Chapter 24, Problem 24.106EP

(a)

Interpretation Introduction

Interpretation: To indicate whether pyruvate 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 ${\text{NAD}}^{+}$.

Pyruvate $\left({\text{CH}}_3{\text{COCOO}}^{-}\right)$ is the conjugate base of pyruvic acid $\left({\text{CH}}_3\text{COCOOH}\right)$.

Answer to Problem 24.106EP

Pyruvate 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:

Lactate is converted to pyruvate in the liver and pyruvate is converted to lactate in the active skeletal muscle cells. Therefore, pyruvate is associated with the Cori cycle.

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

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

Therefore, pyruvate is the final product of glycolysis metabolic pathway.

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, pyruvate is associated with lactate fermentation.

(b)

Interpretation Introduction

Interpretation: To indicate whether ${\text{NADP}}^{+}$ 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 phosphate is associated with the redox reactions in metabolism. Its reduced form is NADPH and oxidized form is ${\text{NADP}}^{+}$. Structurally, ${\text{NADP}}^{+}/\text{NADPH}$ is the phosphorylated version of ${\text{NAD}}^{+}/\text{NADH}$.

Answer to Problem 24.106EP

${\text{NADP}}^{+}$ is involved in (1) the pentose phosphate pathway.

Explanation of Solution

The overall net equation for the pentose phosphate pathway is as follows:

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

Hence, ${\text{NADP}}^{+}$ is encountered as a reactant in the pentose phosphate pathway. Therefore, it is involved in the pentose phosphate pathway.

(c)

Interpretation Introduction

Interpretation: To indicate whether glucose 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 24.106EP

Glucose is associated with (1) the pentose phosphate pathway, (2) the Cori cycle, and (3) glycolysis.

Explanation of Solution

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 present in the form of $\text{glucose 6}-\text{phosphate}$ in the pentose phosphate pathway.

An overview of the Cori cycle is as follows:

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 is encountered as a reactant in is the glycolysis metabolic pathway. Therefore, glucose is involved in the pentose phosphate pathway, the Cori cycle, and (3) glycolysis.

(d)

Interpretation Introduction

Interpretation: To indicate whether $\text{fructose 6}-\text{phosphate}$ 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 24.106EP

$\text{Fructose 6}-\text{phosphate}$ is involved in (1) the pentose phosphate pathway and (3) glycolysis.

Explanation of Solution

The overall net equation for the pentose phosphate pathway is as follows:

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

$\text{Fructose 6}-\text{phosphate}$ is formed as the final product of the pentose phosphate pathway.

An overview of the glycolysis metabolic pathway is as follows:

Isomerization is the second step in the glycolysis process. $\text{Glucose 6}-\text{phosphate}$ is isomerized to $\text{fructose 6}-\text{phosphate}$. $\text{Fructose 6}-\text{phosphate}$ is formed in step 2 of glycolysis. Therefore, $\text{fructose 6}-\text{phosphate}$ is involved in (1) the pentose phosphate pathway and (3) glycolysis.