Chapter 23, Problem 23.103EP

(a)

Interpretation Introduction

Interpretation: To indicate whether oxaloacetate undergoes (1) oxidation but not reduction, (2) reduction but not oxidation, (3) both oxidation and reduction or (4) neither oxidation nor reduction in the common metabolic pathway.

Concept introduction: Common metabolic pathway is the total sum of metabolic reactions that occur in stage third and fourth of the biochemical process or it is defined as the total sum of reactions that occur in the citric acid cycle and electron transport chain and oxidative phosphorylation.

These stages are included in the common metabolic pathway because the reactions in these stages are the same for different kinds of food.

The citric acid cycle is the third stage of the biochemical energy production process. The cycle includes the reactions in which the acetyl part of acetyl CoA is oxidized and leads to the formation of carbon dioxide and $\text{CoA-SH}$.

Electron transport chain is a sequence of biochemical reactions in which electrons and hydrogen atoms from the citric acid cycle are transferred to various intermediate carriers and finally reacts with molecular oxygen to form a water molecule.

Answer to Problem 23.103EP

Oxaloacetate undergoes neither oxidation nor reduction in the common metabolic pathway.

Explanation of Solution

Oxaloacetate undergoes condensation reaction in the first step of the citric acid cycle.

The first step involves the condensation reaction of oxaloacetate and $\text{acetyl CoA}$. They both react in the presence of citrate synthase to form the citrate ion. The reaction of the first step of the citric acid cycle is:

(b)

Interpretation Introduction

Interpretation: To indicate whether ${\text{NAD}}^{+}$ undergoes (1) oxidation but not reduction, (2) reduction but not oxidation, (3) both oxidation and reduction or (4) neither oxidation nor reduction in the common metabolic pathway.

Concept introduction: Common metabolic pathway is the total sum of metabolic reactions that occur in stage third and fourth of the biochemical process or it is defined as the total sum of reactions that occur in the citric acid cycle and electron transport chain and oxidative phosphorylation.

These stages are included in the common metabolic pathway because the reactions in these stages are the same for different kinds of food.

The citric acid cycle is the third stage of the biochemical energy production process. The cycle includes the reactions in which the acetyl part of acetyl CoA is oxidized and leads to the formation of carbon dioxide and $\text{CoA-SH}$.

Electron transport chain is a sequence of biochemical reactions in which electrons and hydrogen atoms from the citric acid cycle are transferred to various intermediate carriers and finally reacts with molecular oxygen to form a water molecule.

Answer to Problem 23.103EP

${\text{NAD}}^{+}$ undergoes reduction in the common metabolic pathway.

Explanation of Solution

Nicotinamide adenine dinucleotide exists in two forms: oxidized form $\left({\text{NAD}}^{+}\right)$ and reduced form $\left(\text{NADH}\right)$. It is employed as an oxidizing agent in various reactions like oxidation of secondary alcohol into a ketone. Its structure consists of three subunits: nicotinamide, ribose, and ADP. The reaction of the reduction of ${\text{NAD}}^{+}$ is:

${\text{NAD}}^{+}+2{\text{H}}^{+}+2{\text{e}}^{-}\to \text{NADH}+{\text{H}}^{+}$

${\text{NAD}}^{+}$ undergoes reduction in step 3, 4 and 8 of the citric acid cycle.

The reaction of step 3 is:

${\text{NAD}}^{\text{+}}$ acts as an oxidizing agent in this step. Isocitrate is oxidized to oxalosuccinate and ${\text{NAD}}^{\text{+}}$ is reduced to form NADH.

The reaction of step 4 is:

${\text{NAD}}^{\text{+}}$ acts as an oxidizing agent in this step. $\alpha \text{-ketoglutarate}$ is oxidized to $\text{succinyl CoA}$ and ${\text{NAD}}^{\text{+}}$ is reduced to form NADH.

The reaction of step 8 is:

${\text{NAD}}^{\text{+}}$ acts as an oxidizing agent in this step. $\text{L-malate}$ is oxidized to oxaloacetate and ${\text{NAD}}^{\text{+}}$ is reduced to form NADH.

(c)

Interpretation Introduction

Interpretation: To indicate whether FADH₂ undergoes (1) oxidation but not reduction, (2) reduction but not oxidation, (3) both oxidation and reduction, or (4) neither oxidation nor reduction in the common metabolic pathway.

Concept introduction: Common metabolic pathway is the total sum of metabolic reactions that occur in stage third and fourth of the biochemical process or it is defined as the total sum of reactions that occur in the citric acid cycle and electron transport chain and oxidative phosphorylation.

These stages are included in the common metabolic pathway because the reactions in these stages are the same for different kinds of food.

The citric acid cycle is the third stage of the biochemical energy production process. The cycle includes the reactions in which the acetyl part of acetyl CoA is oxidized and leads to the formation of carbon dioxide and $\text{CoA-SH}$.

Electron transport chain is a sequence of biochemical reactions in which electrons and hydrogen atoms from the citric acid cycle are transferred to various intermediate carriers and finally reacts with molecular oxygen to form a water molecule.

Answer to Problem 23.103EP

FADH₂ undergoes oxidation in the common metabolic pathway.

Explanation of Solution

FADH₂is the reduced form of flavin adenine dinucleotide. The main function of flavin adenine dinucleotide is to act as an oxidizing agent and used by the cell in oxidation reactions like oxidation of fatty acid. The reaction of the oxidation of FADH₂ is:

${\text{FADH}}_2\to \text{FAD}+2{\text{H}}^{+}+2{\text{e}}^{-}$

FADH₂undergoes oxidation in complex II of the electron transport chain.

Complex II consists of four subunits in its structure. This complex interacts initially with the electrons that are coming after the reduction of FADH₂. FADH₂ is oxidized to form FAD in this reaction. The diagrammatic representation of electron transfer in complex II in the electron transfer chain is:

(d)

Interpretation Introduction

Interpretation: To indicate whether $\text{FeSP}$ undergoes (1) oxidation but not reduction, (2) reduction but not oxidation, (3) both oxidation and reduction or (4) neither oxidation nor reduction in the common metabolic pathway.

Concept introduction: Common metabolic pathway is the total sum of metabolic reactions that occur in stage third and fourth of the biochemical process or it is defined as the total sum of reactions that occur in the citric acid cycle and electron transport chain and oxidative phosphorylation.

These stages are included in the common metabolic pathway because the reactions in these stages are the same for different kinds of food.

The citric acid cycle is the third stage of the biochemical energy production process. The cycle includes the reactions in which the acetyl part of acetyl CoA is oxidized and leads to the formation of carbon dioxide and $\text{CoA-SH}$.

Electron transport chain is a sequence of biochemical reactions in which electrons and hydrogen atoms from the citric acid cycle are transferred to various intermediate carriers and finally reacts with molecular oxygen to form a water molecule.

Answer to Problem 23.103EP

$\text{FeSP}$ undergoes both oxidation and reduction in the common metabolic pathway.

Explanation of Solution

$\text{FeSP}$ undergoes both oxidation and reduction in the complex II, III and IV of the electron transport chain. Iron-sulfur protein $\left(\text{FeSP}\right)$ has iron in its structure that can change its oxidation state from $+2$ to $+3$.

Complex I consists of more than 40 structural subunits. Its structure has B-vitamin-containing flavin mononucleotide $\left(\text{FMN}\right)$ and various iron-sulfur proteins $\left(\text{FeSP}\right)$.

The diagrammatic representation of electron transfer in complex I in the electron transfer chain is as follows:

Complex II consists of four subunits in its structure. This complex interacts initially with the electrons that are coming after the reduction of FADH₂. FADH₂ produced in the citric acid cycle transfers the electron to the complex II. The diagrammatic representation of electron transfer in complex II in the electron transfer chain is:

Complex III consists of 11 structural subunits. Its structural subunits contain iron-sulfur proteins and various cytochromes. $\text{cyt b}$ and ${\text{cyt c}}_1$ are present in the complex III. The diagrammatic representation of electron transfer in complex I in the electron transfer chain is as follows: