogenase is analogous to the pyruvate dehydrogenase mechanism. Draw and show the major intermediates and arrow pushing for the enzyme-catalyze process for the conversion of alpha-ketoglutarate to succinyl CoA. Note: the carbanion of thiamine pyrophosphate nucleophillically attacks C-2 of the alpha-ketoglutarate, i.e., the carbonyl carbon of that substrate.arrow_forward1) Humans do not undergo net synthesis of carbohydrate from acetyl- CoA, yet carbons of acetyl -CoA can be incorporated into glucose and amino acids. Show pathways as to how this can happen. 2) A molecule -X inhibits oxidation of stearoyl-CoA by isolated mitochondria but has no effect on palmitoyl-carnitine oxidation, explain.arrow_forwardWhen the identical subunits of chicken liver fatty acid synthase are dissociated in vitro, all of the activities can be detected in the separated subunits except for the β-ketoacyl synthase reaction and the overall synthesis of palmitate. Explain these observations.arrow_forward
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