S1B4 - Glycolysis

Question 1

What are the energy-expending steps of glycolysis and what energy molecules are expended?

Answer 1

Energy-expending steps in the preparatory phase of glycolysis are the following:

• 1 ATP per glucose: Glucose → G6P
• 1 ATP per glucose: Fructose-6-phosphate → F1,6BP

Question 2

After hexokinase forms glucose-6-phosphate (G6P), what is the next compound formed in glycolysis?

Answer 2

Step 2 (preparatory): G6P is isomerized to fructose-6-phosphate (F6P) by phosphoglucose isomerase. This is a fully reversible reaction driven in either direction by relative concentrations of the substrates.

Question 3

Gene mutations in which glycolytic enzyme can be a cause of maturity-onset diabetes of the young (MODY)?

Answer 3

Gene mutations in glucokinase can be a cause of maturity-onset diabetes of the young (MODY). MODY can present as familial diabetes or gestational diabetes, as well as diabetes with an atypical presentation (e.g. a slim, relatively fit type 2 diabetic).

Question 4

The synthesis of which of the following glycolytic intermediates directly requires ATP?
A) Pyruvate

B) Dihydroxyacetone phosphate

C) Fructose-1,6-bisphosphate

D) Glyceraldehyde 3-phosphate

E) 3-phosphoglygerate

Answer 4

C) Fructose-1,6-bisphosphate

In glycolysis, only 2 steps consume ATP:

• the synthesis of glucose-6-phosphate from glucose, and
• the synthesis of F1,6BP from fructose-6-phosphate.

F6P is made from G6P via an isomerase and doesn’t consume energy.

Question 5

After fructose-1,6-bisphosphate is formed, what is the next compound formed in glycolysis?

Answer 5

Step 4 (preparatory): The unstable F1,6BP is split by aldolase to yield two molecules of the 3-carbon glyceraldehyde 3-phosphate (GA3P).

In reality, fructose is split into GA3P + DHAP (dihydroxyacetone phosphate), but DHAP is rapidly isomerized to GA3P.

Question 6

After 1,3-bisphosphoglycerate (1,3BPG) is formed, what is the next compound formed in glycolysis?

Answer 6

Step 6 (payoff): 1,3BPG is dephosphorylated, yielding 3-phosphoglycerate (3-PG).

Question 7

In what cellular compartment does glycolysis occur?

Answer 7

Glycolysis occurs in the cytosol and has 2 phases: preparatory (requires energy) and payoff (generates energy).

Question 8

After fructose-6-phosphate is formed, what is the next compound formed in glycolysis?

Answer 8

Step 3 (preparatory): F6P is phosphorylated to fructose-1,6-bisphosphate (F1,6BP) by phosphofructokinase-1 (PFK-1).

Question 9

What are the energy-yielding steps of glycolysis and what energy molecules are formed?

Answer 9

Energy-yielding steps in the payoff phase of glycolysis are the following:

• 2 NADH per glucose: GA3P → 1,3BPG
• 2 ATP per glucose: 1,3BPG → 3-PG
• 2 ATP per glucose: PEP → Pyruvate

Question 10

After glyceraldehyde 3-phosphate (GA3P) is formed, what is the next compound formed in glycolysis?

Answer 10

Step 5 (payoff): GA3P forms 1,3-bisphosphoglycerate (1,3BPG).

This yields 2 NADH per glucose (1 per GA3P).

Question 11

After 3-phosphoglycerate (3-PG) is formed, what is the next compound formed in glycolysis?

Answer 11

Step 7: 3-PG is converted to 2-phosphoglycerate (2-PG).

Question 12

After 2-phosphoglyerate (2-PG) is formed, what is the next compound formed in glycolysis?

Answer 12

Step 8 (payoff): 2-PG becomes phosphoenolpyruvate (PEP).

Question 13

Which enzyme converts phosphoenolpyruvate to pyruvate?

Answer 13

Step 9 (payoff): PEP is converted to pyruvate by pyruvate kinase.

This yields 2 ATP per glucose (1 per PEP).

Question 14

In which organs is glucokinase expressed?

Answer 14

Glucokinase is a high-capacity isoenzyme of hexokinase expressed in the liver and pancreas. It is induced by insulin and is a key component of insulin’s hypoglycemic effect.

Question 15

In skeletal muscle, which enzyme sequesters glucose within the cell?

A) Phosphoglucose isomerase

B) Phosphofructokinase-1

C) Glucose-6-phosphatase

D) Hexokinase

E) Glucokinase

Answer 15

D) Hexokinase

To sequester glucose within a cell, it must be phosphorylated to make glucose-6-phosphate. G6P can’t leave the cell without being dephosphorylated.

• In skeletal muscle, hexokinase performs this phosphorylation.
• In the liver, brain, and pancreas, glucokinase performs this function.

Question 16

What is the rate-limiting step of glycolysis?

Answer 16

The formation of F1,6BP by PFK-1 is irreversible and rate-limiting.

Question 17

What enzyme catalyzes the first step of glycolysis, in which glucose is converted to glucose-6-phosphate (G6P)?

Answer 17

Step 1 (preparatory): Hexokinase converts glucose to glucose-6-phosphate (G6P). This sequesters glucose in the cell, as only unphosphorylated glucose can exit the cell. G6P formation also effectively decreases intracellular glucose, driving more glucose into the cell.

Question 18

How does low blood glucose cause fructose-2,6-bisphosphatase activity to predominate over phosphofructokinase-2 in glycolysis?

Answer 18

Phosphofructokinase-2 / fructose-2,6-bisphosphatase is a bifunctional enzyme regulating phosphofructokinase-1. F26BPase is favored under the following pathway:

1. Low blood glucose elevates glucagon secretion
2. Glucagon secretion causes phosphorylation of PFK-2 via cAMP pathway
3. F2,6BPase activity now predominate
4. F26BPase synthesizes fructose-6-phosphate from fructose-2,6-bisphosphonate
5. Lower concentrations of F26BP decreases the activity of PFK-1

Question 19

Which 2 molecules allosterically inhibit pyruvate kinase?

Answer 19

Pyruvate kinase is allosterically inhibited by ATP and alanine.

Question 20

How does fructose-6-phosphate inhibit glucokinase?

Answer 20

Glucokinase is further regulated by fructose-6-phosphate. F6P binds to the same receptor as glucokinase regulatory protein (GKRP), thereby enhancing GKRP’s ability to inhibit glucokinase.

Question 21

What is the difference in the regulation of hexokinase and glucokinase by glucose-6-phosphate?

Answer 21

Hexokinase and glucokinase are regulated by various means. For instance, hexokinase is inhibited by glucose-6-phosphate, whereas glucokinase is not inhibited by G6P. The regulation of hexokinase by G6P prevents overconsumption of ATP during its reaction.

Question 22

Which three compounds decrease the activity of phosphofructokinase-1?

Answer 22

Phosphofructokinase-1 activity is decreased by elevated concentrations of:

• Acetyl-CoA
• Citrate
• ATP

Question 23

How do low levels of glucose affect liver pyruvate kinase activity?

Answer 23

Pyruvate kinase may also be inhibited by low blood glucose levels, allowing for PEP to enter gluconeogenesis via the following pathway:

1. Low glucose levels detected
2. Increased glucagon secretion
3. Increased cAMP and protein kinase A activity
4. Phosphorylation leads to inactivation of pyruvate kinase

Question 24

Which 2 molecules activate pyruvate kinase?

Answer 24

Pyruvate kinase activity is increased by fructose 1,6-bisphosphate (F1,6BP) and phosphoenolpyruvate (PEP).

Question 25

Which two substrates increase the activity of phosphofructokinase-1?

Answer 25

Phosphofructokinase-1 activity is increased in the presence of AMP and fructose-2,6-bisphosphate.

Question 26

How does high blood glucose cause phosphofructokinase-2 to predominate over fructose-2,6-bisphosphatase?

Answer 26

Phosphofructokinase-2 (PFK2) / fructose-2,6-bisphosphatase (F26BPase) is a bifunctional enzyme regulating phosphofructokinase-1. PFK2 is favored under the following pathway: * **High blood glucose causes insulin secretion** * **Insulin activates protein phosphatase** * **Protein phosphatase dephosphorylates PFK2** * **PFK2 then predominates over F26BPase** * **PFK2 synthesizes fructose-2,6-bisphosphate from fructose-6-phosphate** * **F26BP is a potent activator of PFK-1**

Question 27

Does hexokinase have a high or low affinity (K_m) and capacity (V_max) to glucose?

Answer 27

Hexokinase has a **high affinity (decreased Km) and a low capacity (decreased Vmax)** for glucose. The high affinity allows for glycolysis to occur, depsite low blood glucose levels.

Question 28

What are the four key regulatory enzymes of glycolysis?

Answer 28

Glycolysis is regulated by four key enzymes: * **Hexokinase** (most tissues except liver and beta-cells of the pancreas) * **Glucokinase** (liver and beta-cells of the pancreas) * **Phosphofructokinase-1** * **Pyruvate kinase**

Question 29

Which enzyme's genetic mutation may result in maturity-onset diabetes of the young?

Answer 29

**Glucokinase genetic mutations** on chromosome 7 may result in maturity-onset diabetes of the young (MODY). Glucokinase, within the beta-cells of the pancreas, acts as a glucose sensor, thus when a mutation is present, impaired glucose sensation occurs leading to persistant hyperglycemia and impaired insulin secretion.

Question 30

Does glucokinase have a high or low affinity (K_m) and capacity (V_max) to glucose?

Answer 30

Glucokinase has a **low affinity (increased Km) and a high capacity (increased Vmax)** for glucose. Remember, **GLU**cokinase is a **GLU**tton with an insatiable Vmax.

Question 31

How does hexokinase and glucokinase respond to insulin secretion?

Answer 31

Hexokinase is **not triggered by insulin, whereas glucokinase is triggered by insulin.**

Question 32

Phosphofructokinase-1 is a key glycolytic enzyme. Which molecule has the greatest upregulating effect on this enzyme? A) Adenine triphosphate B) Guanosine triphosphate C) Fructose-2,6-bisphosphate D) Guanosine monophosphate E) Fructose-1,6-bisphosphate

Answer 32

C) Fructose-2,6-bisphosphate PFK-1 activity is ↑ by AMP and F2,6BP. F2,6BP is produced by PFK-2.

Question 33

The oxidation of acetyl-CoA by the TCA cycle will account for how many additional molecules of ATP per glucose molecule?

Answer 33

The oxidation of acetyl-CoA by the TCA cycle will account for additional **36** molecules of ATP per glucose molecule.

Question 34

What is the primary purpose of the metabolic reaction involving lactate dehydrogenase?

Answer 34

The primary purpose of **anaerobic glycolysis** is to **regenerate NAD+.**

Question 35

What does the glycerol-3-phosphate shuttle do and why is it necessary?

Answer 35

In anaerobic conditions NAD⁺ must be “regenerated” for glycolysis to continue. Since **NADH cannot cross mitochondrial membrane**, the reducing equivalents from the NADH formed in glycolysis are taken up into mitochondria for oxidation by **shuttle mechanism**.

Question 36

What do you need to know about 2-3BPG and erythrocytes?

Answer 36

2-3BPG **binds to Hb, decreasing its affinity for O₂.**

Question 37

What do you need to know about fructose metabolism and diabetic patients?

Answer 37

In liver, kidney and intestine, fructokinase phosphorylates fructose to fructose-1-phosphate. **Not affected by Insulin** – **diabetic patients can clear fructose.**

Question 38

What is galactosemia the result of?

Answer 38

Galactosemia is the result of a lack of **galactose-1-phosphate uridyltransferase.**

Question 39

What pathway do fructose, galactose, and mannose for metabolism?

Answer 39

Fructose, galactose and mannose enters the **glycolytic pathway** for metabolism.

Question 40

What does galactose enter the glycolytic pathway as?

Answer 40

Galactose will enter the glycolytic pathway as **glucose-6-phosphate.**

Question 41

What does fructose enter the glycolytic pathway as?

Answer 41

Fructose enters the glygolytic pathway as **glyceraldehyde-3-phosphate.**

Question 42

What are the fates of pyruvate produced by glycolysis?

Answer 42

1. **Lactic acid fermentation** in anaerobic conditions 2. **Citric acid cycle** and oxydative phosphorylation in aerobic conditions.