Chapter 23, Problem 23.70EP

(a)

Interpretation Introduction

Interpretation: To determine the number of NADH molecules formed during one turn of the citric acid cycle.

Concept introduction: 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 $\text{CoA-SH}$. An overview of the citric acid cycle is as follows:

$\text{NADH}$ is the reduced form of nicotinamide adenine dinucleotide. 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.

Answer to Problem 23.70EP

Three molecules of NADH are formed in step 3, 4 and 8 of the citric acid cycle.

Explanation of Solution

Step 3 is the first step where both the oxidation and decarboxylation occurs. Step 3 involves the oxidation of isocitrate and formation of CO₂. In this step, firstly isocitrate is oxidized by ${\text{NAD}}^{\text{+}}$ to form oxalosuccinate and then decarboxylation takes place to form $\alpha \text{-ketoglutarate}$. The reaction of this step 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.

Step 4 involves the oxidation of $\alpha \text{-ketoglutarate}$ and formation of CO₂. In this step, $\alpha \text{-ketoglutarate}$ is reacted with $\text{CoA-SH}$ and ${\text{NAD}}^{\text{+}}$. This is the second oxidation reaction of the citric acid cycle and leads to the formation of $\text{succinyl CoA}$. The enzyme employed for oxidation is $\alpha \text{-ketoglutarate dehydrogenase}$. 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.

Step 8 is an oxidation reaction and the last step of the citric acid cycle. In step 8, $\text{L-malate}$ is oxidized to regenerate oxaloacetate with the help of enzyme malate dehydrogenase. ${\text{NAD}}^{\text{+}}$ acts as an oxidizing agent in the reaction.

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.

Hence, three molecules of NADH are formed in the citric acid cycle.

(b)

Interpretation Introduction

Interpretation: To determine the number of GTP molecules formed during one turn of the citric acid cycle.

Concept introduction: 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 $\text{CoA-SH}$. An overview of the citric acid cycle is as follows:

Answer to Problem 23.70EP

One molecule of GTP is formed in step 5 of the citric acid cycle.

Explanation of Solution

Step 5 involves the thioester bond cleavage in $\text{succinyl CoA}$ and phosphorylation of GDP. This step consists of two reactions. Firstly $\text{succinyl CoA}$ is converted into succinyl phosphate by the reaction with ${\text{P}}_{\text{i}}$ and then succinyl phosphate is reacted with GDP to form succinate and GTP. The reaction of step 5 is:

(c)

Interpretation Introduction

Interpretation: To determine the number of time decarboxylation reactions occur during one turn of the citric acid cycle.

Concept introduction: 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 $\text{CoA-SH}$. An overview of the citric acid cycle is as follows:

Answer to Problem 23.70EP

Decarboxylation occurs twice in the citric acid cycle in step 3 and 4.

Explanation of Solution

Step 3 is the first step where both the oxidation and decarboxylation occurs. Step 3 involves the oxidation of isocitrate and formation of CO₂. In this step, firstly isocitrate is oxidized by ${\text{NAD}}^{\text{+}}$ to form oxalosuccinate and then decarboxylation takes place to form $\alpha \text{-ketoglutarate}$. The reaction of this step is:

The final product is $\alpha \text{-ketoglutarate}$ and one molecule of CO₂ is released.

Step 4 involves the oxidation of $\alpha \text{-ketoglutarate}$ and formation of CO₂. In this step, $\alpha \text{-ketoglutarate}$ is reacted with $\text{CoA-SH}$ and ${\text{NAD}}^{\text{+}}$. This is the second oxidation reaction of the citric acid cycle and leads to the formation of $\text{succinyl CoA}$. The enzyme employed for oxidation is $\alpha \text{-ketoglutarate dehydrogenase}$. The reaction of step 4 is:

The final product is $\text{succinyl CoA}$ and one molecule of CO₂ is released.

(d)

Interpretation Introduction

Interpretation: To determine the number of time oxidation-reduction reaction occur during one turn of the citric acid cycle.

Concept introduction: 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 $\text{CoA-SH}$.

An overview of the citric acid cycle is as follows:

Oxidation-reduction reactions are those in which both oxidation and reduction occur simultaneously in the reaction. One species gets oxidized and other gets reduced.

Answer to Problem 23.70EP

The oxidation-reduction reaction occurs four times in the citric acid cycle in step 3, 4, 6 and 8.

Explanation of Solution

Step 3 is the first step where both the oxidation and decarboxylation occurs. Step 3 involves the oxidation of isocitrate and formation of CO₂. In this step, firstly isocitrate is oxidized by ${\text{NAD}}^{\text{+}}$ to form oxalosuccinate and then decarboxylation takes place to form $\alpha \text{-ketoglutarate}$. The reaction of this step is:

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

Step 4 involves the oxidation of $\alpha \text{-ketoglutarate}$ and formation of CO₂. In this step, $\alpha \text{-ketoglutarate}$ is reacted with $\text{CoA-SH}$ and ${\text{NAD}}^{\text{+}}$. This is the second oxidation reaction of the citric acid cycle and leads to the formation of $\text{succinyl CoA}$. The enzyme employed for oxidation is $\alpha \text{-ketoglutarate dehydrogenase}$. 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.

In step 6, oxidation of succinate occurs to form fumarate. The enzyme involved in this step of the citric acid cycle is succinate dehydrogenase. FAD is the oxidizing agent in this step. This reaction takes place in the inner mitochondrial membrane. The reaction of step 6 is:

Step 8 is an oxidation reaction and the last step of the citric acid cycle. In step 8, $\text{L-malate}$ is oxidized to regenerate oxaloacetate with the help of enzyme malate dehydrogenase. ${\text{NAD}}^{\text{+}}$ acts as an oxidizing agent in the reaction.

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.