Bioenergetics L10 Glycogen and gluconeogenesis Flashcards
What tissue is highly dependent on glucose?
The brain
What is blood glucose held constant at?
80 mg/dl (~ 5 mM)
What is excess glucose stored as ?
In the liver and muscle as glycogen
What is sugar stored as in plants?
Starch
What happens when blood glucose level drop?
Liver glycogen is the glucose source
What happens during fasting?
Liver synthesises glucose to maintain blood glucocse levels
Where is glycogen stored?
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
How does glycogen protect osmotic pressure of the cell?
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)
Glycogen structure
Also has reducing and non-reducing ions
The first step for glycogen synthesis (glycogenesis)
Glycogenesis = glycogen-birth
Comes from the hepatic portal vein, into the hepatocyte, through two enzymes:
Hexokinease OR glucokinase (liver, kidney, islet B-cells)
Kinetic properties of two enzymes, Hexokinase and Glucokinase
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).
Second step for glycogen synthesis
Where does the glycosyl group bind?
Glucose is added to non-reducing ends, a-1-4 glucosidic bonds first and then a-1-6 glucosidic bonds (branches)
Glycogen synthesis
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.
Process of how branches in glycogen synthesis are formed
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.
Glycogen breakdown (glycogenolysis)
Glycogen cutting
Enzymes used in glycogenolysis
1) Glycogen phosphorylase
2) Glycogen de-branching enzyme
3) Phosphoglucomutase
Glycogenolysis - the role of glycogen phosphorylase
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.
Inhibition of glycogen phosphorylase
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.
Why phosphorylysis and not hydrolysis?
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
How many glycosyl residues can glycogen phosphorylase work to?
5
Debranching process during glycogenolysis and the role of the debranching enzyme during the final steps of glucose release
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.
Key function of phosphoglucomutase to catalyze the reversible conversion between glucose-1-phosphate (G1P) and glucose-6-phosphate (G6P)
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.
What is an important metabolic efficiency related to glycogen breakdown and glycolysis?
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.
