Glycogen Metabolism I Flashcards
What is the structure of glycogen? How are the chains linked together? ho do they branch? Which end serves as a primer for glycogen synthesis?
Long chain homopolymer of glucose with branches. It has Non-reducing ends that contain a terminal glucose with a free hydroxyl group at carbon four. It has a reducing end that consists of a glucose monomer connected to a protein called glycogenin. Glycogenin creates a short glycogen polymer on itself and serves as a primer for glycogen synthesis. Glycogen is degraded and extended from the non-reducing end.
Where is glycogen stored and how is it stored?
Glycogen is stored in liver and muscle. It is stored as granules that contain not only glycogen but also the enzymes needed for glycogen metabolism.
What is the function of liver glycogen?
regulates blood glucose levels
What is the function of Muscle glycogen?
Provides reservoir of fuel for physical activity.
What are three ways glycogen metabolism is regulated?
Allosteric control
Covalent modification through reversible phosphorylation of key enzymes.
Hormonal control
Walk through the first step of glycogenesis.
Glucose is phosphorylated to glucose-6-phosphate by glucokinase/hexokinase
Phosphoglucomutase then reversibly isomerizes glucose-6-phophate to glucose-1-phosphate.
Uridine-diphosphate (UDP) - glucose pyrophosphorylase then transfers the glucose-1-phosphate to UTP which generates UDP-glucose.
Walk through the second step of glycogenesis.
Preexisting glycogen polymer serves as primer to which glucose units are added.
Glycogen synthase catalyzes transfer of glucose from UDP-glucose to non-reducing end of glycogen chain. This forms alpha1-4 glycosidic bonds between glucose molecules.
Glycogen synthase is the rate limiting enzyme
Walk through the third step of glycogenesis.
When the glycogen chain reaches 11 residues long, a 7 residue long fragment is broken off at the alpha1,4 linke and reattached elsewhere through an alpha1,6 link. This is done by glucosyl(4:6) transferase.
The new branch point must be how at least ___ residues away from a preexisting branch.
four
Why does the glycogen want to branch?
Branching increases solubility of glycogen and increases number of terminal non-reducing end.
Increases rate at which glycogen can be synthesized and degraded.
What are the first three reactions of glycogenolysis?
Glycogen broken down to release glucose-1-phosphate.
Glycogen remnant remodeled to permit further degredation
Glucose-1-phosphate converted to glucose - 6 - phosphate.
What are the four key enzymes of glycogenolysis.
Glycogen phosphate
Phosphoglucomutase
Transferase
alpha-1,6-glucosidase.
Walk through the chain shortening step of glycogenolysis.
Glycogen phosphorylase catalyzes the clevage of glycogen.
The chain shortening occurs at the non-reducing end of the polymer.
GP adds an orthophosphate and releases a glucose residue as glucose-1-phosphate.
Uses pyridoxal phosphate (vitamin B6) as a cofactor.
Phosphorolysis of glucose resiudes continues till the GP gets within four residues of the alpha-1,6 linkage of a branch point.
Walk through the phosphoglucomutase step of glycogenolysis.
Phosphoglucomutase convers gluc-1-phosphate to gluc-6-phosphate.
A phosphoryl group is transferred from the enzyme to the substrate, and a different phosphoryl group is transferred back to restore the enzyme to its initial state.
Walk through the transferase step of glycogenolysis.
Transferase transfers a block of 3 of the 4 remaining glucose to the non-reducing end of the main chain forming an alpha-1,4 bond.
