Regulation Of Glycolysis/Gluconeogenesis And Glycogen Metabolism

Question 1

Describe the concept of the reciprocal regulation of glycolysus and gluconeogenesis

Answer 1

Enzymes of the reversible steps are used in both pathways.
The enzymes catalyzing the irreversible steps and the bifunctional enzyme are regulated. Both pathways are not performed at the same time in the same hepatocyte.

Question 2

How is the hepatic bifunctional enzyme regulated?

Answer 2

The hepatic bifunctional enzyme has a phosphorylation site: Reciprocal regulation of glycolysis and gluconeogenesis

Question 3

What pathways are activated for storage and usage of glucose ?

Answer 3

Glycogen synthesis & glycolysis is activated

Glycigen degradation abd gluconeogenesis is inhibited

Bifunctional enzyme forms F2,6-bisP

Question 4

What pathways used when release of glucose is needed?

Answer 4

Glycogenolysis and Gluconeogenesis is activated

Glycogen synthesis and glycolysus is inhibited

Bifunctional enzyme degrades F2,6-bis-P

Question 5

Contrast the pathway combination in liver And muscle

Answer 5

Liver:
Glycogenolysis and gluconeogenesis take place
at the same time during fasting.

Purpose:
Release of glucose into blood

Muscle

Glycogenolysis and
anaerobic glycolysis take place at the same time during active muscle contraction.
Purpose: ATP formation followed by release of lactate into blood

Question 6

What is the biochemical impact of Ca2+ and AMP in muscle?

Answer 6

Muscle contraction leads to high calcium ions and AMP.

The released calcium ions and the generated AMP allosterically activate muscle glycogen degradation.

Epinephrine can optimize
muscle glycogen degradation by phosphorylation via PKA

Question 7

What happens to skeletal muscle during contraction?

Answer 7

Glycogen degradation is followed by glycolysis.
AMP activates both, muscle glycogen phosphorylase and PFK-1

PFK-1 of glycolysis is activated by low ATP and increased AMP. F2,6-bisP optimizes glycolysis.

ATP needs to be replenished by substrate level phosphorylation
in anaerobic glycolysis and lactate is released into the bloo

Question 8

How does regulation of glycolysis in skeletal muscle during exercise using glycogen degradation?

Answer 8

1. Glycogen phosphorylase provides glucose 1-P which is changed to glucose 6-P which is used for glycolysis. This saves ATP in the investment phase of glycolysis.
2. The committed step of glycolysis is catalyzed by PFK-1 which is active at low ATP during muscle contraction. PFK-1 is in addition activated by AMP formed during muscle contraction and by fructose 2,6-bisP formed by the bifunctional enzyme.
3. Pyruvate kinase isozyme in muscle is always highly active as glycolysis is needed for substrate level phosphorylation. No regulation by alanine or PKA.

Question 9

How is liver glycolysis regulaated?

Answer 9

regulation of glucokinase, PFK-1 and pyruvate kinase isozyme L

Question 10

What is the importance of glucokinase?

Answer 10

Glucokinase is the first enzyme of glycolysis in hepatocytes and b cells of pancreas.

Purpose in hepatocytes: Reduction of high blood glucose levels after a meal

Purpose in beta-cells: Recognition of high blood glucose levels and release of insulin

Question 11

How is hepatic glucokinase regulated?

Answer 11

Glucokinase is active in the cytosol. The enzyme is inactivated by translocation to the nucleus by the glucokinase regulatory protein (GKRP).

At high blood glucose, glucokinase is active in cytosol.

At low cytosolic glucose, glucokinase is inactive and bound in the nucleus

Question 12

How is hepatic PFK-1 regulated?

Answer 12

The healthy liver has always normal ATP levels which inhibit PFK-1.
This inhibition is overcome by fructose 2,6-bisP and AMP.

Hepatic glycolysis is favored after a meal by insulin which leads to formation of F2,6-bisP by the bifunctional enzyme.

AMP signals low ATP and in that case, the liver shall form ATP in glycolysis

Question 13

How is hepatic pyruvate kinase regulated?

Answer 13

The liver contains a specific pyruvate kinase isozyme L which allows optimal glycolysis at normal ATP levels.

At high blood glucose level, the hepatic pyruvate kinase isozyme is allosterically feed-forward activated by fructose 1,6- bisP which overcomes the inhibition by ATP.

Concept: Once the committed step of glycolysis is performed by PFK-1, hepatic pyruvate kinase is optimally active at normal ATP level and is able to finish glycolysis and reduce blood glucose levels. The formed pyruvate will continue with the PDH and TCA cycle

Question 14

How is the Pyruvate kinase L-isoform inhibited?

Answer 14

Pyruvate kinase L-isoform is inhibited by phosphorylation and by alanine.

Glucagon leads to phosphorylation and rapid inhibition which saves PEP for gluconeogenesis

This helps with the hepatic switch from glycolysis to gluconeogenesis.
In addition, hepatic alanine is increased during fasting and inhibits pyruvate kinase allosterically

Question 15

How does insulin and glucagon regulate glycolysis and gluconeogenesis?

Answer 15

Insulin and glucagon regulate glycolysis and gluconeogenesis by dephosphorylation/phosphorylation

• Insulin leads in the fed state to the dephosphorylated bifunctional enzyme which forms fructose 2,6-bisphosphate. This activates PFK-1 (glycolysis) and inhibits fructose 1,6-bisphosphatase (gluconeogenesis).
• Glucagon leads in the fasted state to the phosphorylated bifunctional enzyme which degrades fructose 2,6-bisphosphate. This activates fructose 1,6- bisphosphatase (gluconeogenesis) and inhibits PFK-1 (glycolysis).
• Glucagon leads in the fasted state to phosphorylation and inhibition of the hepatic pyruvate kinase L resulting in the salvage of PEP for gluconeogenesis

Question 16

What is the impact of hepatic glycolysis includes transcriptional control?

Answer 16

Regulation of hepatic glycolysis includes transcriptional control by insulin or glucagon

Insulin induces and glucagon represses
the synthesis of the glycolytic enzymes that catalyze the irreversible steps:

Glukokinase
PFK-1
Pyruvate kinase

Question 17

What is the impact of transcriptional control of gluconeogenesis?

Answer 17

Regulation of hepatic gluconeogenesis includes transcriptional control by glucagon and cortisol.

Glucagon induces specific enzymes of gluconeogenesis:

• PEP carboxykinase
• Fructose 1,6-bisphosphatase
• Glucose 6-phosphatas

Cortisol induces PEP carboxykinase and aminotransferases

Question 18

What is the purpose of glycogen synthesis?

Answer 18

Purpose is the same for liver and muscle

The purpose is the storage of glucose in form of glycogen when blood glucose is abundant

Question 19

What is the purpose of glycogen degradation ?

Answer 19

Purposes are different for liver and muscle

The purpose in liver is the release of glucose into the blood, in muscle it is to generate energy (lactate is released into blood).

Question 20

Describe the hormonal regulation of muscle and liver

Answer 20

using the cAMP-messenger system
Phosphorylation inhibits glycogen synthesis and activates glycogen degradation

Activated protein kinase A phosphorylates directly

• inhibits glycogen synthase
• Glycogen phosphorylase kinase which phosphorylates (regulation)—> glycogen phosphorylase (acts on glycogen) (active)

Question 21

How is glycogen metabolism regulated allosterically?

Answer 21

Regulation of glycogen metabolism by allosteric regulation in muscle and liver.

Glucose 6-P activates glycogen synthase and inhibits glycogen phosphorylase. ATP inhibits glycogen phosphorylase.

Liver only: Glucose inhibits glycogen phosphorylase

Muscle only: AMP activates glycogen phosphorylase.

Question 22

Summarize the glycogen metabolism and their regulation

Answer 22

Glycogen synthase:
1. Is activated by dephosphorylation (protein phosphatase-1) and is inactivated by
protein kinase A (cAMP messenger system).
2. Is allosterically activated by glucose 6-P.

Glycogen phosphorylase kinase (used for regulation of glycogen phosphorylase):

1. Is activated by phosphorylation by protein kinase A (cAMP).
2. Is allosterically activated by calcium.

Glycogen phosphorylase (acts on glycogen and forms glucose 1-P) :
1. Is activated by phosphorylation by glycogen phosphorylase kinase.
2. Is allosterically inactivated by glucose 6-P and ATP (both liver and muscle)
and by free glucose in the liver.
3. In skeletal muscle is allosterically activated by AMP during muscle contraction

Question 23

What happens to glycogen synthase and degradation in the liver after a meal?

Answer 23

Glycogen synthesis is active.
Glycogen synthase is dephosphorylated and active.

Glycogen degradation is inactive
Glycogen phosphorylase kinase is dephosphorylated and inactive. Glycogen phosphorylase is dephosphorylated and inactive

Question 24

How is glucokinase affected in the liver after a meal?

Answer 24

Glycolysis is active.

Glucokinase is in the cytosol (high cytosolic free glucose).
PFK-1 is active: ATP inhibition is overcome by F2,6-bisP.

Pyruvate kinase is dephosphorylated and active: ATP inhibition is overcome by F1,6-bisP (formed by PFK-1) feed-forward activation

Bifunctional enzyme formsxF2,6-P

Question 25

How is gluconeogenesis affected after a meal in the liver?

Answer 25

Gluconeogenesis is inactive.

Pyruvate carboxylase is not active as the TCA cycle is active. PEP carboxykinase lacks substrate in cytosol.

Fructose 1,6-bisphosphatase is inhibited by F2,6-bis P

Question 26

How is Glycogen synthesis and degradation affected in the liver during fasting?

Answer 26

Glycogen synthesis is inactive.
Glycogen synthase is phosphorylated and inactive

Glycogen degradation is active
Glycogen phosphorylase kinase is phosphorylated and active. Glycogen phosphorylase is phosphorylated and active

Question 27

How is glycolysis affected in the liver during fasting?

Answer 27

Glycolysis is inactive:

Glucokinase is in the nucleus.
Bifunctional enzyme is phosphorylated.

PFK-1 is inactive: ATP inhibition due to absence of F2,6bisP.
Pyruvate kinase is phosphorylated and inactive and also allosterically inhibited by ATP and alanine

Bifunctional enzymes degrades F2,6-bis-P

Question 28

How is gluconeogenesis affected in the liver during fasting?

Answer 28

Gluconeogenesis is active:

Pyruvate carboxylase is activated by acetyl CoA (from b-oxidation). PDH and TCA cycle are inhibited.

PEP carboxykinase has oxaloacetate as substrate in cytosol.
Fructose 1,6-bisphosphatase is not inhibited by F2,6bis-P

Question 29

What is the significance Glukokinase deficiency: MODY-2?

Answer 29

Glucokinase deficiency: MODY-2

Deficiency of glucokinase can lead to
Maturity Onset Diabetes of the Young Type 2 (MODY-2).

MODY-2 is characterized by impaired insulin secretion from b-cells of pancreas:
Due to deficiency of glucokinase only a higher blood glucose level (larger than 7- 8 mM) leads to the high ATP level in b-cells that is needed for closing of the K+-channel and insulin release.

Patients will show a chronic mild fasting hyperglycemia without being overweight or having metabolic syndrome.

MODY-2 is a monogenic form of diabetes inherited as an autosomal dominant trait