Glycolysis 2 Flashcards
What are the key features of glycolysis?
- 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
What effect does insulin have in glycolysis?
Insulin up-regulates the 3 rate limiting enzymes in glycolysis
- Glucokinase/Hexokinase
- Phosphofructokinase
- Pyruvate kinase
What is the effect of glucagon on glycolysis?
Down regulates the 3 rate limiting steps of glycolysis
- Glucokinase/Hexokinase
- Phosphofructokinase
- Pyruvate kinase
pat what steps are glycolysis regulated ?
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
What is normal blood glucose level?
Normal blood glucose level around 4.5 Mm, but may increase after a meal
Describe the chemical behavior of Glucokinase
- 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
Describe the chemical behavior of Hexokinase
- 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
What is PFK-1 inhibited by?
- ATP (in muscle)
- Citrate
What chemicals activates PFK-1?
Activated by:
AMP(in muscle )
Fructose-2,6-bid-P —> affects substrate a
What hormones regulate PFK-1?
Insulin and glucagon
-Fructose-2,6-his-P levels regulate glycolysis
Highlight hormonal regulation of glycolysis in a well fed state
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
Highlight glycolysis hormonal regulation in a fasting state
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
What is the function of PFK-2?
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
Explain the regulation of Pyruvate kinase
- 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)
Does Glucose 6-P inhibit Glucokinase?
No
What are the major fates of Pyruvate?
4 major fates in humans
a. Acetyl CoA
b. Oxaloacetate
c. Ethanol(microorganism)
d. Lactate
e. Alanine
The fate of Pyruvate depends on__________
Oxygen availability
What happens to pyruvate in anaerobic conditions?
No mitochondria and less oxygen
Pyruvate —> lactate(lactate dehydrogenase)
Reoxidizes the NADH to allow glycolysis to proceed
What happens to pyruvate in aerobic conditions?
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)
Describe the chemical reaction of lactate reduction
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
The conversion of Pyruvate to lactate is reversible, what does the direction of the reaction depend on?
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
What is hemolytic anemia?
- 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
Describe the fate of lactate in anaerobic glycolysis
-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
What causes cramps during exercise?
Lactate accumulation in muscle causes a drop in pH and may manifest as cramps during intense exercise
Why do alcoholics suffer lactic acidosis?
In alcoholics; increase in NADH due to ethanol metabolism leads to lactic acidosis
Where does lactate often form?
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
