Regulation of Glycolysis

Question 1

Describe glycolysis in muscle

How is the ATP/AMP ratio used to signal the energy state of a cell?

Answer 1

• Muscle contraction converts ATP to ADP.
• ADP can be converted to AMP.

ADP + ADP ⇌ ATP + AMP (y adenylate kinase)

• A drop in ATP and increase in AMP signals “energy demand”.
• AMP increases more than ADP (becuase ADPs recombine as shown above) so AMP is a better signal for energy usage.

Question 2

Describe glycolysis regulation in muscle

Answer 2

•Occurs at the level of phosphofructokinase.

• Inhibited by ATP
• Activated by AMP

From the graph

• At low [ATP] phosphofructokinase acts at a low [fructose-6-phosphate] to catalyse the formation of fructose-1,6,-bisphosphate
• Presence of ATP inhibits the enzyme because you need more substrate to give the same level of enzyme activity
• Even a small amount of AMP alongside the ATP recovers the enzyme activity (shifts graph to the left)

Question 3

Explain how the regulation of glucolysis in muscle occurs at the level of pyruvate kinase

What prevents the accumulation of glucose 6-phosphate at rest?

Answer 3

–Activated by exercise and ATP is an inhibitor

–Fructose 1,6-bisphosphate activates pyruvate kinase in feedforward mechanism

– This ensures different parts of the pathway work at the same rate

•At rest fructose 6-phosphate is not used which “backs up” glucose 6-phosphate and inhibits hexokinase via negative feedback.

–Prevents accumulation of glucose 6-phosphate at rest.

Question 4

Describe how the liver is involved in regulation of blood glucose

What is high/low blood glucose called?

Answer 4

•Blood glucose:

–Normal fasting levels ~4.5 mM.

–Low blood glucose – hypoglycaemia.

–High blood glucose – hyperglycaemia.

–Brain requires > 2 mM blood glucose (or you faint!).

–If glucose is > ~7.5 mM it escapes into the urine.

Question 5

Explain glycolysis regulation in the liver

Answer 5

•Elevation of blood glucose increases glucose phosphorylation in the liver.

Hexokinase: Km < 0.1 mM

• Found in most tissue.
• Strongly inhibited by glucose 6-phosphate (no need to break down more glucose to produce glucose 6-phosphate if its concerntration is already high).

Glucokinase: Km 10 mM

• Found in liver.
• Not inhibited by glucose 6-phosphate (allows continuos uptake of glucose).
• Phosphorylates glucose to glucose-6-phosphate after a meal to prevent blood glucose overshooting.

Question 6

Describe this graph

Answer 6

V_max= The maximum rate of activity of an enzyme

K_m= The concentration of a substrate at which you get half the maximum rate

From the graph:

• Hexokinase will operate even at low concentrations of glucose (to convert it to glucose-6-phosphate)
• Glucokinase has high enzymatic activity at higher glucose concentrations

Question 7

Describe the Regulation of glucose by Fructose 2,6-bisphosphate in the liver

Answer 7

• Increased glucose results in increased F 2,6-BP.
• F 2,6-BP stimulates liver phosphofructokinase.
• This decreases concentration of fructose 6-phosphate required for the pathway.
• This reduces inhibition by ATP.

From the graph:

Addition of higher concentrations of F 2,6-BP reduces the concentration of fructose-6-phosphate for a high rate of enzymatic activity (of phosphofructokinase)

Question 8

Describe the regulation of Liver Pyruvate Kinase

Answer 8

• F 1,6-BP increases with increased glucose.
• Glucagon promotes phosphorylation of pyruvate kinase by cyclic AMP-dependent protein kinase A and inhibition of activity when blood glucose is low.

The phsophorylated form of pyruvate kinase is less active

The dephosphorylated form of pyruvate kinase is more active

Question 9

Draw a diagram to summarise glycolysis regulation in musle

Answer 9

Question 10

Draw a diagram to summarise glycolysis regulation in the liver

Answer 10