Chapter 16 - Glycolysis

Question 1

Lactic acid fermentation and alcoholic fermentation are oxidization-reduction reactions. Identify the ultimate electron donor and electron acceptor.

Answer 1

In both cases, the electron donor is glyceraldehyde 3-phosphate.

In lactic acid fermentation, the electron acceptor is pyruvate converting it to lactate

in alcoholic fermentation, the electron acceptor is acetaldehyde forming ethanol

Question 2

Why is glucose the fuel that is used by all organisms?

Answer 2

Glucose is formed under prebiotic conditions

It is the most stable hexose sugar

has a low tendency to nonenzymatically react with proteins.

Question 3

How much ATP is generated from one molecule of Glucose 6-phosphate as it is processed by glycolysis to lactate?

Answer 3

3 ATP

Question 4

How much ATP is generated from one molecule of Dihydroxyacetone phosphate as it is processed by glycolysis to lactate?

Answer 4

2 ATP

Question 5

How much ATP is generated from one molecule of Glyceraldehyde 3-phosphate as it is processed by glycolysis to lactate?

Answer 5

2 ATP

Question 6

How much ATP is generated from one molecule of Fructose as it is processed by glycolysis to lactate?

Answer 6

2 ATP

Question 7

How much ATP is generated from one molecule of Sucrose as it is processed by glycolysis to lactate?

Answer 7

4 ATP

Question 8

Why is it advantageous for the liver to have both hexokinase and glucokinase to phosphorylate glucose?

Answer 8

glucokinase enables the liver to remove glucose from the blood when hexokinase is saturated, ensuring that glucose is captured for later use.

Question 9

What is the role of thioester in the formation of ATP in glycolysis?

Answer 9

A thioester couples the oxidization of glyceraldehyde 3-phosphate to 3-phosphoglycerate with the formation of 1,3-biphosphoglycerate.

1,3-biphosphoglycerate can subsequently power the formation of ATP.

Question 10

The recommended daily allowance for the vitamin niacin is 15mg per day. How would glycolysis be affected by niacin deficiency?

Answer 10

The conversion of glyceraldehyde 3-phosphate into 1,3-bisphosphoglycerate would be impaired because the reaction requires the niacin-derived coenzyme NAD+.

Glycolysis would be less effective.

Question 11

Which enzyme is the pacemaker of glycolysis?

Answer 11

phosphofructokinase

Question 12

Why is the regulation of phosphofructokinase by energy charge not as important in the liver as it is in muscle?

Answer 12

The energy needs of a muscle cell vary widely, from rest to intense exercise. Consequently, the regulation of phosphofructokinase by energy charge is vital. In other tissues such as the liver, ATP concentration is less likely to fluctuate and will not be a key regulator of phosphofructokinase.

Question 13

Why can’t the reactions of the glycolytic pathway simply be run in reverse to synthesize glucose?

Answer 13

The ^G for the reverse of glycolysis is +90 kJ/mol, far too endergonic to take place.

Question 14

What are the principle fates of pyruvate generated in glycolysis?

Answer 14

Pyruvate can be metabolized to ethanol in alcoholic fermentation, to lactate in lactic acid fermentation, or it can be completely oxidized to CO2 and H2O in cellular respiration.

Question 15

What reactions of glycolysis are irreversible under intracellular conditions?

Answer 15

The conversion of glucose into glucose 6-phosphate by hexokinase

The conversion of fructose 6-phosphate into fructose 1,6-biphosphate by phosphofructokinase

The formation of pyruvate from phosphoenolpyruvate by pyruvate kinase

Question 16

Why is it in the muscle’s best interest to export lactic acid into the bloodstream during intense exercise?

Answer 16

Lactic acid is a strong acid. If it remained in the cell, the pH of the cell would fall, which could lead to the denaturation of muscle protein and result in muscle damage.

Question 17

Describe the pathways by which fructose is prepared for entry into glycolysis.

Answer 17

In the liver, fructokinase converts fructose into fructose 1-phosphate.

Fructose 1-phosphate is cleaved by a specific aldolase to yield glyceraldehyde and dihydroxyacetone phosphate, which is a component of the glycolytic pathway

Glyceraldehyde is converted into the glycolytic intermediate glyceraldehyde 3-phosphate by triose kinase.

In other tissues, fructose is converted into fructose 6-phosphate by hexokinase.

Question 18

The intravenous infusion of fructose into healthy volunteers leads to a two- to five-fold increase in the level of lactate in the blood over glucose.

Why is glycolysis more rapid after the infusion of fructose?

Answer 18

The fructose 1-phosphate pathway forms glyceraldehyde 3-phosphate, bypassing the control by phosphofructokinase

Question 19

Fructose has been used in place of glucose for intravenous feeding. Why is this use of fructose unwise?

Answer 19

Since phosphofructokinase is bypassed, glycolysis proceeds in an unregulated manner. Lactic acidosis may result, and fatty liver may develop.

Question 21

Why is the isomerization of glucose 6-phosphate to fructose 6-phosphate an important step in glycolysis?

How is the conversion of the fructose isomer back into the glucose isomer prevented?

Answer 21

Glucose cannot be cleaved into two three-carbon fragments, whereas fructose can, and three-carbon molecules are metabolized in the second stage of glycolysis.

The conversion of fructose 6-phosphate into fructose 1,6-biphosphate prevents the glucose isomer from being re-formed.

Question 22

In the liver and the pancreas, hexokinase and glucokinase phosphorylate glucose. Glucokinase is active only when the blood concentration of glucose is high. How might glucokinase differ kinetically from hexokinase so as to function only at high glucose levels?

Answer 22

Glucokinase has a higher Km value, which allows this enzyme to become more active at high glucose concentrations - conditions that saturate hexokinase.