WEEK 3 (Glycogen Metabolism) Flashcards
What is the biological importance of Glycogen?
Used as a CARBOHYDRATE RESERVE where glucose phosphates and glucose can be released when needed
Why is Glycogen more useful than Glucose storage?
High concentrations of Glucose within cells would make them STRONGLY HYPOTONIC and would cause an INFLUX OF WATER but insoluble glycogen has a LOW OSMOTIC ACTIVITY
Describe muscle and liver glycogen
- Can store up to 450g of glycogen (1/3 in Liver & 2/3 in Muscle)
- MUSCLE GLYCOGEN provides a readily available source of GLUCOSE-1-PHOSPHATE for GLYCOLYSIS
- LIVER GLYCOGEN functions as a reserve to maintain the blood glucose concentration in the FASTING STATE
Describe the structure of Glycogen
- Branched-chain polysaccharide made from D-GLUCOSE
- Primary glycosidic bond is 1,4 LINKAGE
- After an average of 8-10 GLUCOSYL RESIDUES, there is a branch containing 1,6 LINKAGE
Describe the Glycogen store fluctuation
- Liver glycogen stores increase during the WELL-FED STATE and are DEPLETED during a FAST
- Muscle glycogen is not affected by short periods of fasting (a few days) and is only moderately decreased in prolonged fasting (weeks)
Where does Glycogen synthesis take place?
In the cytosol
What are the properties of Glycogen Synthesis?
- Process occurs in Cytosol
- Requires energy supplied by ATP & UTP
What does Glycogen synthase make?
Alpha (1,4) linkages
What does the Branching enzyme do?
Removes a chain of 6 to 8 GLUCOSYL RESIDUES from glycogen chain & attaches it to a NON-TERMINAL GLUCOSYL RESIDUE by an alpha (1,6) linkage
What are the 4 main steps in Glycogen synthesis?
1) Attaching URIDINE DIPHOSPHATE (UDP) to GLUCOSE
2) Attaching GLUCOSE to GLYCOGENIN which serves as a PRIMER
3) Adding more glucose to the primer
4) Adding branches to the glycogen molecule
What are the enzymes that are used in the conversion of Glucose to Branched Glycogen?
- Hexokinase/Glucokinase
- Phosphoglucomutase
- UDP-Glucose Pyrophosphorylase
- Glycogen Synthase
- Branching Enzyme
Describe the steps of Glycogenesis
1) PHOSPHOGLUCOMUTASE moves the phosphate from the 6th carbon of Glucose to the 1st making GLUCOSE-1-PHOSPHATE. UDP-GLUCOSE PYROPHOSPHORYLASE cuts 2 phosphates off UTP which provides ENERGY for this reaction. UDP-Glucose is formed.
2) GLYCOGEN SYNTHASE catalyses the attachment of the Glucose part of the UDP-Glucose molecule to another glucose residue at the END of the GLYCOGEN BRANCH forming an ALPHA 1,4 GLYCOSIDIC BOND
3) BRANCHING ENZYME shortens the chain and attaches the cut chain to the side of another LINEAR GLYCOGEN CHAIN by creating an ALPHA 1,6 GLYCOSIDIC BOND
4) GLYCOGEN SYNTHASE elongates the chain again creating a BRANCHED GLYCOGEN TREE storing energy
Why is Glycogenin necessary in Glycogen synthesis?
Glycogen synthesis can only occur in an elongating glycogen chain that is already 4 glucose molecules long. Glycogenin attaches 4 glucose molecules to itself mimicking the first step of Glycogen synthesis.
Which part of the Glycogen molecule does Glycogenolysis begin?
The branches
Describe the steps of Glycogenolysis
1) GLYCOGEN PHOSPHORYLASE cleaves ALPHA 1,4 BONDS between individual glucose residues & catalyses the transfer of PHOSPHATE GROUP to the FREE GLUCOSE forming GLUCOSE-1-PHOSPHATE molecules which are released ONE AT A TIME (continues until 4 Glucose are left)
2) DEBRANCHING ENZYME cuts off branches. 4-ALPHA-GLUCANOTRANSFERASE transfers 3/4 glucose molecules off the branch and reattaches to the LINEAR GLYCOGEN CHAIN, extending it. ALPHA 1,6 GLYCOSIDASE cleaves the ALPHA 1,6 BOND which releases a FREE GLUCOSE
3) PHOSPHOGLUCOSEMUTASE converts GLUCOSE-1-PHOSPHATE to GLUCOSE-6-PHOSPHATE
5) IN LIVER, GLUCOSE-6-PHOSPHATASE removes phosphate and releases free glucose into the BLOODSTREAM (increasing blood glucose concentration). IN MUSCLES, NO GLUCOSE-6-PHOSPHATASE so sends Glucose-6-Phosphate into GLYCOLYSIS PATHWAYS to be used for ENERGY