Chapter 16: Glycolysis

Question 1

What is the purpose of phosphorylating glucose in cytosol?

Answer 1

to trap glucose in the cell and to destabilize glucose and facilitate the next series of metabolic steps

Question 2

Which two 3-carbon molecules are generated by the cleavage of fructose-1,6-bisphosphate?

Answer 2

glyceraldehyde-3-phosphate and dihydroxyacetone phosphate

Question 3

What reaction is catalyzed by aldolase?

Answer 3

reversible cleavage of F-1,6-BP to DHAP and GAP

Question 4

What is the function of glyceraldehyde 3-phosphate dehydrogenase?

Answer 4

oxidation by NAD+ and the formation of acyl phosphate

Question 5

What is substrate-level phosphorylation?

Answer 5

ATP synthesis when the phosphate donor is a substrate with high-phosphoryl-transfer potential

Question 6

What are the primary metabolic fates of pyruvate?

Answer 6

ethanol, lactate, and acetyl CoA

Question 7

Fructose can enter glycolysis at two distinct points, depending on the tissue. How is fructose metabolized in adipose tissue?

Answer 7

Fructose is converted to fructose 6-phosphate.

Question 8

How are the glycolytic enzymes regulated?

Answer 8

through transcriptional control, through reversible phosphorylation and through allosteric control

Question 9

Which of the following must be regenerated for glycolysis to proceed?

Answer 9

NAD+

Question 10

What type of organism cannot survive in the presence of oxygen?

Answer 10

obligate anaerobes

Question 11

Which of the following intermediates is needed for the conversion of galactose into glucose by reacting with galactose 1-phosphate?

Answer 11

UDP-glucose

Question 12

Define: Hexokinase

Answer 12

Phosphorylates glucose

Question 13

Define: Phosphoglucose isomerase

Answer 13

Converts glucose 6-phosphate into fructose 6-phosphate

Question 14

Define: Phosphofructokinase

Answer 14

Forms fructose 1,6-bisphosphate

Question 15

Define: Aldolase

Answer 15

Cleaves fructose 1,6-bisphosphate

Question 16

Define: Triose phosphate isomerase

Answer 16

Catalyzes the interconversion of three-carbon isomers

Question 17

Define: Glyceraldehyde 3-phosphate dehydrogenase

Answer 17

Generates the first high- phosphoryl-transfer- potential compound that is not ATP

Question 18

Define: Phosphoglycerate kinase

Answer 18

Generates the first molecule of ATP

Question 19

Define: Enolase

Answer 19

Generates the second

high-phosphoryl- transfer-potential compound that is not ATP

Question 20

Define: Pyruvate Kinase

Answer 20

Generates the second

molecule of ATP

Question 21

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

Answer 21

In both cases, the electron donor is glyceraldehyde 3-phosphate. In lactic acid fermentation, the electron acceptor is pyruvate, converting it into lactate. In alcoholic fermentation, acetaldehyde is the electron acceptor, forming ethanol.

Question 22

Each of the following molecules is processed by glycolysis to lactate. How much ATP is generated from each molecule?

Glucose 6-phosphate

Answer 22

3 ATP

Question 23

Each of the following molecules is processed by glycolysis to lactate. How much ATP is generated from each molecule?

Dihydroxyacetone phosphate

Answer 23

2 ATP

Question 24

Each of the following molecules is processed by glycolysis to lactate. How much ATP is generated from each molecule?

Glyceraldehyde 3-phosphate

Answer 24

2 ATP

Question 25

Each of the following molecules is processed by glycolysis to lactate. How much ATP is generated from each molecule? Fructose

Answer 25

2 ATP

Question 26

Each of the following molecules is processed by glycolysis to lactate. How much ATP is generated from each molecule? Sucrose

Answer 26

4 ATP

Question 27

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

Answer 27

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

Question 28

The interconversion of DHAP and GAP greatly favors the formation of DHAP at equilibrium. Yet the conversion of DHAP by triose phosphate isomerase proceeds readily. Why?

Answer 28

The GAP formed is immediately removed by subsequent reactions, resulting in the conversion of DHAP into GAP by the enzyme

Question 29

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

Answer 29

The conversion of glyceraldehyde 3-phosphate into 1,3-bisphosphoglycerate would be impaired. Glycolysis would be less effective.

Question 30

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

Answer 30

The free energy for the reverse of glycolysis is +90 kJ mol^-1 (+22 kcal mol^-1), far too endergonic to take place.

Question 31

What are the principle fates of pyruvate generated in glycolysis?

Answer 31

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

Question 32

What reactions of glycolysis are irreversible under intracellular conditions?

Answer 32

The conversion of glucose into glucose 6-phosphate by hexokinase; the conversion of fructose 6-phosphate into fructose 1,6-bisphosphate by phosphofructokinase; the formation of pyruvate from phosphoenolpyruvate by pyruvate kinase.

Question 33

Phosphofructokinase converts fructose 6-phosphate to fructose 1,6-bisphosphate, the committed step on the pathway that synthesizes ATP. However, some fructose 6-phosphate is converted into fructose 2,6-biphosphate. Explain why depleting the substrate of PFK to form fructose 2,6-bisphosphate is not a wasteful use of substrate.

Answer 33

Substrates are usually present in much higher concentration than their enzymes. Consequently, converting a small amount of substrate into a potent activator of PFK will lead to a rapid increase in the rate of ATP synthesis.

Question 34

Glucose 6-phosphate:

Answer 34

Inhibits hexokinase

Question 35

Low ATP/AMP ratio:

Answer 35

Stimulates phosphofructokinase

Question 36

Citrate:

Answer 36

Inhibits phosphofructokinase in the liver

Question 37

Low pH:

Answer 37

Inhibits phosphofructokinase

Question 38

Fructose 2,6-bisphosphate;

Answer 38

Stimulates pyruvate kinase

Question 39

Insulin:

Answer 39

Causes the insertion of GLUT4 into cell membranes

Question 40

Has a high KM for glucose:

Answer 40

Glucokinase

Question 41

Transporter specific to liver and pancreas:

Answer 41

GLUT2

Question 42

High ATP/AMP ratio:

Answer 42

Inhibits phosphofructokinase

Question 43

Sucrose is commonly used to preserve fruits. Why is glucose not suitable for preserving foods?

Answer 43

Glucose is reactive because its open-chain form contains an aldehyde group. In other words, glucose is a reducing sugar.

Question 44

In the liver, fructose can be converted into glyceraldehyde 3-phosphate and dihydroxyacetone phosphate without passing through the phosphofructokinase-regulated reaction. Show the reactions that make this conversion possible. Why might ingesting high levels of fructose have deleterious physiological effects?

Answer 44

Fructose +ATP --> fructose 1-phosphate + ADP; Fructokinase Fructose 1-phosphate --> dihydroxyacetone phosphate + glyceraldehyde; Fructose 1-phosphate aldolase Glyceraldehyde +ATP --> Glyceraldehyde 3-Phosphate + ADP; Triose kinase The primary controlling step of glycolysis catalyzed by phosphofructokinase is bypassed by the preceding reactions. Glycolysis will proceed in an unregulated fashion.