Chapter 15, Problem 1E

(a)

Summary Introduction

To explain: The steps of the reactions that have phosphorylation in glycolysis process.

Concept introduction: Glycolysis is a series of 10 enzyme-catalyzed reactions where one molecule of glucose is converted to two molecules of pyruvate. In this process, the total production of 2 ATP molecules and the reduction of 2 NAD⁺ to 2 NADH molecules occurs.

Answer to Problem 1E

Correct answer: The reactions 1, 3, 7, and 10 are the phosphorylation reactions in the glycolysis process.

Explanation of Solution

In glycolysis, 10 steps are involved to convert glucose to pyruvate; out of these, during four reactions, phosphorylation occurs.

In Reaction 1 of glycolysis, a phosphoryl group is transferred from ATP to glucose to form glucose-6-phosphate (G6P) in a catalytic reaction that is continued by hexokinase. In this reaction, kinase acts as an enzyme that transfers the phosphoryl groups between ATP and a metabolite. The metabolite serves as the phosphoryl group acceptor.

Reaction 1: $\text{Glucose}\stackrel{\text{hexokinase}}{\to }\text{Glucose-6-phosphate}$

In Reaction 3 of glycolysis, phosphofructokinase (PFK) phosphorylates fructose-6-phosphate (F6P) to form fructose-1,6-biphosphate (FBP).

Reaction 3: $\text{Fructose-6-phosphate}\stackrel{\begin{array}{c}\text{phosphofructokinase}\text{(PFK)}\\ {\text{Mg}}^{\text{2+}}\end{array}}{\to }\text{Fructose-1,6-bisphosphate}$

Reaction 7 of the glycolysis pathway yields ATP with 3-phosphoglycerate (3GP) in a reaction that is catalyzed by the enzyme phosphoglycerate kinase (PGK). In this reaction, reverse-phosphorylation occurs in the presence of kinase. From 1, 3-Bisphosphoglycerate, one phosphate group is transferred to 3-phosphoglycerate (3GP) and ATP.

Reaction 7:

$\text{1,3-Bisphosphoglycerate}\text{(1,3-BPG)+ATP}\stackrel{\begin{array}{l}\text{phosphoglycerate}\\ \text{kinase}\text{(PGK)}\end{array}}{\rightleftharpoons }\text{3-Phosphoglycerate}\text{(3PG)+ATP}$

In the final reaction, Reaction 10 of glycolysis, pyruvate kinase enhances the phosphoenolpyruvate in the presence of ADP to form pyruvate and ATP. The pyruvate kinase reaction is highly exergonic such that it gives more energy to drive ATP synthesis, a substrate-level phosphorylation reaction.

Reaction 10: $\text{Phosphoenolpyruvate}{\text{(PEP)+ADP+H}}^{\text{+}}\stackrel{\text{pyruvate}\text{kinase}\text{(PK)}}{\to }\text{Pyruvate+ATP}$

(b)

Summary Introduction

To explain: The reaction steps of glycolysis that are isomerization reactions.

Answer to Problem 1E

Correct answer: The reactions 2, 5, and 8 in glycolysis are isomerization reactions.

Explanation of Solution

Reaction 2 of glycolysis explains the conversion of glucose-6-phosphate (G6P) and fructose-6-phosphate (F6P) in the presence of phosphoglucose isomerase (PGI). This reaction represents isomerization of an aldose to a ketone.

Reaction 2: $\text{Glucose-6-phosphate}\stackrel{\text{phosphoglucose}\text{isomerase}}{\rightleftharpoons }\text{Fructose-6-phosphate}$

Reaction 5 represents the products of the aldol cleavage reaction, where glyceraldehyde-3-phosphate is converted to dihydroxyacetone phosphate. In this reaction, interconversion occurs by an isomerization reaction with an enediol intermediate. Triose phosphate isomerase catalyzes this process in this reaction of glycolysis.

Reaction 5:

$\text{Glyceraldehyde-3-phosphate}\text{(GAP)}\rightleftharpoons \text{Enediol}\text{intermediate}\rightleftharpoons \text{Dihydroxyacetone}\text{phosphate}$

Reaction 8 of glycolysis represents the conversion of 3-phosphoglycerate (3PG) to 2-phosphoglycerate in the presence of phosphoglycerate mutase. This reaction also represents the isomerization reaction.

Reaction 8: $\text{3-Phosphoglycerate}\text{(3PG)}\stackrel{\begin{array}{l}\text{phosphoglycerate}\\ \text{mutase}\text{(PGM)}\end{array}}{\rightleftharpoons }\text{2-phosphoglycerate(2PG)}$

(c)

Summary Introduction

To explain: The steps of glycolysis that are the oxidation–reduction reactions.

Answer to Problem 1E

Correct answer: Reaction 6 in glycolysis is oxidation–reduction reaction.

Explanation of Solution

Reaction 6 of glycolysis represents the oxidation and phosphorylation of glyceraldehyde-3-phosphate (GAP) in the presence of NAD⁺ and P_i as catalyzed by glyceraldehyde-3-phosphate dehydrogenase to form 1, 3-Bisphosphoglycerate.

Reaction 6:

$\text{Glyceraldehyde-3-phosphate}{\text{(GAP)+NAD}}^{\text{+}}{\text{+P}}_{\text{i}}\stackrel{\begin{array}{l}\text{glyceraldehyde-3-phosphate}\\ \text{dehydrogenase}\text{(GAPDH)}\end{array}}{\rightleftharpoons }\text{1,3-Bisphosphoglycerate}\text{(1,3-BPG)}$

(d)

Summary Introduction

To explain: The steps of glycolysis that are dehydration reactions.

Answer to Problem 1E

Correct answer: Reaction 9 in glycolysis is a dehydration reaction.

Explanation of Solution

Reaction 9 of glycolysis explains the dehydration reaction in which 2-phosphoglycerate is dehydrated to phosphoenolpyruvate (PEP) in a reaction that is catalyzed by enolase.

Reaction 9: $\text{2-phosphoglycerate(2PG)}\stackrel{\text{enolase}}{\rightleftharpoons }\text{Phosphoenolpyruvate}{\text{(PEP)+H}}_{\text{2}}\text{O}$

(e)

Summary Introduction

To explain: The step of glycolysis that involves carbon–carbon bond cleavage.

Answer to Problem 1E

Correct answer: Reaction 4 of glycolysis is a carbon–carbon bond cleavage reaction.

Explanation of Solution

In Reaction 4 of the glycolysis pathway, aldolase catalyzes the cleavage of fructose-1, 6-biphosphate to form the two trioses: glyceraldehyde-3-phosphate (GAP) and dihydroxyacetone phosphate. This represents the carbon–carbon cleavage that occurs in retro-aldol condensation. The cleavage occurs in between C3 and C4 of fructose-1, 6-biphosphate.

Reaction 4:

$\begin{array}{l}\text{Fructose-1,6-bisphosphate}\stackrel{\text{aldolase}}{\rightleftharpoons }\text{Dihydroxyacetone}\text{phosphate}\text{(DHAP)}\\ \text{+}\\ \text{Glyceraldehyde-3-phosphate}\text{(GAP)}\end{array}$