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