Chapter 23, Problem 23.91EP

(a)

Interpretation Introduction

Interpretation: To determine the electron carrier NADH is associated with which of the protein complexes (I, II, III, and IV) of the ETC.

Concept introduction: 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. There are four complexes associated with the electron transport chain that is present in the inner mitochondrial membrane. The four complexes that help in the electron transfer in the electron transport chain are:

Complex I: $\text{NADH-coenzyme}\text{Q reductase}$

Complex II: $\text{Succinate-coenzyme}\text{Q reductase}$

Complex III: $\text{Coenzyme}\text{Q-cytochrome c reductase}$

Complex IV: $\text{Cytochrome c oxidase}$

An overview of the electron transport chain is as follows:

Answer to Problem 23.91EP

NADH is associated with the complex I of the electron transport chain. Electrons from the citric acid cycle are transported to complex I through NADH molecule.

Explanation of Solution

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 $\left(\text{FeSP}\right)$. NADH from the citric acid cycle initially interacts with the Complex I. The electron movement through this complex is from NADH to $\text{coenzyme Q}\left({\text{CoQH}}_{\text{2}}\right)$. The diagrammatic representation of electron transfer in complex I in the electron transfer chain is as follows:

(b)

Interpretation Introduction

Interpretation: To determine the electron carrier $\text{CoQ}$ is associated with which of the protein complexes (I, II, III, and IV) of the ETC.

Concept introduction: 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. There are four complexes associated with the electron transport chain that is present in the inner mitochondrial membrane. The four complexes that help in the electron transfer in the electron transport chain are:

Complex I: $\text{NADH-coenzyme}\text{Q reductase}$

Complex II: $\text{Succinate-coenzyme}\text{Q reductase}$

Complex III: $\text{Coenzyme}\text{Q-cytochrome c reductase}$

Complex IV: $\text{Cytochrome c oxidase}$

An overview of the electron transport chain is as follows:

Answer to Problem 23.91EP

$\text{CoQ}$ is associated with complex I, complex II and complex III. NADH and FADH₂from complex I and II transfers electrons to $\text{CoQ}$ respectively. $\text{CoQ}$ gets reduced to ${\text{CoQH}}_{\text{2}}$ and transfers electrons to complex III. In complex III, ${\text{CoQH}}_{\text{2}}$ is oxidized to regenerate $\text{CoQ}$.

Explanation of Solution

NADH and FADH₂from complex I and II transfers electrons to $\text{CoQ}$ respectively. $\text{CoQ}$ gets reduced to ${\text{CoQH}}_{\text{2}}$ and transfers electrons to complex III. ${\text{CoQH}}_{\text{2}}$ acts as a mobile electron carrier and transfers electrons from complex I, II to III. The diagrammatic representation of electron transfer from complex I, II to III in the electron transfer chain is as follows:

In complex III, ${\text{CoQH}}_{\text{2}}$ is oxidized to regenerate $\text{CoQ}$. Initially, ${\text{CoQH}}_{\text{2}}$ is oxidized and releases two electrons that are transferred to $\text{2Fe(III)SP}$. The diagrammatic representation of electron transfer in complex I in the electron transfer chain is as follows:

(c)

Interpretation Introduction

Interpretation: To determine the electron carrier $\text{cyt b}$ is associated with which of the protein complexes (I, II, III, and IV) of the ETC.

Concept introduction: 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. There are four complexes associated with the electron transport chain that is present in the inner mitochondrial membrane. The four complexes that help in the electron transfer in the electron transport chain are:

Complex I: $\text{NADH-coenzyme}\text{Q reductase}$

Complex II: $\text{Succinate-coenzyme}\text{Q reductase}$

Complex III: $\text{Coenzyme}\text{Q-cytochrome c reductase}$

Complex IV: $\text{Cytochrome c oxidase}$

An overview of the electron transport chain is as follows:

Answer to Problem 23.91EP

$\text{Cyt b}$ is associated with the complex III of the electron transport chain. $\text{Cyt b}$ transfers an electron to $\text{FeSP}$ unit in the complex III.

Explanation of Solution

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. ${\text{CoQH}}_{\text{2}}$ from complex I and complex II transfers electrons to complex III and start the reactions of complex III. The diagrammatic representation of electron transfer in complex I in the electron transfer chain is as follows:

Initially, ${\text{CoQH}}_{\text{2}}$ is oxidized and releases two electrons that are transferred to $\text{FeSP}$ and then to $\text{cyt b}$. The electrons from $\text{cyt b}$ is then again transferred to another unit of $\text{FeSP}$ and then to ${\text{cyt c}}_1$.

(d)

Interpretation Introduction

Interpretation: To determine that the electron carrier $\text{cyt a}$ is associated with which of the protein complexes (I, II, III, and IV) of the ETC.

Concept introduction: 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. There are four complexes associated with the electron transport chain that is present in the inner mitochondrial membrane. The four complexes that help in the electron transfer in the electron transport chain are:

Complex I: $\text{NADH-coenzyme}\text{Q reductase}$

Complex II: $\text{Succinate-coenzyme}\text{Q reductase}$

Complex III: $\text{Coenzyme}\text{Q-cytochrome c reductase}$

Complex IV: $\text{Cytochrome c oxidase}$

An overview of the electron transport chain is as follows:

Answer to Problem 23.91EP

$\text{Cyt a}$ is associated with the complex III of the electron transport chain. $\text{Cyt}\text{c}$ transfers electrons to $\text{cyt}\text{a}$ in the complex IV.

Explanation of Solution

Complex IV contains 13 structural subunits. It has two cytochrome units that have copper along with iron. The copper also participates in the redox reactions and changes its oxidation state from $+2$ to $+1$. $\text{Cyt c}$ carries one electron from complex III to complex IV. In the complex IV, $\text{cyt}\text{c}$ transfers electrons to $\text{cyt}\text{a}$ and $\text{cyt}\text{a}$ transfers it to $\text{cyt}{\text{a}}_3$. In the last step, electrons from $\text{cyt}{\text{a}}_3$ and hydrogen ions react with molecular oxygen to form water.

The diagrammatic representation of the electron transfer in complex IV in the electron transport chain is: