Glycogen Metabolism in Muscle and Liver Flashcards
What is glycogen and where is it normally stored?
- storage form of polysaccharides
- liver and skeletal muscle
What is the main role of liver glycogen in the body?
- maintain plasma glucose levels between meals
- done through glycogenolysis
Is glycogenolysis catabolic (breaking down) or anabolic (building things up)?
- catabolic
- Exergonic = energy Exits the reaction
- provide energy for anabolic reactions (coupling)
What is the main role of skeletal muscle glycogen in the body?
- maintain muscle contraction
- glucose broken down in skeletal muscle stays in skeletal muscles
What is glycogenesis?
- glycogen is formed from dietary glucose
Glucose is a highly branched polysaccharide of glucose consisting of α1-4 linked glucose molecules with an α1-6 branch every 8-14 glucose residues. Why is this important?
- provides a large number of ends at which phosphorylase and glycogen synthase can act to ensure rapid breakdown and re-synthesis
In a highly branched polysaccharide of glucose consisting what does α1-4 and α1-6 linked glucose molecules mean?
α1-4 - glucose molecules are linked at 1 and 4 carbons
α1-6 - glucose molecules are linked at 1 and 4 carbons
What is the role of phosphorylase and glycogen synthase with glucose?
- phosphorylase = breaks down and removes glucose = Glycogenolysis
- glycogen synthase = adds glucose to glucose chains = Glycogenesis
What % of the liver and skeletal muscle weight is glycogen storage in the fed state?
- liver = 10%
- skeletal muscle = 2%
- BUT more glycogen is stored in the muscle as there is more total skeletal muscle mass in the body
In the image below what do the numbers 1 and 2 denote in terms of a1-4 and a1-6 glucose bonds?
- 1 = a1-4
- 2 = a1-6
In times of metabolic need glycogen is broken down, often called mobilisation. Glycogen is broken down into glucose 1 phosphate (phosphate group is attached to carbon 1) and then it can be reversibly broken down into glucose 6 phosphate (phosphate group is attached to carbon 6). What enzyme is responsible for this and what happens to the molecule when converting it from glucose 1 phosphate into glucose 6 phosphate?
- phosphoglucomutase
- moves the phosphate group from carbon 1 to carbon 6
In times of metabolic need glycogen is broken down, often called mobilisation. Glycogen is broken down into glucose 1 phosphate (phosphate group is attached to carbon 1) and then it can be reversibly broken down into glucose 6 phosphate (phosphate group is attached to carbon 6). Phosphoglucomutase is responsible for this and moves the phosphate group from carbon 1 to carbon 6. Why is this important in ATP generation?
- glucose 6 phosphate is the 1st stage of glycolysis
- so glycogen breakdown can initiate glycolysis
In times of metabolic need glycogen is broken down, often called mobilisation. Glycogen is broken down into glucose 1 phosphate (phosphate group is attached to carbon 1) and then it can be reversibly broken down into glucose 6 phosphate (phosphate group is attached to carbon 6). Phosphoglucomutase is responsible for this and moves the phosphate group from carbon 1 to carbon 6. Why is glucose 6 phosphate important in glycogen breakdown in the liver?
- glucose 6 phosphotase can remove phosphate and glucose can be pumped into the blood plasma
- important for maintaining blood glucose levels
In the liver glycogen can be broken down into glucose 1 phosphate (G-1-P) (phosphate group is attached to carbon 1) and then it can be reversibly broken down into glucose 6 phosphate (G-6-P) (phosphate group is attached to carbon 6). This means it can provide energy to glycolysis (G-6-P) and help maintain blood glucose levels (G-1-P). Does skeletal muscle produce G-6-P and G-1-P?
- no only G-6-P
- goes straight into energy metabolism
The liver is able to provide energy from glycogen that enters glycolysis, but importantly can also contribute to plasma glucose levels. To do this glucose-6-phosphate can be phosphorylised straight into glucose and enter the blood. What enzyme facilites this in the liver?
- glucose 6-phosphatase (removes phosphate)
- reverse of hexokinase (adds phosphate)
If fasting continues overnight, how long would our liver glycogen stores generally last?
- 12-24 hours
PHOSPHOROLYSIS is the process of breaking down α1-4 linkages in glycogen. What enzyme is responsible for this, and what is formed when a single unit of glycogen is broken off?
- glycogen phosphorylase breaks the bonds in glycogen by adding a phosphate to the glucose molecule
- glucose-1-phosphate is formed
PHOSPHOROLYSIS is the process of breaking down α1-4 linkages in glycogen. GLYCOGEN PHOSPHORYLASE is responsible for this. Does it break off big chunks of the glycogen or does it nibble off and take 1 glucose molecule at a time?
- 1 molecule at a time
- but due to structure of glycogen phosphorylase can act on multiple ends, releasing multiple glucose
Is glycogen phosphorylase able to break down both α1-4 and α1-6 bonds of glycogen?
- no
- only α1-4 bonds
Glycogen phosphorylase is only able to break down α1-4 bonds of glycogen. In the second phase of glycogenolysis (breaking down glycogen) another enzyme is required to break the branches of the α1-6 bonds of glycogen. What is this enzyme called?
- de-branching enzyme
Glycogen phosphorylase is only able to break down α1-4 bonds of glycogen. Glycogen phosphorylase is also unable to break down any α1-4 bonds when it is within how many bonds from a α1-6 bond?
- 4 bonds (inlcuding the α1-6 bond)
Glycogen phosphorylase is only able to break down α1-4 bonds of glycogen. Glycogen phosphorylase is also unable to break down any α1-4 bonds which are within 4 bonds from a α1-6 bond. The de-branching enzyne is able to do what with 3 of the α1-4 bonds before the α1-6 bind?
- the 3 of the α1-4 bonds are moved to the non-reducing end of the chain