Glycogenolysis and glycogenesis Flashcards
Glycogenesis
Synthesis of glycogen from glucose
Glycogenolysis
Breakdown of glycogen
Where is glycogen stored
Liver : for maintenance of blood glucose levels
Muscle : for glycolysis during prolonged muslce contraction
What is glucose stored as in the body
Glucose is stored as glycogen
The 2 types of glycogen synthesized
- Linear chained glycogen
- Branched chain glycogen
Step 1a synthesis of linear chain glycogen
Activation
Glucose is converted into glucose 6 phosphate via hexokinase, while converting ATP to ADP.
Step 1b synthesis of linear chain glycogen
Activation, phospho-gluco-mutase
Glucose 6 phosphate is isomerized into glucose 1 phosphate via phospho-gluco-mutase.
Step 1c synthesis of linear chain glycogen
UDP glucose pyro-phosphorylase Activation
Glucose 1 phosphate is converted into UDP glucose via UDP glucose pyro-phosphorylase. UTP becomes PI in the process.
Step 2 synthesis of linear chain glycogen. Initiation step. Current and new glycogen chain.
Current chain - Activated glucose aka UDP glucose will add on to glycogen via glycogen synthase, producing glycogen and UDP. UDP glucose can add on to the glycogen chain one by one.
New chain - UDP glucose can add to glycogenin to make a new chain of glycogen, forming a new chain of glycogen. Glycogenin is the starting point of glycogenolysis where UDP glucose can anchor on to it.
How is glucose added to a new current chain?
Glucose (from UDP-glucose) is then transferred to the 4-OH at non-reducing end of glycogen core by glycogen synthase.
Alpha 1,4 glycosidic bond is formed, for linear chain
UDP is attached to carbon 1 of UDP glucose
Branching enzymes
alpha 1,6 branch linkage, alpha 1,6 glycosidic bond
Cleave the linear chain and form branches from there.
Glycogen synthase is still needed
Glycogenolysis
Breakdown of glycogen by phosphorolytic cleavage.. Sequential removal of glucose residues from glycogen.
Step 1 Glycogenolysis
Glycogen + inorganic phosphate via glycogen phosphorylase release glucose from glycogen chain.
Glycogen (n-1) + glucose-1-phosphate is formed.
alpha 1,4 Glycosidic bond is broken to release 1 glucose molecule.
Step 2 Glycogenolysis
Glucose-1-phosphate from step 1 becomes glucose 6 phosphate via phosphoglucomutase
Step 3 Glycogenolysis
Glucose 6 phosphate can enter liver to become glucose via gluconeogenesis to diffuse into the blood.
Glucose 6 phosphate can enter muscles to synthesize ATP via glycolysis.
Glycogen phosphorylase
- Uses inorganic phosphate to cleave a glucose residue from glycogen.
- Breaks alpha-1-4 glycosidic bond.
- Results in the release of glucose-1-phosphate.
Glycogen (n-1) + Glucose-1-phosphate is formed.
Debranching enzymes
to debranch the glycogen chain
Where do glycogenolysis occur
Liver and muscles
Liver - maintain blood glucose levels. In the liver, there is glucose 6 phosphatase as an extra step to form glucose.
Muscles - dont require glucose 6 phosphatase and enter glycolysis directly
Glycogenolysis in muscles
Glycogenolysis depends on availability or (demand for) ATP for muscle function.
Small amount of free glucose is produced, bulk is Glucose-1-phosphate.
G-1-P converted to G-6-P, able to undergo glycolysis to produce pyruvate, pyruvate can undergo lactic acid fermentation for energy.
Absence of glucose-6-phosphatase in muscle. Therefore, not converted to blood glucose.
Regulation of muscle glycogen storage during rest
Insulin decrease, glycogen synthesis decrease and glucose transport decrease
Regulation of muscle glycogen storage after meal and resting
Insulin increase, glycogen synthesis increase and glucose transport increase
Regulation of muscle glycogen storage during exercise
Epinephrine increase, glycogen synthesis decrease, glycogen degradation increase and glycolysis increase.
Regulation of liver glycogen storage during rest
Glucagon increase, insulin decrease. Glycogen degradation increase, glycogen synthesis decrease
Regulation of liver glycogen storage after carbohydrate meal
Glucagon decrease, insulin increase, glucose increase.
Glycogen degradation decrease, glycogen synthesis increase
Regulation of liver glycogen storage after exercise and stress
Epinephrine increase. Glycogen degradation increase, glycogen synthesis decrease
