Regulation of Glycolysis

Question 1

What does a low Km mean?

Answer 1

High affinity for glucose

Question 2

Which enzyme has a low Km?

Answer 2

HEXOKINASE has a low Km meaning a high affinity for glucose, so it ramps up activity quickly and plateaus, it is a very active enzyme

This is important because it means that even between meals, glucose is still circulating in our blood and hexokinase is going to be active.

Question 3

Which enzyme has a high Km?

Answer 3

GLUCOKINASE has a high Km meaning a LOW affinity for glucose.

This is a protective function, meaning that the liver only uses glucose at a significant rate when BG levels are VERY HIGH, allowing peripheral tissues first access to glucose when BG levels are LOW.

Question 4

How is GLUCOKINASE regulated?

Answer 4

In liver cells

Inhibited by FRUCTOSE-6-P

Activated by FRUCTOSE-1-P (comes from the metabolism of fructose)

Question 5

Define inducible enzyme

Answer 5

Glucokinase– meaning that the liver can up-regulate the amount of glucokinase that is made in liver cells.

This does not occur with hexokinase

Question 6

How is HEXOKINASE regulated?

Answer 6

All peripheral tissues, except the liver

Strongly inhibited by his own product - Glucose-6-P

Question 7

What regulates the inducible enzyme function of glucokinase?

Answer 7

It is controlled by HORMONES.

An increase in BG —> Increase in INSULIN —> Insulin stimulates the transcription of glucokinase gene and thus increases the amount of the enzyme

The absence of insulin, makes the diabetic patient deficient in glucokinase, even with higher BG levels

Question 8

What negatively inhibits the activity of PFK-1?

Answer 8

1. ATP
2. CITRATE (1st step in FA synthesis, this means that there is adequate energy in the cytoplasm).

Question 9

What positively activates PFK-1?

Answer 9

1. AMP
2. FRUCTOSE 2,6-BISPHOSPHATE

Question 10

What compound is a powerful regulator for PFK-1?

Answer 10

FRUCTOSE 2,6-BISPHOSPHATE will positively allerosterically promote PFK-1, which drives GLYCOLYSIS forward

Question 11

What is a bifunctional enzyme?

Answer 11

An enzyme that carries out both the synthesis and degradation of fructose 2,6-bisP

Example: Phosphofructokinase 2/Fructose 2,6-bisphosphatase (this is one GIANT example) with kinase and phosphatase activity

Question 12

How is PFK-2/F 2,6-biphosphatase regulated?

Answer 12

BY COVALENT MODULATION (which is the addition or removal of a phosphate group)

Question 13

What happens when PFK-2/F 2,6 bisphosphatase is PHOSPHORYLATED?

Answer 13

1. It will INACTIVATE the PFK-2 site (kinase portion), which will reduce the synthesis of fructose 2,6-bisP
2. It will ACTIVATE the F 2,6-bisP site (phosphatase portion), resulting in hydrolysis of F 2,6-bisP back to Fructose-6-P

Question 14

What happens when PFK-2/F 2,6-bisphosphatase is DEphosphorylated?

Answer 14

It will have the opposite effect.

1. The kinase portion will be ACTIVATED
2. The phosphatase portion will be INACTIVATED

Question 15

What is covalent modulation?

Answer 15

The addition or removal of a phosphate group

aka: phosphorylation or dephosphorylation

Question 16

What controls covalent modulation with PFK-2/F 2,6-bisphosphase enzyme?

Answer 16

It is controlled by GLUCAGON and EPINEPHRINE

Question 17

In the liver, when BG levels are LOW, what is produced? What does this result in?

Answer 17

GLUCAGON is produced which stimulates the cAMP production

ATP ———————-> cAMP + ppi
via ADENYLATE CYCLASE

Question 18

What is adenylate cyclase?

Question 19

cAMP stimulates what enzyme (in the liver)?

Answer 19

PROTEIN KINASE

Which phosphorylates the bifunctional enzyme (PFK-2/F 2,6-bisphosphatase)- this causes an inactivation of PFK-2 and an activation of F 2,6-bisphosphatase which DECREASES FRUCTOSE 2,6-BISP, thus REDUCING GLYCOLYSIS.

Question 20

How does a decrease in F 2,6-bisP effect PFK-1?

Answer 20

It makes PFK-1 LESS effective but makes F 1,6-biphosphatase MORE effective

This inhibits glycolysis, but STIMULATES gluconeogenesis, resulting in an INCREASE in BG levels.

Question 21

What causes BG levels to rise in the liver?

Answer 21

1. Glucagon stimulating cAMP
2. cAMP stimulating PROTEIN KINASE, causing phosphorylation of the bifunctional enzyme, as the PFK-2 portion is INACTIVATED and the F 2,6-bisphosphatase is ACTIVATED, causing F 2,6-biP levels to decrease, resulting in an inhibition of glycolysis, but stimulating F 1,6-bisphosphatase to drive gluconeogenesis

Question 22

(TRUE/FALSE)

Insulin stimulates glycolysis.

Answer 22

TRUE

Question 23

What type of ratio of insulin : glucagon causes decreased cAMP and reduced levels of active protein kinase A?

Answer 23

High insulin : Low glucagon

Question 24

When insulin levels are high, what does this lead to?

Answer 24

Decreased PROTEIN KINASE A activity

Question 25

What does decreased protein kinase A activity favor?

Answer 25

DEPHOSPHORYLATION of PFK-2/FBP-2 which causes PFK-2 portion to be ACTIVE.

This favors the formation of F 2,6-bisP, STIMULATES PFK-1, which DRIVES GLYCOLYSIS forward

Question 26

(TRUE/FALSE)

Glucagon and epinephrine are secreted when BG levels decrease.

Answer 26

TRUE

Question 27

Glycolysis is inhibited/stimulated by epinephrine in the liver?

Answer 27

Glycolysis is INHIBITED in the LIVER, to conserve glucose for use by other tissues

Question 28

Glycolysis is inhibited/stimulated by epinephrine in the HEART muscle?**

Answer 28

**Glycolysis is STIMULATED in the HEART, as part of a mechanism to meet the increased demand for ATP caused by epinephrine-signaled increase in work load.

Question 29

(TRUE/FALSE)**

In the heart muscle, phosphorylation by PROTEIN KINASE occurs at a different site that ACTIVATES rather than inhibits PFK-2.

Answer 29

TRUE**

This produces an increase rather than a decrease in FRUCTOSE 2,6-bisP levels (which results in an increase in PFK-1 and INCREASED GLYCOLYSIS).