Bioenergetics L10 Glycogen and gluconeogenesis

Question 1

What tissue is highly dependent on glucose?

Answer 1

The brain

Question 2

What is blood glucose held constant at?

Answer 2

80 mg/dl (~ 5 mM)

Question 3

What is excess glucose stored as ?

Answer 3

In the liver and muscle as glycogen

Question 4

What is sugar stored as in plants?

Answer 4

Starch

Question 5

What happens when blood glucose level drop?

Answer 5

Liver glycogen is the glucose source

Question 6

What happens during fasting?

Answer 6

Liver synthesises glucose to maintain blood glucocse levels

Question 7

Where is glycogen stored?

Answer 7

In cytosol, in most tissues (not just liver and muscles, but at lower conc)
Synthesis by insulin - driving the uptake of glucose in the storage and formation of glycogen
Breakdown driven by glucagon
Large glycogen polymers do not attract as much water as glucose

Question 8

How does glycogen protect osmotic pressure of the cell?

Answer 8

Only glycogen in liver (and a little bit in kidney) can release glucose to other tissues
Liver can store 8-10% of wet mass as glycogen
Muscles 1-2% (space limits in muscle)

Question 9

Glycogen structure

Answer 9

Also has reducing and non-reducing ions

Question 10

The first step for glycogen synthesis (glycogenesis)

Answer 10

Glycogenesis = glycogen-birth
Comes from the hepatic portal vein, into the hepatocyte, through two enzymes:
Hexokinease OR glucokinase (liver, kidney, islet B-cells)

Question 11

Kinetic properties of two enzymes, Hexokinase and Glucokinase

Answer 11

Hexokinase (0.1 mM Km):

Hexokinase, with a low Km value (0.1 mM), shows high affinity for glucose, meaning it reaches half of its maximum velocity (Vmax/2) at low glucose concentrations.
It is active even at lower glucose levels, suggesting it operates efficiently under normal glucose conditions.
The curve (a) rises sharply and plateaus quickly, indicating that Hexokinase is saturated at lower glucose levels.
Glucokinase (10 mM Km):

Glucokinase has a much higher Km (10 mM), which reflects a lower affinity for glucose compared to Hexokinase.
It responds primarily to higher glucose concentrations, as seen after meals, and does not saturate as quickly. The curve (b) shows a gradual rise, indicating that Glucokinase becomes more active when glucose levels are high.
This enzyme is significant in the liver for regulating glucose storage and metabolism after a meal.
Summary:
Hexokinase is active at lower glucose concentrations, ensuring glucose utilization in tissues like muscle and brain.
Glucokinase is more active in the liver when glucose is abundant, particularly after eating, to help in glucose storage (as glycogen).

Question 12

Second step for glycogen synthesis

Answer 12

Question 13

Where does the glycosyl group bind?

Answer 13

Glucose is added to non-reducing ends, a-1-4 glucosidic bonds first and then a-1-6 glucosidic bonds (branches)

Question 14

Glycogen synthesis

Answer 14

Glycogenesis: The process by which glucose molecules are added to chains of glycogen for storage in the liver and muscle cells.

UDP-glucose and glycogen synthase: UDP-glucose is a glucose donor in glycogenesis, and glycogen synthase is the key enzyme that helps in adding glucose residues (in the form of UDP-glucose) to the growing glycogen chain. The note on the image, “11 x UDP glucose & glycogen synthase,” likely refers to 11 glucose residues being added sequentially by the enzyme glycogen synthase.

Branching enzyme: This enzyme, referred to here as Amylo α(1,4) to α(1,6)-transglycosylase, creates branches in the glycogen molecule by converting some of the α(1,4) glycosidic bonds to α(1,6) bonds. Branching increases the number of terminal glucose units, which can be quickly mobilized during glycogen breakdown.

Core of glycogen: The arrow labeled “to core” likely points to the central structure of glycogen, which starts from a glycogenin protein and continues to grow outward.

Efficient storage: The branching structure of glycogen, which creates many terminal points, is an efficient way to store energy. The image notes that this structure allows for “97% efficient storage,” implying that glycogen’s highly branched nature enables compact and dense energy storage.

Question 15

Process of how branches in glycogen synthesis are formed

Answer 15

Branch creation: The branching enzyme creates new branches by transferring glycosyl residues from one part of the glycogen chain to another. These residues are added to form α(1,6)-glycosidic linkages, creating the branching structure.

Branch growth requirement: Each new branch must grow to about 11 glucose residues before it can be transferred to form a new branch. This ensures that each branch has a sufficient number of glucose units before forming further branches.

Branching distance: New branches are placed exactly 4 residues away from each other. Additionally, the new branches are oriented to move towards the glycogen core, which maintains the compact, dense structure of the glycogen granule. This is crucial for efficient energy storage, as more branches mean more terminal points for rapid glucose release when needed.

Question 16

Glycogen breakdown (glycogenolysis)

Answer 16

Glycogen cutting

Question 17

Enzymes used in glycogenolysis

Answer 17

1) Glycogen phosphorylase
2) Glycogen de-branching enzyme
3) Phosphoglucomutase

Question 18

Glycogenolysis - the role of glycogen phosphorylase

Answer 18

Glycogen phosphorylase: This enzyme breaks down glycogen by cleaving α(1,4)-glycosidic linkages through a process known as phosphorylysis, which does not require ATP. Instead, it uses inorganic phosphate (Pi) to release glucose-1-phosphate (G1P) from glycogen.

Glucose-1-phosphate (G1P): The primary product released by glycogen phosphorylase during glycogen breakdown. G1P can be converted to glucose-6-phosphate, which can enter glycolysis or be converted into free glucose in the liver.

Phosphorylase limitation: Glycogen phosphorylase can only act up until the 5th glycosyl residue from a branch point, leaving behind a “limit dextrin” structure where 4 residues remain before the branch. At this point, the enzyme cannot proceed further, and de-branching enzymes are needed to continue the breakdown process.

Question 19

Inhibition of glycogen phosphorylase

Answer 19

Inhibitors of glycogen phosphorylase:
ATP: As a product of cellular respiration, high levels of ATP signal that the cell has sufficient energy, thus inhibiting further breakdown of glycogen.
G6P (Glucose-6-phosphate): A key product of glycogen breakdown and a precursor for glycolysis and other metabolic pathways. High levels of G6P indicate that the cell does not need more glucose for energy, thus inhibiting glycogen breakdown.
Glucose: Directly inhibits glycogen phosphorylase in liver cells. When glucose is abundant, there is no need to mobilize more glucose from glycogen stores.

Question 20

Why phosphorylysis and not hydrolysis?

Answer 20

1) Hydrolysis will leave an un-phosphorylated glucose
2) Ensures released glucose is charged and trapped in cells (important in muscles)
3) Saves ATP each time - Pi is used directly

Question 21

How many glycosyl residues can glycogen phosphorylase work to?

Answer 21

5

Question 22

Debranching process during glycogenolysis and the role of the debranching enzyme during the final steps of glucose release

Answer 22

Debranching enzyme: The debranching enzyme has two distinct activities:

Transferase activity: It transfers three glycosyl units (glucose residues) from the branch to the main chain.
α-1,6-glucosidase activity: This hydrolyzes the remaining α(1,6)-glycosidic bond, releasing a free glucose molecule from the branch point.

Remaining glucose trapping: After the α(1,6)-glycosidic bond is cleaved and free glucose is released, this glucose is phosphorylated (or “trapped”) by hexokinase in a reaction that requires ATP. This phosphorylation turns glucose into glucose-6-phosphate (G6P), enabling it to enter glycolysis or other metabolic pathways.

Question 23

Key function of phosphoglucomutase to catalyze the reversible conversion between glucose-1-phosphate (G1P) and glucose-6-phosphate (G6P)

Answer 23

Glucose-1-phosphate (G1P): This molecule is produced by glycogen phosphorylase during glycogen breakdown (glycogenolysis). It is not directly usable in glycolysis, so it must be converted to G6P.

Phosphoglucomutase: This enzyme catalyzes the transfer of a phosphate group between the 1-position and the 6-position on the glucose molecule. It facilitates the shift from G1P to G6P, which can then enter glycolysis or be further processed, depending on the cell’s needs.

Glucose-6-phosphate (G6P): This is a key intermediate in carbohydrate metabolism, as it can enter glycolysis for energy production, be converted into free glucose in the liver, or be used in other biosynthetic pathways, such as the pentose phosphate pathway.

Question 24

What is an important metabolic efficiency related to glycogen breakdown and glycolysis?

Answer 24

Glycolysis Overview:
Normally, in glycolysis, glucose is converted to pyruvate in a series of steps that produce ATP.
The first step of glycolysis is the conversion of glucose to glucose-6-phosphate (G6P), catalyzed by the enzyme hexokinase. This step requires 1 ATP to attach a phosphate group to glucose.

Glycogen Breakdown (Glycogenolysis):
When glycogen is broken down, the first product is glucose-1-phosphate (G1P). This is converted to glucose-6-phosphate (G6P) by the enzyme phosphoglucomutase.
In this case, no ATP is required to make G6P because it is formed directly from glycogen breakdown, bypassing the step where ATP would normally be used.

ATP Yield Difference:
When glucose comes from free glucose (like from the blood), glycolysis requires 1 ATP in the first step to convert glucose into G6P.
However, when glucose comes from glycogen breakdown, the first step of glycolysis (which uses ATP) is bypassed because G6P is already formed from glycogen without needing ATP.

Energy Efficiency:
Since G6P enters glycolysis without using an ATP, the total ATP yield from glycolysis is higher. Normally, glycolysis produces 2 ATP per glucose molecule, but when starting with G6P, the net ATP yield increases to 3 ATP because no ATP was “spent” in the initial step.

Question 25

How is glucose 6-phosphatase produced?

Answer 25

Produced during the breakdown of glycogen (glycogenolysis) and from the phosphorylation of glucose during glycolysis. In most tissues, G6P is used directly in glycolysis for energy production.

Question 26

What is the role of Glucose-6-Phosphatase (G6Pase)?

Answer 26

G6Pase is an enzyme found only in the liver and kidneys, but not in muscle or other tissues.
The enzyme dephosphorylates G6P (removes the phosphate group), converting it into free glucose.
This free glucose can then be exported from liver cells into the bloodstream for distribution to other tissues that rely on glucose as an energy source, such as the brain and muscles.

Question 27

Why is G6Pase essential?

Answer 27

Liver function: The liver plays a key role in maintaining blood glucose levels, especially during fasting or between meals. By converting G6P into glucose, the liver can release glucose into the blood to prevent hypoglycemia.

Muscles lack G6Pase: In contrast, muscle cells do not have G6Pase, meaning they cannot release free glucose into the blood. Instead, muscle cells use G6P for internal energy production (glycolysis) to fuel muscle activity.

Question 28

Overview of glycogen metabolism

Answer 28

Glycogen degradation: Glycogen is broken down into glucose-1-phosphate (G1P) by the enzyme glycogen phosphorylase.
Phosphoglucomutase: G1P is converted to glucose-6-phosphate (G6P) by the enzyme phosphoglucomutase.
Glucose-6-phosphatase (liver only): In the liver, G6P can be converted to free glucose by glucose-6-phosphatase. This glucose is then released into the bloodstream to maintain blood glucose levels.
Glucose-6-phosphatase is only present in the liver (and kidneys), which allows the liver to supply glucose to other tissues when blood glucose levels are low.
In muscles, G6P is not converted into free glucose; instead, it is used directly in glycolysis to provide energy for muscle activity.

Glycogen Synthesis (Glycogenesis):
Glucose capture: Free glucose is first phosphorylated by hexokinase (in most tissues) or glucokinase (in the liver) to form glucose-6-phosphate (G6P).
Phosphoglucomutase: G6P is converted into glucose-1-phosphate (G1P) by the enzyme phosphoglucomutase.
UDP-glucose: G1P is activated to form UDP-glucose (UDPG) by the enzyme UDP-glucose pyrophosphorylase. This is a high-energy intermediate necessary for glycogen synthesis.
Glycogen synthase: UDP-glucose is then added to the growing glycogen chain by the enzyme glycogen synthase, resulting in the storage of glucose as glycogen.

Question 29

How are glycogen degradation (glycogenolysis) and glycogen synthesis (glycogenesis) tightly regulated and when do they occur?

Answer 29

If Blood Glucose is High (after a meal):
Glycogen Synthesis (Glycogenesis) occurs first.
After a meal, when blood glucose levels are high, insulin is released from the pancreas.
Insulin stimulates glycogen synthesis, causing excess glucose to be stored as glycogen in the liver and muscles.
Free glucose is taken up by cells, phosphorylated into glucose-6-phosphate (G6P), and converted into glucose-1-phosphate (G1P). G1P is then used to form UDP-glucose, which is added to the growing glycogen chain by glycogen synthase.

If Blood Glucose is Low (fasting or energy demand):
Glycogen Degradation (Glycogenolysis) occurs first.
When blood glucose levels are low, such as during fasting or between meals, the hormone glucagon (or adrenaline during stress/exercise) is released.
Glucagon triggers the breakdown of glycogen in the liver into glucose-1-phosphate (G1P).
G1P is converted into glucose-6-phosphate (G6P). In the liver, G6P is dephosphorylated by glucose-6-phosphatase, releasing free glucose into the bloodstream to maintain normal blood glucose levels.
In muscle cells, G6P enters glycolysis to provide energy locally, but it is not released into the bloodstream.

Question 30

How does the coordinated hormonal regulation of glycogen metabolism work?

Answer 30

Glycogen Breakdown (Left Pathway):
This is the pathway that activates glycogenolysis (glycogen breakdown).

Epinephrine or Glucagon: Hormones like epinephrine (released during stress or exercise) or glucagon (released when blood glucose is low) activate the pathway.

Adenylate Cyclase: The hormone binds to its receptor on the cell surface, activating adenylate cyclase, which converts ATP into cyclic AMP (cAMP).

Protein Kinase A (PKA): cAMP activates protein kinase A (PKA), a key enzyme that initiates a cascade of phosphorylation reactions.

Phosphorylase Kinase: PKA phosphorylates and activates phosphorylase kinase.

Phosphorylase b to Phosphorylase a: Phosphorylase kinase phosphorylates glycogen phosphorylase b, converting it into its active form, phosphorylase a.

Active Glycogen Phosphorylase: Once active, glycogen phosphorylase catalyzes the breakdown of glycogen into glucose-1-phosphate, which is then converted to glucose-6-phosphate for energy use or free glucose release (in the liver).

Glycogen Synthesis Inhibition (Right Pathway):
This pathway shows how glycogen synthesis (glycogenesis) is inhibited under the same conditions.

Epinephrine or Glucagon: When these hormones activate adenylate cyclase, the cAMP and PKA pathway is activated just like in glycogen breakdown.

Glycogen Synthase Inactivation: PKA phosphorylates glycogen synthase, converting the active form (glycogen synthase a) into the inactive form (glycogen synthase b).

Inactive Glycogen Synthase: This stops the enzyme from catalyzing the addition of glucose to the growing glycogen chain, effectively halting glycogen synthesis.

Coordinated Control:
These two pathways ensure that glycogen synthesis is inhibited while glycogen breakdown is activated. This prevents a futile cycle, where glycogen would be simultaneously synthesized and degraded, wasting energy.

Both processes are tightly regulated to respond to the body’s immediate needs:

When energy is needed (low glucose, stress, exercise), glycogen breakdown is favored.
When glucose is abundant (after a meal), glycogen synthesis is favored, and breakdown is inhibited.

Question 31

How do we find out how things worked?

Answer 31

Through disease

Question 32

What is the Cori’s disease?

Answer 32

Mutations in the debranching enzyme

Question 33

What is the von Gierke’s disease?

Answer 33

Glucose 6 - phosphatase mutation (this is the enzyme specific to the liver [kidney], can’t release glucose from liver into the blood)
Glucose6 phosphatase deficiency

Question 34

Symptoms of von gierke’s disease

Answer 34

• Enlarged liver and kidneys
• Treatment fructose and other carbohydrates
• Elevated lactate during fasting, acidosis
• Gout (hyperuricaemia)
• Hypoglycaemia

Question 35

What is Mc Ardle’s disease?

Answer 35

• Excess Glycogen in muscle, but severe muscle cramps
• Lack of glucose release, little glycogen phosphorylase activity in muscle

Question 36

What is glucose usage in an average resting male?

Answer 36

Question 37

What is the Cori cycle?

Answer 37

Cori cycle is a metabolic pathway that helps maintain energy supply during anaerobic conditions, such as intense exercise. It involves the liver and muscles working together to convert lactate (produced in muscles during anaerobic glycolysis) back into glucose, which can then be used again for energy.

Question 38

Steps of the Cori Cycle

Answer 38

Steps of the Cori Cycle:
Anaerobic Glycolysis in Muscle:

During intense exercise or anaerobic conditions, muscles rely on glycolysis for quick energy production.
Glucose is broken down into pyruvate, but without sufficient oxygen, pyruvate is converted into lactate by the enzyme lactate dehydrogenase.
Lactate builds up in the muscle and can cause muscle fatigue and soreness.
Transport of Lactate to the Liver:

Lactate produced in the muscle is transported via the bloodstream to the liver.
Lactate Conversion to Glucose in the Liver:

In the liver, lactate is converted back into pyruvate by lactate dehydrogenase.
The liver then uses gluconeogenesis to convert pyruvate back into glucose.
This glucose is released back into the bloodstream and can be used again by the muscles for energy.

Glucose Return to Muscle:
The glucose produced in the liver is transported back to the muscles, where it can be used for glycolysis, completing the cycle.

Answer 39

1. Lactate (Cori Cycle):
   Lactate is produced during anaerobic glycolysis (especially in muscle cells) and is transported to the liver via the Cori cycle.
   In the liver, lactate is converted back into pyruvate by the enzyme lactate dehydrogenase, and pyruvate is then used as a substrate for gluconeogenesis to produce glucose.
2. Amino Acids (except leucine and lysine):
   Most amino acids (except leucine and lysine, which are ketogenic) can be used for gluconeogenesis.
   These amino acids are broken down into intermediates that enter the TCA cycle or are directly converted to pyruvate or oxaloacetate, which can then be used to make glucose.
3. Glycerol:
   Glycerol is derived from the breakdown of triglycerides (fats). While fatty acids themselves cannot be converted into glucose, glycerol can.
   Glycerol is phosphorylated to glycerol-3-phosphate and then converted to dihydroxyacetone phosphate (DHAP), which is an intermediate in gluconeogenesis and glycolysis.
4. TCA (Citric Acid Cycle) Intermediates:
   Several intermediates from the TCA cycle (also known as the Krebs or citric acid cycle) can be used for gluconeogenesis.
   Oxaloacetate, malate, and citrate can be used as starting points for the production of glucose, either directly or by conversion into phosphoenolpyruvate (PEP) and entering the gluconeogenic pathway.
   Important Note on Fatty Acids:
   Fatty acids cannot be converted into glucose in humans because they are broken down into acetyl-CoA, which enters the TCA cycle but cannot be used to make glucose. The enzyme pyruvate dehydrogenase (PDH) irreversibly converts pyruvate into acetyl-CoA, meaning the reverse reaction (acetyl-CoA to pyruvate) is not possible.
   As a result, fatty acids are ketogenic rather than glucogenic, meaning they can be used to produce ketone bodies for energy but not glucose.