Chp 22: Glycolysis

Question 1

1. What are the names for aerobic glycolysis?

Answer 1

Glycolysis or glycolytic pathway

Question 2

1. What are the functions of aerobic glycolysis?

Answer 2

To produce energy and substrates for other anabolic pathways

Question 3

1. What are the substrates of aerobic glycolysis?

Answer 3

• Glucose
• ADP
• NAD⁺
• Pi

Question 4

1. What are the products of aerobic glycolysis?

Answer 4

• Pyruvate
• NADH
• H⁺
• ATP
• H₂O

Question 5

1. What is the control enzyme in aerobic glycolysis?

Answer 5

Phosphofructokinase-1 (PFK-1)

Question 6

1. What are the regulators of aerobic glycolysis?

Answer 6

• Fructose 2,6-bisphosphate
• Ratio of ATP/ADP&AMP

Question 7

1. Where in the cell does aerobic glycolysis take place?

Answer 7

Cytosol

Question 8

1. What are the tissues of interest for aerobic glycolysis?

Answer 8

Every cell type

Question 9

2. What is the first enzyme in glycolysis in muscle?

What reaction is catalyzed by this enzyme?

Is this reaction reversible?

What is the isozyme of this enzyme in liver?

Answer 9

• Hexokinase in muscle
• Glucose + ATP → Glucose-6-P + ADP
• Irreversible
• Glucokinase in liver

Question 10

3. What is the enzyme that transfers a phosphate group to fructose-6-phosphate in glycolysis in liver?

What reaction is catalyzed by this enzyme?

Is this reaction reversible?

Answer 10

• Phosphofructokinase-1 (PFK-1) transfers a phosphate group to fructose-6-phosphate in the liver, and it catalyzes the following reaction:
• Fructose 6-phosphate + ATP → Fructose 1,6-bisphosphate + ADP
• Irreversible

Other info: PFK-1 is considered the committed step in glycolysis and is the control enzyme. It is controlled by fructose 2,6-bisphosphate and the ratio of ATP/ADP&AMP

Question 11

4. What is the enzyme that produces NADH from a triose phosphate in the glycolytic pathway?

What reaction does this enzyme catalyze?

Is the reaction reversible?

Answer 11

• Glyceraldehyde 3-phosphate dehydrogenase
• Glyceraldehyde 3-P + P_i + NAD⁺ ⇔ 1,3 Bisphosphoglycerate + NADH + H⁺
• Reversible

Other info: Glyceraldehyde 3-phosphate is formed when fructose 1,6-bisphosphate is cleaved to two triose phosphates (glyceraldehyde 3-phosphate and dihydroxyacetone phosphate (DHAP) by aldolase. DHAP is then isomerized to a second glyceraldehyde 3-phosphate by triose phosphate isomerase. Thus, net 2 glyceraldehyde 3-phosphates are formed from fructose 1,6-bisphosphate

Question 12

5. What is the enzyme that produces ATP from 1,3 bisphosphoglycerate in the glycolytic pathway?

What reaction does this enzyme catalyze?

Is the reaction reversible?

Is this substrate level phosphorylation?

Answer 12

• Phosphorglycerate kinase
• 1,3 Bisphosphoglycerate + ADP ⇔ 3-Phosphoglycerate + ATP
• Reversible
• Yes

Question 13

6. What is the enzyme that produces ATP from phosphoenolpyruvate in the glycolytic pathway?

What reaction does this enzyme catalyze?

Is the reaction reversible?

Answer 13

• Pyruvate kinase
• Phosphoenolpyruvate + ADP → Pyruvate + ATP
• Irreversible

Question 14

7. Is glycolysis reversible?

Answer 14

• No, glycolysis is irreversible. Like all pathways, this one has evolved to be irreversible:
• Glucose + 2 ADP + 2 NAD⁺ + 2 P_i → 2 Pyruvate + 2 NADH + 2 H⁺ + 2 ATP + 2 H₂O
• The above reaction for glycolysis has a standard free energy change of approximately a -22 kcal/mol. We decided that a -7 kcal/mol is usually irreversible. Another way to put it: one could not raise the concentrations of glycolysis products high enough to cause the equation to reverse directions

Note! 2 ATPs that were used by hexokinase/glucokinase and phosphofructokinase-1 do not appear on the left side of the equation because 2 ATPs have been subtracted from both sides of the equation

Question 15

8. What is the function of the malate-aspartate shuttle?

Answer 15

• The function of the malate-aspartate shuttle is to get the electrons on NADH from glycolysis to NADH in the mitochondria so they can be used by the NADH dehydrogenase of the ETC. Without this shuttle, aerobic glycolysis would be inhibited by NADH and the lack of substrate NAD⁺
• The purpose of the malate-aspartate shuttle is to make NADH in the matrix of the mitochondria from NADH in the cytosol. The NADH in the matrix can then be oxidized by the ETC. Because NAD⁺ and NADH cannot cross the mitochrondrial membrane, another way must be found to transfer the reducing electrons in NADH into the mitochrondria.

Question 16

8. What are the substrates, products, and enzymes of the reactions necessary to transfer the electrons from the product of the glyceraldehyde 3-phosphate reaction to the substrate for the NADH dehydrogenase in the ETC?

Answer 16

Malate dehydrogenase catalyzes the following reaction in the cytosol:

• NADH + H⁺ + oxaloacetate ⇔ Malate + NAD⁺

In the matrix, malate dehydrogenase catalyzes the reverse reaction:

• NAD⁺ + malate⇔ oxaloacetate + NADH + H⁺

Question 17

9. What are the substrates and products of the LDH reaction? Is the reaction readily reversible?

Answer 17

• Lactate dehydrogenase (LDH) reaction: Pyruvate + NADH + H⁺ ⇔ Lactate + NAD⁺
• Reversible

Other info: If this reaction were not reversible, the lactate made by red blood cells and striated muscle could not be used by the liver or heart. This is the last reaction in anaerobic glycolysis, but is not a part of aerobic glycolysis

Question 18

10. What are the two major factors determining whether a cell oxidizes glucose by aerobic glycolysis or by anaerobic glycolysis?

Answer 18

• Mitochondria: a cell that lacks mitochondria (such as a red blood cell) must utilize the anaerobic pathway and make lactate. Different muscle types have different mitochondrial densities, so they have different abilities to pyruvate
• Oxygen supply: Without oxygen, the ETC, TCA, and PDC cannot use pyruvate, so the cell would have to run anaerobic glycolysis. The more limited the oxygen supply, the more the cell would have to rely on anaerobic metabolism. With a good oxygen supply, most cells will run aerobic glycolysis

Question 19

11. Compare the energy produced from glucose during anaerobic glycolysis with the energy produced in the conversion of glucose to pyruvate during aerobic glycolysis.

Answer 19

Anaerobic glycolysis produces 2 ATPs:
Glucose + 2 ADP + 2 P_i → 2 Lactate + 2 ATP + 2H₂O

Aerobic glycolysis produces 2 ATPs and 2 NADHs, or 7 ATPs:
Glucose + 2 ADP + 2 NAD⁺ + 2 P_i → 2 Pyruvate + 2 NADH + 2 H⁺ + 2 ATP + 2 H₂O

The difference is that the two NADHs produced in aerobic glycolysis are converted into ATP while the NADH produced in anaerobic glycolysis is used to reduce pyruvate

1 NADH = 2.5 ATP

Question 20

12. Compare the energy produced from glucose during anaerobic glycolysis with the energy produced if glucose is completely oxidized to CO2 by glycolysis, pyruvate dehydrogenase, and the TCA cycle?

Answer 20

Anaerobic glycolysis (i.e., the conversion of glucose to lactate) = 2 ATP

Complete oxidation of glucose to CO2 and H2O = 32 ATP

• Aerobic glycolysis = 2 NADH + 2 ATP = 7 ATP
• Pyruvate dehydrogenase = 2 NADH = 5 ATP (for two pyruvate)
• TCA cycle = 6 NADH (15 ATP) + 2 FADH2 (3 ATP) + 2 GTP (2 ATP) = 20 ATP (for two acetyl CoA)

Conclusion: Aerobic oxidation of glucose to CO2 and H2O yields 16 times more energy for the cell than anaerobic glycolysis

Other info: It is interesting that anaerobic glycolysis has to run approximately 16X faster to produce the same amount of ATP as aerobic glycolysis. Anaerobic glycolysis can go more than 100X faster in some muscle cells

Question 21

13. What is lacticacidosis?

Answer 21

Lactic acidosis is a metabolic acidosis occurring as a result of excess anaerobic glycolysis and the release of excess lactic acid into the blood

This occurs when enough cells of the body are forced to anaerobically instead of aerobically metabolize glucose. The lactic acid dissociated and the lactate and H+ are transported out of the cell and eventually into the blood. If there is too much lactic acid and it can’t be buffered, it will lower the pH of the blood

Any disease process that interferes with the ETC, TCA, or PDC is liable to abnormally increase metabolic anaerobic glycolysis. Some possible causes of lactic acidosis include:

• Poor oxygen uptake by blood in the lungs
• Poor transport of oxygen to the tissues
• Inhibition of the ETC
• Congenital deficiency of lactate dehydrogenase in the liver so the normal lactate released from muscle and red blood cells cannot be removed from blood

Other info: Lactic acidosis can be a life-threatening condition. Remember that that structure of proteins and the activity of enzymes are dependent upon the pH. The key signs of lactic acidosis include unusually deep and rapid breathing (hyperventilation to try to rid the body of excess CO2 to increase blood pH), vomiting, and abdominal pain

Question 22

14. Explain the Cori Cycle!

Answer 22

A red blood cell, muscle cell, or any other cell undergoing anaerobic glycolysis releases lactate, which travels to the liver to undergo gluconeogenesis. In the liver, 2 lactates are converted to 1 glucose. This requires 6 ATPs. The glucose produced is returned to the bloodstream and transported back to a cell to be used again for anaerobic glycolysis, generating 2 ATPs and 2 lactate – so on and so forth

Question 23

15. Which of the glycolytic enzymes are activated when the cellular ratio of ATP to ADP or ATP to AMP is decreased?

Answer 23

• Phosphofrutokinase-1 is activated by AMP and inhibited by ATP. This is the control enzyme in glycolysis. We don’t have to remember which exact nucleotide inhibits or activates, just the effect of the ratio of ATP to ADP&AMP. This is an energy producing pathway, so you would expect that when you don’t need energy (i.e., high ATP), the pathway would be activated
• Pyruvate kinase is also inhibited by a high ATP to ADP&AMP ratio. However, since this is not the control step, we won’t focus here

Question 24

16. What reaction is catalyzed by adenylate kinase?

Answer 24

Adenylate kinase (myokinase): ATP + AMP ⇔ 2 ADP

When a muscle cell is working very hard, it produces a lot of ADP from ATP. Thus, the ATP concentration drops and the ADP concentration rises. Some of the ADP produced is used to produce more ATP and AMP. This provides additional ATP for muscle contraction. The reaction is the reverse of the myokinase reaction

Question 25

17. Why is AMP concentration a better measure of energy utilization than ATP concentration?

Answer 25

AMP is normally present in such a small concentration in the cytosol compared to ATP. During energy utilization, [ATP] will decrease only about 20% while [AMP] will increase by 300%. The significantly greater change in the concentration of AMP provides for a more sensitive indicator for enzymes monitoring ATP utilization in the cell

Question 26

18. In liver, high glucagon and high cAMP will activate protein kinase A. High protein kinase A activity will phosphorylate the enzyme phosphofructokinase-2/fructose-2,6-bisphospahtase. How will this effect the activity of phosphofructokinase-1 and the rate of glycolysis?

Answer 26

* Phosphorylation of the dual enzyme phosphofructokinase-2/fructose 2,6-bisphosphatase will both activate the fructose 2,6-bisphosphatase enzyme and deactivate the phosphofructokinase-2 enzyme. Thus, fructose 2,6-bisphosphate will be destroyed and no more will be synthesized * Glycolysis is activated by fructose 2,6-bisphophsate so it will be inhibited by its absence. The rate of glycolysis will stop * Other info: In the fasting state, glucagon inhibits glycolysis and activates gluconeogenesis so that it can release glucose into the blood. Also, you know that control PFK-1 is activated by fructose 2,6-bisphosphate. So, you should suspect a pathway (cAMP cascade) activation by glucagon will remove fructose 2,6-bisphosphate from the cell. * In the fed state, insulin is high and glucagon is low in the blood. The second messenger pathway for insulin will dephosphorylate the dual enzyme phosphofructokinase-2/fructose 2,6-bisphosphatase. This will both inactivate the fructose 2,6-bisphosphatase enzyme and activate the phosphofructokinase-2 enzyme. This will increase the concentration of fructose 2,6-bisphosphate so glycolysis will be activated * It may help to remember that fructose 2,6 bisphosphate is not itself an intermediate or an enzyme in glycolysis. It is a regulator whose concentration in the blood reflects the levels of insulin and glucagon * Epinephrine has the same effect as glucagon on dual enzyme phosphofructokinase-2/fructose 2,6-bisphosphatase in the liver. In muscle tissue, epinephrine has the opposite effect, causing the synthesis of fructose 2,6-bisphosphate

Question 27

19. In cardiac muscle, high adrenalin and high cAMP will activate protein kinase A. High protein kinase A activity will phosphorylate the enzyme phosphofructokinase-2/fructose-2,6-bisphospahtase. How will this effect the activity of phosphofructokinase-1 and the rate of glycolysis?

Answer 27

* In muscle, when protein kinase A phosphorylates the dual enzyme phosphofructokinase-2/fructose 2,6 bisphosphatase, the result is just the opposite of what happens in the liver. In muscle, protein kinase A will both inactivate the fructose 2,6-bisphosphatase enzyme and activate the phosphofructokinase-2 enzyme. Thus, fructose 2,6-bisphosphate will be synthesized and no more will be destroyed * Glycolysis will be activated by the increased concentration of fructose 2,6-bisphosphate * Epinephrine therefore increases rate of glycolysis, producing more ATP and enabling the muscle to better contract Other info: When you want to run, you would expect the epinephrine in the blood to result in an activation of glycolysis in muscle since this is needed to provide energy for muscle contraction. Insulin and epinephrine both activate glycolysis in muscle. Insulin activates but epinephrine inhibits glycolysis in liver. Protein kinase A phosphorylates the dual enzyme at different R-groups in muscle and liver. The different conformations result in different enzyme activities

Question 28

20. In liver, high glucagon and high cAMP will activate protein kinase A. How will this affect the activity of pyruvate kinase and the rate of glycolysis?

Answer 28

* In the presence of high glucagon, glucagon binds to receptors on liver cells and activates the cAMP cascade and protein kinase A. Protein kinase A phosphorylates and inhibits pyruvate kinase. Since pyruvate kinase is a necessary step in glycolysis, the activation of protein kinase A inhibits glycolysis * Other info: The inhibitions of pyruvate is required if gluconeogenesis is to take place. It makes sense that when glucagon activates gluconeogenesis, it also inhibits pyruvate kinase. * Glucagon has receptors on liver, adipose tissue, and heart cells – but not striated muscle cells * Adrenaline does the same thing in liver

Question 29

21. Concerning Lopa Fusor: Explain why hemorrhage, anemia, COPD, or any combination of these three might result in lactic acidosis.

Answer 29

* Hemorrhage may cause low perfusion in the tissue as a result of low blood volume. This in turn results in inadequate perfusion of the tissues and inadequate oxygen delivery * Anemia is the lower-than-normal oxygen-carrying capacity in blood due to either low red blood cells and/or dysfunctional hemoglobin. It may cause hypoxia reduction of oxygen supply to a tissue despite adequate perfusion of the tissue by blood * COPD (chronic obstructive pulmonary disorder) results in lower exchange of atmospheric oxygen with the blood. The blood does not get enough oxygen even though there may be more blood cells and hemoglobin than in a normal person. It may cause hypoxia reduction of oxygen supply to a tissue despite adequate perfusion of the tissue by blood Low oxygen delivery to the tissues would lead to the following: * Inhibition of the electron transport chain * A decrease in the ratio of ATP to ADP&, and an increase in NADH and FAD(2H) * Inhibition of the TCA cycle * Increase in acetyl CoA * Inhibition of pyruvate dehydrogenase * Increased glycolysis * Increased conversion of pyruvate to lactate * Increased release of lactic acid into the blood – lactic acidosis

Question 30

22. Concerning Otto Shape, compare his metabolism when walking slowly with sprinting. Consider the following:

Answer 30

* Total ATP used by muscle per second: More with sprinting * Rate of ATP generated by oxidative phosphorylation: More with sprinting. It plateaus as he reaches sprinting speeds due to a lack of oxygen availability * The rate of pyruvate oxidized by pyruvate dehydrogenase and the TCA cycle. Why? Faster with sprinting. While sprinting, there is a higher demand for ATP; thus, glycolysis is running at a faster rate to satisfy this need and in the process is producing more pyruvate. This pyruvate is then oxidized via pyruvate dehydrogenase and TCA. Eventually the mitochondria’s ability to produce ATP via oxidative phosphorylation is maxed out, increasing NADH levels, and subsequently inhibiting the oxidation of pyruvate. As a result, conversion to lactic acid will increase * The rate of glycolysis. Why? Faster with sprinting since the muscle is performing much more work with greater utilization of ATP. There is a decrease in the ratio of ATP to ADP/AMP. ATP is an inhibitor of the control enzyme phosphofructokinase-1, while AMP is an activator of the same enzyme. Thus as ATP decreases and AMP increases, the enzyme is activated and the rate of the pathway is increased * The rate of conversion of pyruvate to lactate. Why? Much greater with sprinting. Pyruvate is made much faster than it can be used by the PDC, TCA, and ETC. Increase in pyruvate that can’t be used and increase in NADH that can’t be used both drive the lactate dehydrogenase (LDH or LH) reaction towards lactate

Question 31

23. Concerning Ivan Applebod: Streptococcus mutans needs energy for growth and cell division. As a result, they also cause dental caries. What is the relationship between the production of energy and dental caries?

Answer 31

Oral bacteria acquire the majority of their energy through anaerobic glycolysis. This process produces lactic as well as other acids. When the pH around the bacterium drops sufficiently, acid begins dissolving the tooth Other info: Steptococcus mutans plays a big role in smooth surface caries because it secretes dextran. Dextran is an insoluble polysaccharide that forms the base for plaque. It is a sticky water-insoluble substance that mediates the attachment of attachment of Streptococcus mutans to the tooth surface and keeps acid from diffusing away