MMT: glycogen metabolism I and II Flashcards
Where is glycogen stored in a cell?
In glycogen granules in the cytoplasm.
Describe the basic structure of glycogen.
It is a branched molecule with a-1,4 bonds and branching a-1,6 attachments with one reducing end.
What is the significance of the reducing end in glycogen?
It is where we initiate synthesis.
Where do we build up or break down the glycogen molecule?
At the nonreducing ends.
What are the rate limiting enzymes for glycogen metabolism?
Glycogen phosphorylase and glycogen synthase.
What are major differences in the function of glycogen metabolism between the liver and muscle?
The liver breaks and builds glycogen to buffer our blood glucose, while muscle holds onto glucose as glycogen to be able to use it for energy when needed. As a result, muscle has glucose-6P but cannot convert it to free glucose.
Where do we find glucokinase?
In the liver; used for glycogenesis.
Describe glycogenesis in the liver.
- Glucose is trapped in the cell via glucokinase. 2. Phosphoglucomutase forms glucose 1-P from glucose 6-P. 3. UDP glucose is formed. 4. Glycogen synthase forms glycogen.
Describe glycogenolysis in the liver.
- Phosphorylase breaks glycogen into glucose 1-P. 2. Phosphoglucomutase turns glucose 1-P to glucose 6P. 3. Glucose-6-phosphatase converts glucose 6P back to glucose that can leave the liver.
What tissues express glucose-6-phosphatase?
The adrenal cortex and liver only.
What is the difference in glycogen synthesis in muscle cells?
They use hexokinase instead of glucokinase (aka hexokinase IV).
Does hexokinase or glucokinase have a higher affinity for glucose?
Hexokinase.
Describe MODY2 in terms of glycogen metabolism.
Mutations in glucokinase prevent proper export of insulin from beta cells by preventing glucose use in generation of ATP to fuel K+ channels that result in calcium influx and insulin secretion.
Does glycogen synthesis require energy?
Yes!
What is glycogenin?
An autocatalytic enzyme that synthesizes glycogen de novo; it adds glucose residues to itself using UDP glucose.
Glycogen synthase creates __ linkages.
a-1,4.
How does branching of glucose happen?
A branching enzyme moves a 7-glucose unit by breaking an a-1,4 bond and creating an a-1,6 bond.
Why is glycogen so branched?
To increase the solubility and create more terminal residues to enhance the rate of synthesis and degradation.
What powers glycogenolysis?
Inorganic phosphate.
Why does fasting only deplete liver glycogen?
Skeletal muscle does not express glucagon receptors and thus does not respond to glucagon.
Insulin ___ glycogenolysis.
Inhibits.
Glucocorticoids ___ glycogenolysis.
Inhibit.
In glycogenolysis, what is the fate of glucose 6P in the muscle and brain?
Formation of pyruvate that will lead to lactate and ATP.
In glycogenolysis, what is the fate of glucose 6P in the liver?
Formation of free glucose for use in other tissues or entering the pentose phosphate pathway.
