Chapter 25, Problem 25.48EP

(a)

Interpretation Introduction

Interpretation:

Whether dehydrogenase is involved in (1) glycerol metabolism to dihydroxyacetone phosphate, (2) fatty acid metabolism to acetyl CoA, or (3) both glycerol metabolism and fatty acid metabolism has to be determined.

Concept introduction:

Triacylglycerol mobilization is an ongoing process in which triacylglycerols that are stored in the adipose tissue are hydrolyzed. Fatty acids and glycerol are the products of triacylglycerol mobilization. The products are released into the bloodstream.

After entering the bloodstream, the glycerol travels to the kidneys or liver. The first stage of glycerol metabolism occurs in the liver or kidney. The first stage of glycerol metabolism is a two-step process. After the first stage, the remaining stages of glycerol metabolism are the same as glucose pathways.  The overall equation for glycerol metabolism is as follows:

$\text{Glycerol}+\text{ATP}+{\text{NAD}}^{+}\to \text{Dihydroxyacetone phosphate}+\text{ADP}+\text{NADH}+{\text{H}}^{+}$

Fatty acids are molecules that are long hydrocarbon chain of carboxylic acid. They are building blocks of fat in humans and animals.

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. Acetyl CoA, FADH₂, and NADH are produced in this pathway.

Answer to Problem 25.48EP

Dehydrogenase is involved in (3) both glycerol metabolism and fatty acid metabolism.

Explanation of Solution

The first stage of glycerol metabolism is a two-step process. In step 1, glycerol-3-phosphate is formed as the intermediate compound that further reacts to form dihydroxyacetone phosphate in step 2. The reaction for the conversion of glycerol is as follows:

Here, represents ${\text{PO}}_3^{2-}$ unit.

In step 2 of glycerol metabolism, glycerol-3-phosphate dehydrogenase enzymes catalyzed the conversion of glycerol-3-phosphate to dihydroxyacetone phosphate. Therefore, dehydrogenase is involved in glycerol metabolism.

The reaction in step 1 of a turn of the β-oxidation pathway is a dehydrogenation reaction in which 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. The reaction for step 1 is as follows:

The reaction in step 1 of a turn of the β-oxidation pathway is catalyzed by acyl CoA dehydrogenase enzyme.

Hence, dehydrogenase is involved in (3) both glycerol metabolism and fatty acid metabolism.

(b)

Interpretation Introduction

Interpretation:

Whether AMP is involved in (1) glycerol metabolism to dihydroxyacetone phosphate, (2) fatty acid metabolism to acetyl CoA, or (3) both glycerol metabolism and fatty acid metabolism has to be determined.

Concept introduction:

Triacylglycerol mobilization is an ongoing process in which triacylglycerols that are stored in the adipose tissue are hydrolyzed. Fatty acids and glycerol are the products of triacylglycerol mobilization. The products are released into the bloodstream.

After entering the bloodstream, the glycerol travels to the kidneys or liver. The first stage of glycerol metabolism occurs in the liver or kidney. The first stage of glycerol metabolism is a two-step process. After the first stage, the remaining stages of glycerol metabolism are the same as glucose pathways.  The overall equation for glycerol metabolism is as follows:

$\text{Glycerol}+\text{ATP}+{\text{NAD}}^{+}\to \text{Dihydroxyacetone phosphate}+\text{ADP}+\text{NADH}+{\text{H}}^{+}$

Fatty acids are molecules that are long hydrocarbon chain of carboxylic acid. They are building blocks of fat in humans and animals.

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. Acetyl CoA, FADH₂, and NADH are produced in this pathway.

AMP (adenosine monophosphate) is an important nucleotide that is used in the metabolic processes in the living cells. It is interconverted to adenosine triphosphate (ATP) or to ADP (adenosine diphosphate).

Answer to Problem 25.48EP

AMP is involved in (2) fatty acid metabolism to acetyl CoA.

Explanation of Solution

The first stage of fatty acid metabolism is the activation of fatty acids in the outer mitochondrial membrane. The fatty acid is activated by coenzyme A and ATP. The activation reaction of fatty acid is as follows:

ATP is converted to AMP. Hence, AMP is involved in fatty acid metabolism to acetyl CoA.

(c)

Interpretation Introduction

Interpretation:

Whether acyl CoA is involved in (1) glycerol metabolism to dihydroxyacetone phosphate, (2) fatty acid metabolism to acetyl CoA, or (3) both glycerol metabolism and fatty acid metabolism has to be determined.

Concept introduction:

Triacylglycerol mobilization is an ongoing process in which triacylglycerols that are stored in the adipose tissue are hydrolyzed. Fatty acids and glycerol are the products of triacylglycerol mobilization. The products are released into the bloodstream.

After entering the bloodstream, the glycerol travels to the kidneys or liver. The first stage of glycerol metabolism occurs in the liver or kidney. The first stage of glycerol metabolism is a two-step process. After the first stage, the remaining stages of glycerol metabolism are the same as glucose pathways.  The overall equation for glycerol metabolism is as follows:

$\text{Glycerol}+\text{ATP}+{\text{NAD}}^{+}\to \text{Dihydroxyacetone phosphate}+\text{ADP}+\text{NADH}+{\text{H}}^{+}$

Fatty acids are molecules that are long hydrocarbon chain of carboxylic acid. They are building blocks of fat in humans and animals.

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. Acetyl CoA, FADH₂, and NADH are produced in this pathway.

Answer to Problem 25.48EP

Acyl CoA is involved in (2) fatty acid metabolism to acetyl CoA.

Explanation of Solution

The first stage of fatty acid metabolism is the activation of fatty acids in the outer mitochondrial membrane. The fatty acid is activated by coenzyme A and ATP. ATP is converted to AMP. The activation reaction of fatty acid is as follows:

The activated fatty acid-CoA that is formed after the activation of the fatty acid molecule in the oxidation process of fatty acids is called acyl CoA.

The enzymes that are needed for the oxidation of fatty acid are located in the mitochondrial matrix. Acyl CoA cannot pass through the inner mitochondrial membrane to the mitochondrial matrix because it is too large. A shuttle mechanism that involves the molecule carnitine effects the entry of acyl CoA into the mitochondrial matrix. After this process, acyl CoA is degraded to acetyl CoA through a series of four biochemical reactions. This pathway is termed as a β-oxidation pathway. Acetyl CoA, FADH₂, and NADH are produced in this pathway.

Hence, acyl CoA is involved in fatty acid metabolism to acetyl CoA.

(d)

Interpretation Introduction

Interpretation:

Whether glycerol-3-phosphate is involved in (1) glycerol metabolism to dihydroxyacetone phosphate, (2) fatty acid metabolism to acetyl CoA, or (3) both glycerol metabolism and fatty acid metabolism has to be determined.

Concept introduction:

Triacylglycerol mobilization is an ongoing process in which triacylglycerols that are stored in the adipose tissue are hydrolyzed. Fatty acids and glycerol are the products of triacylglycerol mobilization. The products are released into the bloodstream.

After entering the bloodstream, the glycerol travels to the kidneys or liver. The first stage of glycerol metabolism occurs in the liver or kidney. The first stage of glycerol metabolism is a two-step process. After the first stage, the remaining stages of glycerol metabolism are the same as glucose pathways.  The overall equation for glycerol metabolism is as follows:

$\text{Glycerol}+\text{ATP}+{\text{NAD}}^{+}\to \text{Dihydroxyacetone phosphate}+\text{ADP}+\text{NADH}+{\text{H}}^{+}$

Fatty acids are molecules that are long hydrocarbon chain of carboxylic acid. They are building blocks of fat in humans and animals.

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. Acetyl CoA, FADH₂, and NADH are produced in this pathway.

Answer to Problem 25.48EP

Glycerol-3-phosphate is involved in (1) glycerol metabolism to dihydroxyacetone phosphate.

Explanation of Solution

The first stage of glycerol metabolism is a two-step process. In step 1, glycerol-3-phosphate is formed as the intermediate compound that further reacts to form dihydroxyacetone phosphate in step 2. The reaction for the conversion of glycerol is as follows:

Here, represents ${\text{PO}}_3^{2-}$ unit.

Therefore, glycerol-3-phosphate is involved in glycerol metabolism.