Chapter 24, Problem 24.46EP

(a)

Interpretation Introduction

Interpretation: To identify acetaldehyde is an intermediate in which the fate of pyruvate- $\text{acetyl CoA}$, lactate, and ethanol.

Concept introduction: Pyruvate is the end product in the glycolysis. The production of the fate of pyruvate varies with the nature of the organism and the cellular conditions. The common fates of pyruvate are as follows:

Aerobic reactions need oxygen while anaerobic reactions don’t need oxygen. Pyruvate forms $\text{acetyl CoA}$ under oxygen-rich conditions. The fermentation process is an alternative method that oxidized NADH to ${\text{NAD}}^{+}$ under anaerobic conditions.

An intermediate is defined as the transient species that is formed from the reactants in the preceding step and gets consumed in the subsequent steps to generate the products. An intermediate is formed within a multi-step reaction mechanism.

Answer to Problem 24.46EP

Acetaldehyde is an intermediate in the formation of ethanol.

Explanation of Solution

Reason for correct choice:

In the ethanol fermentation process, pyruvate is converted to ethanol and carbon dioxide $\left({\text{CO}}_2\right)$ by enzymes under the anaerobic conditions. The process of ethanol fermentation takes place in two steps. In step 1, the pyruvate molecule is converted to acetaldehyde by pyruvate decarboxylase enzymes. Carbon dioxide molecule is produced in this step. The chemical reaction is as follows:

In step 2, acetaldehyde is reduced to ethanol by alcohol dehydrogenase enzymes.

The chemical reaction is as follows:

The ethanol fermentation equation is as follows:

$\text{Pyruvate}+\text{NADH}+{\text{2H}}^{+}\stackrel{\text{Two steps}}{\to }\text{Ethanol}+{\text{CO}}_2+{\text{NAD}}^{+}$

Therefore, acetaldehyde is formed as an intermediate in the fermentation of ethanol.

Reason for incorrect choice:

The reaction equation for the conversion of pyruvate to $\text{acetyl CoA}$ is as follows:

$\text{Pyruvate}+{\text{NAD}}^{+}+\text{CoA}-\text{SH}\stackrel{\text{pyruvate dehydrogenase complex}}{\to }\text{Acetyl}-\text{CoA}+\text{NADH}+{\text{CO}}_2$

Acetaldehyde is not involved in the oxidation of pyruvate to $\text{acetyl CoA}$.

The chemical equation for the formation of lactate is as follows:

$\text{Pyruvate}+\text{NADH}+{\text{H}}^{+}\stackrel{\text{Lactate dehydrogenase}}{\to }\text{lactate}+{\text{NAD}}^{+}$

Acetaldehyde is not involved in lactate fermentation.

(b)

Interpretation Introduction

Interpretation: To identify NADH is a product in which fate of pyruvate- $\text{acetyl CoA}$, lactate, and ethanol.

Concept introduction: Pyruvate is the end product in the glycolysis. The production of the fate of pyruvate varies with the nature of the organism and the cellular conditions. The common fates of pyruvate are as follows:

Aerobic reactions need oxygen while anaerobic reactions don’t need oxygen. Pyruvate forms $\text{acetyl CoA}$ under oxygen-rich conditions. The fermentation process is an alternative method that oxidized NADH to ${\text{NAD}}^{+}$ under anaerobic conditions.

Nicotinamide adenine dinucleotide is associated with the redox reactions in metabolism. Its reduced form is NADH and oxidized form is ${\text{NAD}}^{+}$.

Answer to Problem 24.46EP

NADH is formed along with $\text{acetyl CoA}$ from pyruvate under aerobic conditions in humans.

Explanation of Solution

Reason for correct choice:

Under aerobic conditions, pyruvate is converted to $\text{acetyl CoA}$ by the involvement of pyruvate dehydrogenase complex enzymes in the human body. This conversion involves oxidation as well as decarboxylation reaction. The overall reaction equation for the conversion of pyruvate to $\text{acetyl CoA}$ is as follows:

$\text{Pyruvate}+{\text{NAD}}^{+}+\text{CoA}-\text{SH}\stackrel{\text{pyruvate dehydrogenase complex}}{\to }\text{Acetyl}-\text{CoA}+\text{NADH}+{\text{CO}}_2$

Therefore, NADH is formed along with $\text{acetyl CoA}$ from pyruvate under aerobic conditions in humans.

Reason for incorrect choice:

The ethanol fermentation equation is as follows:

$\text{Pyruvate}+\text{NADH}+{\text{2H}}^{+}\stackrel{\text{Two steps}}{\to }\text{Ethanol}+{\text{CO}}_2+{\text{NAD}}^{+}$

NADH is encountered as a reactant in the fermentation of ethanol.

The lactate fermentation equation is as follows:

$\text{Pyruvate}+\text{NADH}+{\text{H}}^{+}\stackrel{\text{Lactate dehydrogenase}}{\to }\text{lactate}+{\text{NAD}}^{+}$

NADH is encountered as a reactant in lactate fermentation.

(c)

Interpretation Introduction

Interpretation: To identify ${\text{NAD}}^{+}$ is a product in which fate of pyruvate- $\text{acetyl CoA}$, lactate, and ethanol.

Concept introduction: Pyruvate is the end product in the glycolysis. The production of the fate of pyruvate varies with the nature of the organism and the cellular conditions.

The common fates of pyruvate are as follows:

Aerobic reactions need oxygen while anaerobic reactions don’t need oxygen. Pyruvate forms $\text{acetyl CoA}$ under oxygen-rich conditions. The fermentation process is an alternative method that oxidized NADH to ${\text{NAD}}^{+}$ under anaerobic conditions.

Nicotinamide adenine dinucleotide is associated with the redox reactions in metabolism. Its reduced form is NADH and oxidized form is ${\text{NAD}}^{+}$.

Answer to Problem 24.46EP

${\text{NAD}}^{+}$ is formed as a product in the lactate and ethanol production from pyruvate.

Explanation of Solution

Reason for correct choice:

In the absence of oxygen, pyruvate is converted to lactate by lactate dehydrogenase enzymes in the human body. In this reaction, NADH is oxidized to ${\text{NAD}}^{+}$. This anaerobic reduction is called lactate fermentation. The overall reaction equation for the conversion of pyruvate to lactate is as follows:

$\text{Pyruvate}+\text{NADH}+{\text{H}}^{+}\stackrel{\text{Lactate dehydrogenase}}{\to }\text{lactate}+{\text{NAD}}^{+}$

The ethanol fermentation equation is as follows:

$\text{Pyruvate}+\text{NADH}+{\text{2H}}^{+}\stackrel{\text{Two steps}}{\to }\text{Ethanol}+{\text{CO}}_2+{\text{NAD}}^{+}$

Reason for incorrect choice:

The overall reaction equation for the conversion of pyruvate to $\text{acetyl CoA}$ is as follows:

$\text{Pyruvate}+{\text{NAD}}^{+}+\text{CoA}-\text{SH}\stackrel{\begin{array}{l}\text{pyruvate dehydrogenase }\\ \text{complex}\end{array}}{\to }\text{Acetyl}-\text{CoA}+\text{NADH}+{\text{CO}}_2$

NADH is formed along with $\text{acetyl CoA}$ from pyruvate under aerobic conditions in humans.

(d)

Interpretation Introduction

Interpretation: To identify the end product is a ${\text{C}}_2$ molecule in which fate of pyruvate- $\text{acetyl CoA}$, lactate, and ethanol.

Concept introduction: Pyruvate is the end product in the glycolysis. The production of the fate of pyruvate varies with the nature of the organism and the cellular conditions. The common fates of pyruvate are as follows:

Aerobic reactions need oxygen while anaerobic reactions don’t need oxygen. Pyruvate forms $\text{acetyl CoA}$ under oxygen-rich conditions. The fermentation process is an alternative method that oxidized NADH to ${\text{NAD}}^{+}$ under anaerobic conditions.

Pyruvate $\left({\text{CH}}_3{\text{COCOO}}^{-}\right)$ is the conjugate base of pyruvic acid $\left({\text{CH}}_3\text{COCOOH}\right)$. Three carbon atoms are present in pyruvate, therefore, pyruvate is a ${\text{C}}_3$ molecule.

Answer to Problem 24.46EP

The end product of the ethanol fermentation is ${\text{C}}_2$ ethanol molecule and the end product in the formation of $\text{acetyl CoA}$ is a ${\text{C}}_2$ acetyl group molecule.

Explanation of Solution

Reason for correct choice:

In the ethanol fermentation process, pyruvate is converted to ethanol and carbon dioxide by enzymes under the anaerobic conditions. The process of ethanol fermentation takes place in two steps. In step 1, the pyruvate molecule is converted to acetaldehyde by pyruvate decarboxylase enzymes. Carbon dioxide molecule is produced in this step. The chemical reaction is as follows:

In step 2, acetaldehyde is reduced to ethanol by alcohol dehydrogenase enzymes. The chemical reaction is as follows:

The ethanol fermentation equation is as follows:

$\text{Pyruvate}+\text{NADH}+{\text{2H}}^{+}\stackrel{\text{Two steps}}{\to }\text{Ethanol}+{\text{CO}}_2+{\text{NAD}}^{+}$

Ethanol $\left({\text{CH}}_3{\text{CH}}_2\text{OH}\right)$ contains two carbon atoms. Therefore, it is a ${\text{C}}_2$ molecule.

Pyruvate is converted to $\text{acetyl CoA}$ under oxygen-rich conditions by pyruvate dehydrogenase complex enzymes in the human body. The chemical reaction for the formation of $\text{acetyl CoA}$ is as follows:

Acetyl group $\left({\text{CH}}_3{\text{COO}}^{-}\right)$ contains two carbon atoms. Therefore, ${\text{CH}}_3{\text{COO}}^{-}$ is a ${\text{C}}_2$ molecule.

Reason for incorrect choice:

The chemical reaction for the formation of lactate is as follows:

Lactate contains three carbon atoms. Therefore, lactate is a ${\text{C}}_3$ molecule.