L2 - Anaerobic Metabolism (Glycolysis)

Question 1

Key Facts about Glycolysis Pathway:

1. Where does it occur (i.e. which cell type and which species)?
2. What is its main function and minor role?
3. What other biological macromolecules can be used as fuel except for glucose?

Answer 1

1. This is the most important metabolic pathway - as this occurs in all cells in all organisms.
2. Main Function: ATP synthesis by using glucose as a fuel.
   Minor role: it can also produce metabolic intermediates for a.a. and FA synthesis.
3. FAs can also be used as fuel; this is more reduced than glucose so when oxidise it, it can generate more energy than glucose.

Question 2

Structure of Glucose:

- Talk about the different types/isomers of sugars (4 types)

Answer 2

Sugars can either be aldoses or ketoses - depending on whether they have aldehyde or ketone groups.

Glucose can either exist in the linear (Fischer projection) or ring (Haworth projection) form.

Glucose has D- and L-isomers. The main one in our body is D-glucose as our enzymes will only metabolise D-glucose. (a.a are in L-form; sugars are in D-form).

Glucose can either have an alpha or beta form - OH group is either below or above.

Question 3

What are the sources of glucose for glycolysis:

Answer 3

1. Sugar and starch from diet
2. Breakdown of stored glycogen
3. Recycled glucose - from a.a, lactate or glycerol

Question 4

Key Points of Glycolysis:

• Definition (start -> end)
• Location
• Tissues (where it occurs)
• Function

Answer 4

• Glucose (6C) is converted to pyruvate (3C)
• Cytosol
• All cells
• Function: ATP synthesis by using glucose as fuel and to produce metabolic intermediates for a.a. and fat synthesis.

Question 5

The 10 reactions of glycolysis can be grouped into four stages:

Answer 5

• Activation (uses ATP hydrolysis)
• Splitting 6C to two 3C
• Oxidation (uses NAD+)
• Synthesis of ATP

Question 6

Activation Stages of Glycolysis:

Answer 6

1) Glucose + ATP —> Glucose-6-phosphate + ADP
HEXOKINASE (Muscle)/GLUCOKINASE (Liver)

2) Glucose-6-Phosphate –> Fructose 6-phosphate
PHOSPHOGLUCOSE ISOMERASE

3) Fructose-6-Phosphate + ATP —> Fructose 1,6-bisphosphate + ADP
PHOSPHOFRUCTOKINASE

--> = reversible reactions 
—> = irreversible reactions

Question 7

Splicing of 6C to 3C stages:

Answer 7

4 & 5) Fructose 1,6 Bisphosphate –> Dihydroxyacetone phosphate or glyceraldehyde 3-phosphate
(Interconvert between each other with TRIOSE PHOSPHATE ISOMERASE)
ALDOLASE

--> = reversible reactions 
—> = irreversible reactions

Question 8

Oxidation Step

Answer 8

6) Glyceraldehyde-3-Phosphate + Pi + NAD+ –> 1,3-Bisphosphoglycerate + NADH + H+
GLYCERALDEHYDE 3-PHOSPHATE DEHYDROGENASE

--> = reversible reactions 
—> = irreversible reactions

Question 9

ATP Synthesis Stages:

Answer 9

7) 1,3-BPG + ADP –> ATP + 3-phosphoglycerate
3-PHOSPHOGLYCERATE KINASE

8) 3-PG –> 2-phosphoglycerate
PHOSPHOGLYCERATE MUTASE

9) 2-PG –> phosphoenolpyruvate + H2O
ENOLASE

10) PEP + ADP –> Pyruvate + ATP
PYRUVATE KINASE

--> = reversible reactions 
—> = irreversible reactions

Question 10

Role of NAD as H acceptor:

Answer 10

NAD is a cofactor as it gets modified in the reaction. NADH stores energy and is needed for ATP synthesis in OXPHOS.

Question 11

Yield of ATP in Glycolysis:

Answer 11

• 2 ATP is used
• 4 ATP is made

Net yield: 2 ATP

Further ATP production is done by TCA and OXPHOS.

Question 12

Anaerobic Glycolysis (what happens to pyruvate):

Answer 12

• When oxygen supplies are low, pyruvate is not completely oxidised to CO2.
• Instead, pyruvate is converted to lactate - this recycles NAD+ as NADH is converted back to NAD+ in this process. This allows there to be enough NAD+ for glycolysis - allows glycolysis to be continuous.

Pyruvate + NADH + H+ —> Lactate + NAD+

This is catalysed by lactate dehydrogenase in both direction. Lactate can enter blood and is taken back into liver where it is converted back to pyruvate.

Question 13

Metabolic Fate of Pyruvate:

Answer 13

1) Pyruvate to lactate when no oxygen or mitochondria (in RBCs).
2) Pyruvate to ethanol in microorganisms only.

3) Pyruvate to Acetyl CoA.
Acetyl CoA to fatty acids if excess calories intake (done by liver when excess glucose exceeds glycogen stores).
Acetyl CoA to CO2 via TCA cycle when oxygen present.

Question 14

Summarise the role of glycolysis in different tissues e.g. skeletal muscle, red cells, brain.

Answer 14

Skeletal Muscle: In intense exercise, it hugely depends on glycolysis for energy due to lack of O2.

Red Blood cells: Don’t have mitochondria so can only use glucose as fuel to produce energy.

Brain: Glucose is a major source of ATP (fatty acids can’t pass through BBB).

Question 15

Regulation of glycolysis:

Answer 15

Increases rate of glycolysis:

• high carbohydrate diet
• intense exercise/muscle work

Decreases rate of glycolysis:
- fasting state have high glucagon - inhibits glucose metabolism

Question 16

Feedback inhibition of glycolysis:

Answer 16

This occurs at PFK-1 catalysed reaction where F6P is converted to F1,6BP.

High ATP and citrate will inhibit this enzyme and inhibit glycolysis.
High AMP will stimulate this enzyme and hence glycolysis.

Question 17

Summary of Glycolysis:

Answer 17

• Main catabolic pathway using glucose, present in all tissues.
• Only pathway which can produce energy in aerobic and anaerobic conditions (useful for RBCs and muscles)
• Energy yields are low (2 ATP) but pyruvate can enter mito to undergo further ATP production
• Pathway produces intermediates for fats and a.a, etc.