Chapter 24, Problem 24.105EP

(a)

Interpretation Introduction

Interpretation: To indicate whether lactate is involved in (1) the pentose phosphate pathway, (2) the Cori cycle, (3) glycolysis, or (4) lactate fermentation.

Concept introduction: The pentose phosphate pathway is defined as the metabolic pathway in which NADPH, $\text{ribose 5}-\text{phosphate}$, and other sugar phosphates are produced from $\text{glucose 6}-\text{phosphate}$.

Glucose is converted to pyruvate by glycolysis metabolic pathway; pyruvate is further converted to lactate in the skeletal muscle cells by anaerobic reactions. The lactate is diffused into the bloodstream, by which it is transported to the liver. Lactate is reconverted to pyruvate. Gluconeogenesis metabolic pathway uses this pyruvate to synthesize glucose in the liver cells. Glucose is diffused into the bloodstream and is transported back to the active skeletal muscle cells. This cycle is known as the Cori cycle.

Glycolysis is the metabolic pathway that breaks down a glucose molecule and converts it into two pyruvate molecules along with the production of two ATP molecules and NADH coenzymes.

Fermentation is defined as the biochemical anaerobic process by which NADH is oxidized to ${\text{NAD}}^{+}$. In the lactate fermentation, NADH is oxidized to ${\text{NAD}}^{+}$.

The structure of lactate is as follows:

Answer to Problem 24.105EP

Lactate is associated with (2) the Cori cycle and (4) the lactate fermentation.

Explanation of Solution

An overview of the Cori cycle is as follows:

Lactate is converted to pyruvate in the liver. Therefore, lactate is associated with the Cori cycle.

Pyruvate is converted to lactate under oxygen-poor conditions by lactate dehydrogenase enzymes in the human body. This anaerobic reduction of pyruvate to form lactate by enzymes is called lactate fermentation. The chemical reaction for the formation of lactate is as follows:

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

Therefore, lactate is associated with the Cori cycle and the lactate fermentation.

(b)

Interpretation Introduction

Interpretation: To indicate whether ${\text{NAD}}^{+}$ is involved in (1) the pentose phosphate pathway, (2) the Cori cycle, (3) glycolysis, or (4) lactate fermentation.

Concept introduction: The pentose phosphate pathway is defined as the metabolic pathway in which NADPH, $\text{ribose 5}-\text{phosphate}$, and other sugar phosphates are produced from $\text{glucose 6}-\text{phosphate}$.

Glucose is converted to pyruvate by glycolysis metabolic pathway; pyruvate is further converted to lactate in the skeletal muscle cells by anaerobic reactions. The lactate is diffused into the bloodstream, by which it is transported to the liver. Lactate is reconverted to pyruvate. Gluconeogenesis metabolic pathway uses this pyruvate to synthesize glucose in the liver cells. Glucose is diffused into the bloodstream and is transported back to the active skeletal muscle cells. This cycle is known as the Cori cycle.

Glycolysis is the metabolic pathway that breaks down a glucose molecule and converts it into two pyruvate molecules along with the production of two ATP molecules and NADH coenzymes.

Pyruvate is converted to lactate under oxygen-poor conditions by lactate dehydrogenase enzymes in the human body. This anaerobic reduction of pyruvate to form lactate by enzymes is called lactate fermentation.

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 24.105EP

${\text{NAD}}^{+}$ is associated with (2) the Cori cycle, (3) glycolysis, and (4) the lactate fermentation.

Explanation of Solution

An overview of the Cori cycle is as follows:

In the Cori cycle, ${\text{NADH/NAD}}^{+}$ are associated with the conversion of pyruvate to lactate and vice-versa. Therefore, ${\text{NAD}}^{+}$ is associated with the Cori cycle.

Pyruvate is converted to lactate under oxygen-poor conditions by lactate dehydrogenase enzymes in the human body. This anaerobic reduction of pyruvate to form lactate by enzymes is called lactate fermentation. The chemical reaction for the formation of lactate is as follows:

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

In the lactate fermentation, NADH is oxidized to ${\text{NAD}}^{+}$. Therefore, ${\text{NAD}}^{+}$ is associated with the lactate fermentation.

The net overall equation for the glycolysis process is as follows:

$\text{Glucose}+2\text{ADP}+2{\text{P}}_i+2{\text{NAD}}^{+}\to 2\text{pyruvate}+2\text{NADH}+2\text{ATP}+2{\text{H}}^{+}+2{\text{H}}_2\text{O}$

${\text{NAD}}^{+}$ are encountered as reactant in the glycolysis process. Therefore, ${\text{NAD}}^{+}$ is involved in glycolysis.

(c)

Interpretation Introduction

Interpretation: To indicate whether $\text{glucose 6}-\text{phosphate}$ is involved in (1) the pentose phosphate pathway, (2) the Cori cycle, (3) glycolysis, or (4) lactate fermentation.

Concept introduction: The pentose phosphate pathway is defined as the metabolic pathway in which NADPH, $\text{ribose 5}-\text{phosphate}$, and other sugar phosphates are produced from $\text{glucose 6}-\text{phosphate}$.

Glucose is converted to pyruvate by glycolysis metabolic pathway; pyruvate is further converted to lactate in the skeletal muscle cells by anaerobic reactions. The lactate is diffused into the bloodstream, by which it is transported to the liver. Lactate is reconverted to pyruvate. Gluconeogenesis metabolic pathway uses this pyruvate to synthesize glucose in the liver cells. Glucose is diffused into the bloodstream and is transported back to the active skeletal muscle cells. This cycle is known as the Cori cycle.

Glycolysis is the metabolic pathway that breaks down a glucose molecule and converts it into two pyruvate molecules along with the production of two ATP molecules and NADH coenzymes.

Pyruvate is converted to lactate under oxygen-poor conditions by lactate dehydrogenase enzymes in the human body. This anaerobic reduction of pyruvate to form lactate by enzymes is called lactate fermentation.

Answer to Problem 24.105EP

$\text{Glucose 6}-\text{phosphate}$ is involved in (1) the pentose phosphate pathway, (2) the Cori cycle and (3) glycolysis.

Explanation of Solution

The first stage of the pentose phosphate pathway is the oxidative stage. In the oxidative stage (involves three steps) of the pentose phosphate pathway, $\text{glucose 6}-\text{phosphate}$ is converted to $\text{ribulose 5}-\text{phosphate}$ and ${\text{CO}}_2$. NADPH is formed along with $\text{ribulose 5}-\text{phosphate}$ and ${\text{CO}}_2$. The net equation for the oxidative stage is as follows:

$\text{Glucose 6}-\text{phosphate}+{\text{2NADP}}^{+}+{\text{H}}_2\text{O}\to \text{ribulose 5}-\text{phosphate}+2\text{NADPH}+{\text{CO}}_2+2{\text{H}}^{+}$

The general equation for the entire pentose phosphate pathway is as follows:

$\begin{array}{l}\text{3Glucose 6}-\text{phosphate}+{\text{6NADP}}^{+}+3{\text{H}}_2\text{O}\to \\ 2\text{fructose 6}-\text{phosphate}+6\text{NADPH}+3{\text{CO}}_2+6{\text{H}}^{+}+\text{glyceraldehyde 3}-\text{phosphate}\end{array}$

Therefore, $\text{glucose 6}-\text{phosphate}$ is encountered as the reactant in the Cori cycle.

The net overall equation for the glycolysis process is as follows:

$\text{Glucose}+2\text{ADP}+2{\text{P}}_i+2{\text{NAD}}^{+}\to 2\text{pyruvate}+2\text{NADH}+2\text{ATP}+2{\text{H}}^{+}+2{\text{H}}_2\text{O}$

Glucose enters the glycolysis metabolic pathway in the form of $\text{glucose 6}-\text{phosphate}$. In the Cori cycle, glucose enters the glycolysis metabolic pathway in the form of $\text{glucose 6}-\text{phosphate}$. Therefore, $\text{glucose 6}-\text{phosphate}$ is involved in the pentose phosphate pathway, the Cori cycle and glycolysis.

(d)

Interpretation Introduction

Interpretation: To indicate whether $\text{ribose 5}-\text{phosphate}$ is involved in (1) the pentose phosphate pathway, (2) the Cori cycle, (3) glycolysis, or (4) lactate fermentation.

Concept introduction: The pentose phosphate pathway is defined as the metabolic pathway in which NADPH, $\text{ribose 5}-\text{phosphate}$, and other sugar phosphates are produced from $\text{glucose 6}-\text{phosphate}$. The overall net equation for the pentose phosphate pathway is as follows:

$\begin{array}{l}\text{3Glucose 6}-\text{phosphate}+{\text{6NADP}}^{+}+3{\text{H}}_2\text{O}\to \\ 2\text{fructose 6}-\text{phosphate}+6\text{NADPH}+3{\text{CO}}_2+6{\text{H}}^{+}+\text{glyceraldehyde 3}-\text{phosphate}\end{array}$

Glucose is converted to pyruvate by glycolysis metabolic pathway; pyruvate is further converted to lactate in the skeletal muscle cells by anaerobic reactions. The lactate is diffused into the bloodstream, by which it is transported to the liver. Lactate is reconverted to pyruvate. Gluconeogenesis metabolic pathway uses this pyruvate to synthesize glucose in the liver cells. Glucose is diffused into the bloodstream and is transported back to the active skeletal muscle cells. This cycle is known as the Cori cycle.

Glycolysis is the metabolic pathway that breaks down a glucose molecule and converts it into two pyruvate molecules along with the production of two ATP molecules and NADH coenzymes.

Pyruvate is converted to lactate under oxygen-poor conditions by lactate dehydrogenase enzymes in the human body. This anaerobic reduction of pyruvate to form lactate by enzymes is called lactate fermentation.

Answer to Problem 24.105EP

$\text{Ribose 5}-\text{phosphate}$ is involved in (1) the pentose phosphate pathway.

Explanation of Solution

The first stage of the pentose phosphate pathway is the oxidative stage. In the oxidative stage (involves three steps) of the pentose phosphate pathway, $\text{glucose 6}-\text{phosphate}$ is converted to $\text{ribulose 5}-\text{phosphate}$ and ${\text{CO}}_2$. NADPH is formed along with $\text{ribulose 5}-\text{phosphate}$ and ${\text{CO}}_2$. The net equation for the oxidative stage is as follows:

$\text{Glucose 6}-\text{phosphate}+{\text{2NADP}}^{+}+{\text{H}}_2\text{O}\to \text{ribulose 5}-\text{phosphate}+2\text{NADPH}+{\text{CO}}_2+2{\text{H}}^{+}$

The first step of the second stage i.e. the non-oxidative stage of the pentose phosphate pathway is the isomerization of $\text{ribulose 5}-\text{phosphate}$ to $\text{ribose 5}-\text{phosphate}$. The isomerization equation for the first step of the second stage is as follows:

$\text{Ribulose 5}-\text{phosphate}\stackrel{\text{Phosphopentose isomerase}}{\to }\text{Ribose 5}-\text{phosphate}$

$\text{Ribose 5}-\text{phosphate}$ is a pentose ribose that is formed in the first step of the second stage. Therefore, $\text{ribose 5}-\text{phosphate}$ is involved in the pentose phosphate pathway.