Chapter 25, Problem 25.30EP

(a)

Interpretation Introduction

Interpretation:

The type of reaction that occurs in step 1 of a turn of the $\beta -\text{oxidation}$ pathway has to be determined.

Concept Introduction:

The $\beta -\text{oxidation}$ pathway is defined as a repetitive series of four biochemical reactions in which $\text{acyl CoA}$ is degraded to $\text{acetyl CoA}$ by the removal of two carbon atoms at a time. NADH and ${\text{FADH}}_2$ are also produced in this pathway.

Dehydrogenation reaction, hydration reaction, and thiolysis reactions occur in the $\beta -\text{oxidation}$ pathway.

In dehydrogenation reaction, hydrogen molecule $\left({\text{H}}_2\right)$ is removed from an organic substance; in hydration reaction, a water molecule $\left({\text{H}}_2\text{O}\right)$ is added to an unsaturated substrate; in thiolysis reaction, thiol $\left(-\text{SH}\right)$ group cleaves one compound into two compounds.

Answer to Problem 25.30EP

Dehydrogenation reaction occurs in step 1 of a turn of the $\beta -\text{oxidation}$ pathway.

Explanation of Solution

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

The hydrogen molecule is removed from $\text{acyl CoA}$, therefore; the reaction in step 1 of a turn of the $\beta -\text{oxidation}$ pathway is a dehydrogenation reaction.

(b)

Interpretation Introduction

Interpretation:

The type of reaction that occurs in step 2 of a turn of the $\beta -\text{oxidation}$ pathway has to be determined.

Concept Introduction:

The $\beta -\text{oxidation}$ pathway is defined as a repetitive series of four biochemical reactions in which $\text{acyl CoA}$ is degraded to $\text{acetyl CoA}$ by the removal of two carbon atoms at a time. NADH and ${\text{FADH}}_2$ are also produced in this pathway.

Dehydrogenation reaction, hydration reaction, and thiolysis reactions occur in the $\beta -\text{oxidation}$ pathway.

In dehydrogenation reaction, hydrogen molecule $\left({\text{H}}_2\right)$ is removed from an organic substance; in hydration reaction, a water molecule $\left({\text{H}}_2\text{O}\right)$ is added to an unsaturated substrate; in thiolysis reaction, thiol $\left(-\text{SH}\right)$ group cleaves one compound into two compounds.

Answer to Problem 25.30EP

Hydration reaction occurs in step 2 of a turn of the $\beta -\text{oxidation}$ pathway.

Explanation of Solution

In step 2, a water $\left({\text{H}}_2\text{O}\right)$ molecule is added across the trans double bond in $\text{trans-enoyl CoA}$, therefore; secondary alcohol is produced at $\beta$ carbon.  This reaction is catalyzed by $\text{enoyl CoA hydratase}$ enzyme.

The reaction for step 2 is as follows:

A water molecule is added in step 2, therefore; the reaction in step 2 of a turn of the $\beta -\text{oxidation}$ pathway is a hydration reaction.

(c)

Interpretation Introduction

Interpretation:

The type of reaction that occurs in step 3 of a turn of the $\beta -\text{oxidation}$ pathway has to be determined.

Concept Introduction:

The $\beta -\text{oxidation}$ pathway is defined as a repetitive series of four biochemical reactions in which $\text{acyl CoA}$ is degraded to $\text{acetyl CoA}$ by the removal of two carbon atoms at a time. NADH and ${\text{FADH}}_2$ are also produced in this pathway.

Dehydrogenation reaction, hydration reaction, and thiolysis reactions occur in the $\beta -\text{oxidation}$ pathway.

In dehydrogenation reaction, hydrogen molecule $\left({\text{H}}_2\right)$ is removed from an organic substance; in hydration reaction, a water molecule $\left({\text{H}}_2\text{O}\right)$ is added to an unsaturated substrate; in thiolysis reaction, thiol $\left(-\text{SH}\right)$ group cleaves one compound into two compounds.

Answer to Problem 25.30EP

Dehydrogenation reaction occurs in step 3 of a turn of the $\beta -\text{oxidation}$ pathway.

Explanation of Solution

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

The hydrogen $\left({\text{H}}_2\right)$ molecule is removed from $\text{L-}\beta \text{-hydroxyacyl CoA}$. Therefore, the reaction in step 3 of a turn of the $\beta -\text{oxidation}$ pathway is a dehydrogenation reaction.

(d)

Interpretation Introduction

Interpretation:

The type of reaction that occurs in step 4 of a turn of the $\beta -\text{oxidation}$ pathway has to be determined.

Concept Introduction:

The $\beta -\text{oxidation}$ pathway is defined as a repetitive series of four biochemical reactions in which $\text{acyl CoA}$ is degraded to $\text{acetyl CoA}$ by the removal of two carbon atoms at a time. NADH and ${\text{FADH}}_2$ are also produced in this pathway.

Dehydrogenation reaction, hydration reaction, and thiolysis reactions occur in the $\beta -\text{oxidation}$ pathway.

In dehydrogenation reaction, hydrogen molecule $\left({\text{H}}_2\right)$ is removed from an organic substance; in hydration reaction, a water molecule $\left({\text{H}}_2\text{O}\right)$ is added to an unsaturated substrate; in thiolysis reaction, thiol $\left(-\text{SH}\right)$ group cleaves one compound into two compounds.

Answer to Problem 25.30EP

Thiolysis reaction occurs in step 4 of a turn of the $\beta -\text{oxidation}$ pathway.

Explanation of Solution

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

The carbon-carbon bond in $\beta \text{-ketoacyl CoA}$ is cleaved by $\text{CoA-SH}$ molecule. $\text{Acyl CoA}$ and $\text{acetyl CoA}$ is formed in this reaction.  Therefore, thiolysis reaction occurs in step 4 of a turn of the $\beta -\text{oxidation}$ pathway.