Overview
This lecture covers glycogen's structure, its role as an energy storage molecule, and key pathways of glycogen synthesis (glycogenesis) and breakdown (glycogenolysis) in the body.
What is Glycogen?
- Glycogen is a large, branched polysaccharide made of glucose units linked by glycosidic bonds.
- It serves as the primary storage form of carbohydrate energy in animals.
- Glycogen is mainly stored in the liver and skeletal muscles.
- The brain does not store glycogen but utilizes glucose and ketone bodies for energy.
Glycogen Synthesis (Glycogenesis)
- Glycogenesis is the process of converting glucose to glycogen for storage.
- Key steps: glucose is phosphorylated to glucose-6-phosphate, then to glucose-1-phosphate, and then to UDP-glucose.
- Glycogen synthase adds glucose units from UDP-glucose to the growing glycogen chain.
- The branching enzyme creates Ξ±-1,6 linkages, increasing glycogenβs solubility and storage capacity.
Glycogen Breakdown (Glycogenolysis)
- Glycogenolysis is the process of breaking down glycogen to release glucose.
- Glycogen phosphorylase removes glucose units as glucose-1-phosphate from the non-reducing ends.
- The debranching enzyme resolves branch points by transferring and releasing extra glucose units.
- This process provides glucose during fasting, exercise, and between meals.
Regulation of Glycogen Metabolism
- Glycogen metabolism is regulated by hormones (insulin, glucagon, epinephrine).
- Insulin stimulates glycogenesis, promoting glycogen storage after eating.
- Glucagon and epinephrine stimulate glycogenolysis, releasing glucose when blood sugar drops.
- Allosteric regulation and covalent modification (phosphorylation) also control enzyme activity.
Glycogen Storage and Function
- Liver glycogen maintains blood glucose levels during fasting.
- Muscle glycogen provides rapid energy during physical activity.
- Glycogen cannot be stored in unlimited amounts and is used up in prolonged fasting or strenuous exercise.
Key Terms & Definitions
- Glycogen β Branched glucose polymer for carbohydrate energy storage in animals.
- Glycogenesis β Synthesis of glycogen from glucose.
- Glycogenolysis β Breakdown of glycogen to release glucose.
- Glycogen synthase β Main enzyme for adding glucose to glycogen.
- Branching enzyme β Creates branches in the glycogen molecule.
- Glycogen phosphorylase β Enzyme that removes glucose units from glycogen.
- UDP-glucose β Activated form of glucose used in glycogen synthesis.
- Glucagon/Insulin β Hormones regulating glucose and glycogen metabolism.
Action Items / Next Steps
- Review the pathways of glycogenesis and glycogenolysis in detail.
- Practice drawing and labeling the glycogen synthesis and breakdown processes.
- Study the hormonal regulation and related metabolic conditions (e.g., diabetes, glycogen storage diseases).