Chapter 25, Problem 25.99EP

(a)

Interpretation Introduction

Interpretation:

The process that has four distinct reaction steps among (1) the β-oxidation pathway, (2) ketogenesis, or (3) lipogenesis has to be identified.

Concept introduction:

Lipogenesis is the process employed for the synthesis of fatty acid. The starting precursor for the synthesis is acetyl CoA. The enzyme employed for the process is fatty acid synthase. It is a multienzyme complex that ties the reaction responsible for the synthesis of fatty acid. The fatty acid is synthesized in two parts. In the first part, there is citrate-malate shuttle system and in the second part there is a cyclic process to synthesize saturated fatty acid.

The fatty acids are broken down to provide energy. The breakdown of fatty acids is a three parts process. In the first part, the fatty acid is activated. In the second part, the transportation of fatty acid into the mitochondrial matrix is facilitated by a shuttle mechanism. In the third part, the fatty acid is readily oxidized, cycling through a series of four reactions. In these series of reactions, acyl CoA is degraded to acetyl CoA. This pathway is termed as a β-oxidation pathway.

Ketogenesis is a metabolic process by which ketone bodies are produced by the breakdown of fatty acids and ketogenic amino acids. This metabolic process supplies our organs with needed energy under certain circumstances such as starvation. Fatty acid molecules degrade into acetyl CoA which are utilized as reactants in the process of ketogenesis. These molecules of acetyl CoA undergo the process of condensation twice, followed by chain cleavage and hydrogenation to produce ketone bodies.

Answer to Problem 25.99EP

The β-oxidation pathway, ketogenesis and chain process of lipogenesis involve four distinct reaction steps.

Explanation of Solution

All three processes involve four distinct reactions.

The four distinct reactions involved in β-oxidation pathway is:

The four distinct reactions involved in lipogenesis is:

The four distinct reactions involved in ketogenesis is:

(b)

Interpretation Introduction

Interpretation:

The process that two different hydrogenation reactions among (1) the β-oxidation pathway, (2) ketogenesis, or (3) lipogenesis has to be identified.

Concept introduction:

Lipogenesis is the process employed for the synthesis of fatty acid. The starting precursor for the synthesis is acetyl CoA. The enzyme employed for the process is fatty acid synthase. It is a multienzyme complex that ties the reaction responsible for the synthesis of fatty acid. The fatty acid is synthesized in two parts. In the first part, there is citrate-malate shuttle system and in the second part there is a cyclic process to synthesize saturated fatty acid.

The fatty acids are broken down to provide energy. The breakdown of fatty acids is a three parts process. In the first part, the fatty acid is activated. In the second part, the transportation of fatty acid into the mitochondrial matrix is facilitated by a shuttle mechanism. In the third part, the fatty acid is readily oxidized, cycling through a series of four reactions. In these series of reactions, acyl CoA is degraded to acetyl CoA. This pathway is termed as a β-oxidation pathway.

Ketogenesis is a metabolic process by which ketone bodies are produced by the breakdown of fatty acids and ketogenic amino acids. This metabolic process supplies our organs with needed energy under certain circumstances such as starvation. Fatty acid molecules degrade into acetyl CoA which are utilized as reactants in the process of ketogenesis. These molecules of acetyl CoA undergo the process of condensation twice, followed by chain cleavage and hydrogenation to produce ketone bodies.

In hydrogenation reaction, a hydrogen molecule (H₂) is added to an organic substance.

Answer to Problem 25.99EP

Lipogenesis has two different hydrogenation reactions.

Explanation of Solution

Lipogenesis involves two different hydrogenation reactions in step 2 and 4 of the cyclic process of lipogenesis.

Step 2 is the first hydrogenation reaction step of the cyclic process and it involves the hydrogenation of acetoacetyl ACP to synthesis β-hydroxybutyryl ACP with the help of reducing agent NADPH. The reaction of this step is:

Step 4 involves the hydrogenation reaction. In this step, crotonyl ACP is converted to butyryl ACP with the help of reducing agent NADPH. The reaction of this step is:

(c)

Interpretation Introduction

Interpretation:

The process that two different dehydrogenation reactions among (1) the β-oxidation pathway, (2) ketogenesis, or (3) lipogenesis has to be identified.

Concept introduction:

Lipogenesis is the process employed for the synthesis of fatty acid. The starting precursor for the synthesis is acetyl CoA. The enzyme employed for the process is fatty acid synthase. It is a multienzyme complex that ties the reaction responsible for the synthesis of fatty acid. The fatty acid is synthesized in two parts. In the first part, there is citrate-malate shuttle system and in the second part there is a cyclic process to synthesize saturated fatty acid.

The fatty acids are broken down to provide energy. The breakdown of fatty acids is a three parts process. In the first part, the fatty acid is activated. In the second part, the transportation of fatty acid into the mitochondrial matrix is facilitated by a shuttle mechanism. In the third part, the fatty acid is readily oxidized, cycling through a series of four reactions. In these series of reactions, acyl CoA is degraded to acetyl CoA. This pathway is termed as a β-oxidation pathway.

Ketogenesis is a metabolic process by which ketone bodies are produced by the breakdown of fatty acids and ketogenic amino acids. This metabolic process supplies our organs with needed energy under certain circumstances such as starvation. Fatty acid molecules degrade into acetyl CoA which are utilized as reactants in the process of ketogenesis. These molecules of acetyl CoA undergo the process of condensation twice, followed by chain cleavage and hydrogenation to produce ketone bodies.

In dehydrogenation reaction, a hydrogen molecule (H₂) is removed from an organic substance.

Answer to Problem 25.99EP

The β-oxidation pathway has two different dehydrogenation reactions.

Explanation of Solution

The β-oxidation pathway has two different dehydrogenation reactions in step 1 and 3.

In step 1 of a turn of the β-oxidation pathway, hydrogen atoms from $\alpha$ and $\beta$ carbons of acyl CoA are removed; a double bond is formed between $\alpha$ and $\beta$ carbons. FAD oxidizes acyl CoA. This reaction is catalyzed by acyl CoA dehydrogenase enzyme. The reaction for step 1 is as follows:

In step 3 of a turn of the β-oxidation pathway, two hydrogen atoms are removed from L- β-hydroxyacyl CoA. In this reaction, β-hydroxy group is converted to a keto group. NAD⁺ is used as an oxidizing agent. This reaction is catalyzed by β-hydroxyacyl CoA dehydrogenase enzyme. The reaction for step 3 is as follows:

(d)

Interpretation Introduction

Interpretation:

The process that involves thiolysis reaction among (1) the β-oxidation pathway, (2) ketogenesis, or (3) lipogenesis has to be identified.

Concept introduction:

Lipogenesis is the process employed for the synthesis of fatty acid. The starting precursor for the synthesis is acetyl CoA. The enzyme employed for the process is fatty acid synthase. It is a multienzyme complex that ties the reaction responsible for the synthesis of fatty acid. The fatty acid is synthesized in two parts. In the first part, there is citrate-malate shuttle system and in the second part there is a cyclic process to synthesize saturated fatty acid.

The fatty acids are broken down to provide energy. The breakdown of fatty acids is a three parts process. In the first part, the fatty acid is activated. In the second part, the transportation of fatty acid into the mitochondrial matrix is facilitated by a shuttle mechanism. In the third part, the fatty acid is readily oxidized, cycling through a series of four reactions. In these series of reactions, acyl CoA is degraded to acetyl CoA. This pathway is termed as a β-oxidation pathway.

Ketogenesis is a metabolic process by which ketone bodies are produced by the breakdown of fatty acids and ketogenic amino acids. This metabolic process supplies our organs with needed energy under certain circumstances such as starvation. Fatty acid molecules degrade into acetyl CoA which are utilized as reactants in the process of ketogenesis. These molecules of acetyl CoA undergo the process of condensation twice, followed by chain cleavage and hydrogenation to produce ketone bodies.

In thiolysis reaction, thiol (-SH) group cleaves one compound into two compounds.

Answer to Problem 25.99EP

The β-oxidation pathway involves thiolysis reaction.

Explanation of Solution

In step 4 of a turn of the β-oxidation pathway, the fatty acid carbon chain between $\alpha$ and $\beta$ carbons is broken by CoA-SH molecule. The reaction for step 4 is as follows:

The carbon-carbon bond in β-ketoacyl CoA is cleaved by CoA-SH molecule. Acyl CoA and acetyl CoA is formed in this reaction.