Metabolism of Glycogen
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What is glycogenesis?
Formation of glycogen from glucose
What is glycogenolysis?
Breakdown of glycogen to glucose
What becomes of fructose in the muscle and adipose tissue?
Produces fructose-6-phosphate – which is a glycolytic intermediate using hexokinase
What becomes of fructose in the liver?
Fructose-1-phosphate is produced from fructokinase which eventually forms GAP (glyceraldehyde-3-phosphate)
What is glycogen?
storage form of glucose
Where is glycogen made?
Made primarily in liver and muscle cells; can also occur in the brain and stomach
What regulates glycogen metabolism?
Insulin, glucagon, and epinephrine
What activates and inhibits glycogenolysis?
- Activates: glucagon, epinephrine (need energy quick)
- Inhibits: Insulin
What activates and inhibits glycogenesis?
- Activates: insulin (too much glucose)
- Inhibits: glucagon and epinephrine
How many steps in glycogenesis?
What are the steps of glycogenesis?
- Synthesis of glucose-1-phosphate from glucose-6-phosphate
- Synthesis of UDP-glucose from glucose-1-phosphate
- Synthesis of glycogen from UDP-glucose
What converts glucose-6-phosphate to glucose-1-phosphate?
What is the active site of Phosphoglucomutase made of?
Serine residue with a phosphoryl group that is transferred to the C-1 position and then regenerated by removing the phosphoryl group at the C-6 position
How is UDP-Glucose made?
UDP-glucose phosphorylase catalyzes the transfer of uridine and one phosphate onto glucose-1-phosphate forming UDP-glucose
What drives step 2 of glycogenesis ?
Rxn is driven to completion b/c of the irreversible hydrolysis of pyrophosphate (PPi) to 2 Pi
What is a glycogenin?
Homodimer protein bound to glycogen
What is a glycogen primer?
4 or more residues formed by glycogenin and remains covalently bounded to this enzyme
How is the glycogen primer (glycogenin) extended?
The primer is extended using glycogen synthase by hydrolysis of the UDP-glucose bond
What type of bonds are made with glycogen synthase?
- Glucose polymerization
How are branches created in glycogen?
- With a branching enzyme that creates alpha-1,6) linkages
- between one chain and a “branch” chain of 6-7 residues taken from the end
Where can the highly branched glycogen be found?
As large granules in the cytoplasm of liver and muscle cells
Where are the nonreducing ends located on glycogen?
The edges of the glycogen
Where are the reducing ends of the glycogen found?
Attached to the glycogenin
What removes residues from the nonreducing ends of glycogen and where does it cleave?
- Glycogen phosphorylase
- only cleaves the alpha(1,4) linkages
Where does the glycogen phosphorylase stop?
On branches this enzyme stops 4 residues from a branch point
How are branches broken down?
- The debranching enzyme removes 3 of the 4 residues and places them on the nearest nonreducing end of the glycogen chain
- The enzyme removes the remaining a(1,6) linkage and releases the free glucose molecule and an unbranched glycogen chain
What is the debranching enzyme called?
What is limit dextrin?
Glycogen that has been degraded to its branch points
Glycogen and glucagon?
Glucagon is released by the pancreas when blood sugar levels are low to signal the breakdown of liver glycogen and release of glucose into bloodstream
Insulin and glycogen?
Insulin is released by pancreas when blood sugar levels are high to signal uptake of glucose by several types of cells
Epinephrine and glycogen?
Epinephrine is used during stress to free up muscle glucose for use in energy production
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