Glycolysis 2

Question 1

What are the key features of glycolysis?

Answer 1

• Oxidation of glucose to 2 molecules of Pyruvate
• Cytosolic pathway
• three irreversible reactions(regulatory steps)
  - Glucokinase/Hexokinase
  - Phosphofructokinase-1
  - Pyruvate kinase

Two substrate level phosphorylation reactions

   - Phosphoglycerate kinase
   - Pyruvate kinase

Question 2

What effect does insulin have in glycolysis?

Answer 2

Insulin up-regulates the 3 rate limiting enzymes in glycolysis

• Glucokinase/Hexokinase
• Phosphofructokinase
• Pyruvate kinase

Question 3

What is the effect of glucagon on glycolysis?

Answer 3

Down regulates the 3 rate limiting steps of glycolysis

• Glucokinase/Hexokinase
• Phosphofructokinase
• Pyruvate kinase

Question 4

pat what steps are glycolysis regulated ?

Answer 4

The 3 irreversible steps and by transport of glucose in the cell

• GLUT 4 regulated by insulin( muscle, adipocytes)
• Formation of glucose 6-phosphate (Glucokinase/Hexokinase)
• formation of fructose-1,6 bisP(PFK-1)
  - main regulatory point

-Regulation of Pyruvate kinase

Question 5

What is normal blood glucose level?

Answer 5

Normal blood glucose level around 4.5 Mm, but may increase after a meal

Question 6

Describe the chemical behavior of Glucokinase

Answer 6

• Km ~ 10Mm(HIGH)
• Liver, B-cells of pancreas
• NOT inhibited by glucose 6-P
• Liver uses blood glucose after a meal
• B-cells act as glucose sensor

Question 7

Describe the chemical behavior of Hexokinase

Answer 7

• Km~0.1 Mm (LOW)
• Insulin responsive peripheral tissues(muscle)
• Muscle and adipose only take glucose when blood glucose is high(GLUT-4).
• Inhibited by Glucose 6-P

Question 8

What is PFK-1 inhibited by?

Answer 8

• ATP (in muscle)

- Citrate

Question 9

What chemicals activates PFK-1?

Answer 9

Activated by:

AMP(in muscle )

Fructose-2,6-bid-P —> affects substrate a

Question 10

What hormones regulate PFK-1?

Answer 10

Insulin and glucagon

-Fructose-2,6-his-P levels regulate glycolysis

Question 11

Highlight hormonal regulation of glycolysis in a well fed state

Answer 11

Insulin dephosphorylates FBP-2/PFK-2(bifunctional enzyme ) thereby activating PFK-2 activity.

PFK-2 converts fructose 6-P to fructose 2,6-BP.

This leads to activation of PFK-1 and enhances glycolysis

Question 12

Highlight glycolysis hormonal regulation in a fasting state

Answer 12

Glucagon activates protein kinase A which phosphorylates FBP-2/PFK-2 activity. This results in the conversion of fructose 2,6-BP back to fructose 6-P. Glycolysis is inhibited

Question 13

What is the function of PFK-2?

Answer 13

PFK-2 forms 2,6 bisphosphate(not a substrate of glycolysis; but a regulator of glycolysis)

Fructose 2,6-bisphosphate is a potent regulator of PFK-1

Question 14

Explain the regulation of Pyruvate kinase

Answer 14

• Inhibited by ATP and Acetyl-CoA
  
  • High energy intermediate (ATP)
  • Abundent fuel from FA oxidation (Acetyl-CoA)
• Activated by AMP
  
  • Low energy signal
• Activated by fructose-1,6-bisphosphate
  - Feed forward mechanism from committed step
• Glucagon —> leads to covalent phosphorylation by a protein kinase A
  
  • Inhibition of Pyruvate kinase(liver only)

Question 15

Does Glucose 6-P inhibit Glucokinase?

Answer 15

No

Question 16

What are the major fates of Pyruvate?

Answer 16

4 major fates in humans

a. Acetyl CoA
b. Oxaloacetate
c. Ethanol(microorganism)
d. Lactate
e. Alanine

Question 17

The fate of Pyruvate depends on__________

Answer 17

Oxygen availability

Question 18

What happens to pyruvate in anaerobic conditions?

Answer 18

No mitochondria and less oxygen

Pyruvate —> lactate(lactate dehydrogenase)

Reoxidizes the NADH to allow glycolysis to proceed

Question 19

What happens to pyruvate in aerobic conditions?

Answer 19

Aerobic conditions (oxygen and mitochondria)

• a Pyruvate is transported into the mitochondria by Pyruvate dehydrogenase complex(PDH complex)
• NADH is also reoxidized in the mitochondria(shuttle pathway allows NADH equivalents to enter mitochondria)

Question 20

Describe the chemical reaction of lactate reduction

Answer 20

During anaerobic glycolysis, Pyruvate is converted to lactate to regenerate NAD+

Pyruvate converted to lactate
Enzyme: lactate dehydrogenase

This converts to NADH, and a proton to NAD+

Regenerated NAD+ is now free to participate in the Glyceraldehyde-3-P-dehydrogenase reaction

Question 21

The conversion of Pyruvate to lactate is reversible, what does the direction of the reaction depend on?

Answer 21

The direction of the reaction ( Pyruvate to lactate(and vice versa) ) depends on NADH/NAD+ ratio. Higher the NADH levels, the reaction is driven towards lactate formation

Question 22

What is hemolytic anemia?

Answer 22

• Erythrocyte Pyruvate kinase deficiency
• Mature RBCs lack mitochondria and are completely dependent on glycolysis for ATP formation
• defects in glycolysis would result in hemolysis and hemolytic anemia due to decreased ATP generation

Question 23

Describe the fate of lactate in anaerobic glycolysis

Answer 23

-Lactate formed in muscle and red blood cells, eventually diffuses out of the cells and is taken to the liver where it is used for gluconeogenesis

Question 24

What causes cramps during exercise?

Answer 24

Lactate accumulation in muscle causes a drop in pH and may manifest as cramps during intense exercise

Question 25

Why do alcoholics suffer lactic acidosis?

Answer 25

In alcoholics; increase in NADH due to ethanol metabolism leads to lactic acidosis

Question 26

Where does lactate often form?

Answer 26

In an actively contracting skeletal muscle, NADH formed as a result of glycolysis, exceeds to oxidative capacity of the ETC(Low oxygen tension), resulting in a high NADH/NAD+ ratio. This favors the conversion of Pyruvate to lactate via lactate dehydrogenase