Chapter 24, Problem 24.17EP

(a)

Interpretation Introduction

Interpretation: To identify the number of steps in glycolysis that produce ATP.

Concept introduction: In the glycolysis metabolic pathway, a glucose molecule breaks down into two pyruvate molecules. Two ATP molecules and NADH coenzymes are produced along with pyruvate.

The block diagram to represent an overview of glycolysis is as follows:

Adenosine triphosphate (ATP) is a molecule that is defined as the energy currency of life and provides energy to carry out the metabolic processes in the living cells. It is converted either to adenosine monophosphate (AMP) or to adenosine diphosphate (ADP) after the consumption in the metabolic processes.

Answer to Problem 24.17EP

In glycolysis, two steps produce ATP.

Explanation of Solution

In step 7 and step 10, two ATP molecules are produced in each step. Hence, in the glycolysis pathway, four ATP molecules are produced is two steps.

(b)

Interpretation Introduction

Interpretation: To identify the number of steps in glycolysis that involve phosphorylation.

Concept introduction: In the glycolysis metabolic pathway, a glucose molecule breaks down into two pyruvate molecules. Two ATP molecules and NADH coenzymes are produced along with pyruvate.

The block diagram to represent an overview of glycolysis is as follows:

In the phosphorylation reaction, the molecule is attached to the phosphoryl group. The transfer of a phosphoryl group $\left({\text{PO}}_3^{2-}\right)$ is catalyzed by the kinase enzymes. In the glycolysis process, the phosphate group comes from ATP, ADP or some higher energetic phosphate molecules.

Answer to Problem 24.17EP

Phosphorylation is involved in five steps.

Explanation of Solution

The first step in the glycolysis process is the phosphorylation of glucose using ATP. Glucose is converted to $\text{glucose 6}-\text{phosphate}$. The phosphoryl group $\left({\text{PO}}_3^{2-}\right)$ is transferred from ATP to glucose.  The hexokinase enzyme catalyzes this reaction. The word equation for the phosphorylation reaction is as follows:

$\text{Glucose}+\text{ATP}\stackrel{\text{Hexokinase}}{\to }\text{Glucose 6}-\text{phosphate}+\text{ADP}$

In the third step, $\text{fructose 6}-\text{phosphate}$ is converted to $\text{fructose 1,6}-\text{biphosphate}$ by phosphofructokinase enzymes. The type of reaction is phosphorylation. The phosphoryl group $\left({\text{PO}}_3^{2-}\right)$ is transferred from ATP to $\text{fructose 6}-\text{phosphate}$. The word equation for the phosphorylation reaction is as follows:

$\text{Fructose 6}-\text{phosphate}+\text{ATP}\stackrel{\text{phosphofructokinase}}{\to }\text{Fructose 1,6}-\text{biphosphate}+\text{ADP}$

In step 6, $\text{glyceraldehyde 3}-\text{phosphate}$ is oxidized by using ${\text{NAD}}^{+}$ and phosphorylated using ${\text{P}}_{\text{i}}$. $\text{Glyceraldehyde 3}-\text{phosphate}$ is converted to $1,3-\text{biphosphoglycerate}$. $\text{Glyceraldehyde 3}-\text{phosphate dehydrogenase}$ enzymes are involved in this reaction. The reaction is as follows:

$\text{Glyceraldehyde 3}-\text{phosphate}+{\text{NAD}}^{+}{\text{+P}}_{\text{i}}\stackrel{\begin{array}{l}\text{Glyceraldehyde}\\ \text{ 3}-\text{phosphate}\\ \text{ dehydrogenase}\end{array}}{\rightleftharpoons }\text{1,3}-\text{Biphosphoglycerate}+{\text{NADH+H}}^{+}$

Therefore, step 6 proceeds through the oxidation and phosphorylation reaction using ${\text{P}}_{\text{i}}$.

The seventh step in the glycolysis process is the phosphorylation of $\text{1,3}-\text{Biphosphoglycerate}$ using ADP. In step 7,$\text{1,3}-\text{biphosphoglycerate}$ is converted to $\text{3}-\text{phosphoglycerate}$ by phosphoglycerokinase. The chemical reaction is as follows:

$\text{1,3}-\text{Biphosphoglycerate}+\text{ADP}\stackrel{\text{Phosphoglycerokinase}}{\rightleftharpoons }\text{3}-\text{phosphoglycerate}+\text{ATP}$

Step 10 is the final step of the glycolysis process. In step 10, phosphoenolpyruvate is converted to pyruvate using ADP, thus, ADP is converted to ATP. Pyruvate kinase enzymes are involved in this reaction. The type of reaction is phosphorylation. The word equation for the phosphorylation reaction is as follows:

$\text{Phosphoenolpyruvate}+\text{ADP}\stackrel{\text{Pyruvate kinase}}{\to }\text{Pyruvate}+\text{ADP}$

Therefore, step 1, step 2, step 6, step 7 and step 10 involved the phosphorylation reaction.

(c)

Interpretation Introduction

Interpretation: To identify the number of steps in glycolysis that involve ${\text{NAD}}^{+}$ as a reactant.

Concept introduction: In the glycolysis metabolic pathway, a glucose molecule breaks down into two pyruvate molecules. Two ATP molecules and NADH coenzymes are produced along with pyruvate.

The block diagram to represent an overview of glycolysis is as follows:

A reactant is defined as the substance that is initially present in the chemical reaction and gets consumed to form a new substance.

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 24.17EP

One step involves ${\text{NAD}}^{+}$ as a reactant.

Explanation of Solution

In step 6, $\text{glyceraldehyde 3}-\text{phosphate}$ is oxidized by using ${\text{NAD}}^{+}$ and phosphorylated using ${\text{P}}_{\text{i}}$. $\text{Glyceraldehyde 3}-\text{phosphate dehydrogenase}$ enzymes are involved in this reaction. The reaction is as follows:

$\text{Glyceraldehyde 3}-\text{phosphate}+{\text{NAD}}^{+}+{\text{P}}_{\text{i}}\stackrel{\begin{array}{l}\text{Glyceraldehyde}\\ \text{ 3}-\text{phosphate}\\ \text{ dehydrogenase}\end{array}}{\rightleftharpoons }\text{1,3}-\text{Biphosphoglycerate}+\text{NADH}+{\text{H}}^{+}$

Therefore, ${\text{NAD}}^{+}$ is encountered as a reactant in step 6.

(d)

Interpretation Introduction

Interpretation: To identify the number of steps in glycolysis that involve a compound with a high-energy bond as a reactant.

Concept introduction: In the glycolysis metabolic pathway, a glucose molecule breaks down into two pyruvate molecules. Two ATP molecules and NADH coenzymes are produced along with pyruvate.

The block diagram to represent an overview of glycolysis is as follows:

A reactant is defined as the substance that is initially present in the chemical reaction and gets consumed to form a new substance.

High energy compounds are those compounds that release a large amount of energy upon hydrolysis. These compounds consist of highly strained bonds that are responsible for the release of a high amount of energy. The compounds containing a phosphate group are examples of high energy compounds.

A high-energy phosphate group is formed when a phosphate group is attached to a carbon atom participating in carbon-oxygen or carbon-carbon double bond.

Answer to Problem 24.17EP

Two steps involve a compound with a high-energy bond as a reactant.

Explanation of Solution

The seventh step in the glycolysis process is the phosphorylation of $\text{1,3}-\text{Biphosphoglycerate}$ using ADP. $\text{1,3}-\text{Biphosphoglycerate}$ is a high-energy compound because the phosphate group is attached to that carbon atom which is involved in $\text{C}=\text{O}$ bond. In step 7,$\text{1,3}-\text{biphosphoglycerate}$ is converted to $\text{3}-\text{phosphoglycerate}$ by phosphoglycerokinase. The chemical reaction is as follows:

Here,  denotes the ${\text{PO}}_3^{2-}$ unit. Therefore, in step 7, a high-energy bond is encountered as a reactant.

In step 10, phosphoenolpyruvate is converted to pyruvate using ADP, thus, ADP is converted to ATP. Pyruvate kinase enzymes are involved in this reaction. Phosphoenolpyruvate is a compound with a high-energy phosphate group because the phosphate group is attached to that carbon atom which is involved in carbon-carbon double bond. The chemical reaction is as follows:

Therefore, in step 10, a high-energy bond is encountered as a reactant.