Chapter 13, Problem 13.45EP

(a)

Interpretation Introduction

Interpretation: To identify ${\text{CO}}_2$ is produced 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.

Answer to Problem 13.45EP

Carbon dioxide $\left({\text{CO}}_2\right)$ is produced in the $\text{acetyl CoA}$ formation and ethanol fermentation.

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:

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

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. In step 2, acetaldehyde is reduced to ethanol by alcohol dehydrogenase enzymes. 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, ${\text{CO}}_2$ is produced in the $\text{acetyl CoA}$ formation and ethanol fermentation.

Reason for incorrect choice:

The reaction equation for lactate fermentation is as follows:

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

${\text{CO}}_2$ is not associated with lactate fermentation.

(b)

Interpretation Introduction

Interpretation: To identify NADH is a reactant 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.

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

A reactant is defined as the substance that is initially present in the chemical reaction and gets consumed to form a new substance.

Answer to Problem 13.45EP

NADH is encountered as a reactant 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:

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

Ethanol fermentation process occurs in some microorganisms (for example yeast) under the anaerobic conditions. 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, NADH is encountered as a reactant in the lactate and ethanol production from pyruvate.

Reason for incorrect choice:

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

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

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

(c)

Interpretation Introduction

Interpretation: To identify ${\text{NAD}}^{+}$ is a reactant 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.

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

A reactant is defined as the substance that is initially present in the chemical reaction and gets consumed to form a new substance.

Answer to Problem 13.45EP

In the production of $\text{acetyl CoA}$ from pyruvate, ${\text{NAD}}^{+}$ is encountered as a reactant.

Explanation of Solution

Reason for correct choice:

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

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

Therefore, ${\text{NAD}}^{+}$ is reduced to NADH when pyruvate is converted to $\text{acetyl CoA}$ under aerobic conditions.

Reason for incorrect choice:

The reaction equation for the conversion of pyruvate to lactate is as follows:

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

Ethanol fermentation process occurs in some microorganisms (for example yeast) under the anaerobic conditions. 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, ${\text{NAD}}^{+}$ is the product in the lactate and ethanol production from pyruvate.

(d)

Interpretation Introduction

Interpretation: To identify the end product is a ${\text{C}}_3$ molecule 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.

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 13.45EP

In the absence of oxygen, pyruvate is converted to ${\text{C}}_3$ lactate molecule.

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. This anaerobic reduction is called lactate fermentation. The chemical reaction for the formation of lactate is as follows:

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

Reason for incorrect choice:

In the ethanol fermentation process, pyruvate is converted to ethanol and carbon dioxide by enzymes under the anaerobic conditions. 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:

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

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.