Chapter 21: Glycogen Flashcards
Know the structure of glycogen
Alpha 1,4 links
Alpha 1,6 branches
Know which tissues store glycogen and how each uses that glycogen
Muscle and Liver
Explain why phosphorolysis is more efficient than hydrolysis
If hydrolysis were to occur in the active site of glycogen phosphorylase it would create glucose but in order to use that glucose it would need a phosphate group to be kept in the membrane.
Phosphorolysis adds a phosphate group to a glucose molecule at carbon 1. Then a mutase can convert G-1-P to G-6-P.
Describe the mechanism of Glycogen Phosphorylase
Draw it out.
This enzyme active site prevents water from entering the active site and doing a hydrolysis reaction.
Describe why the cell needs transferase and debranching enzymes
The cell needs these enzymes because glycogen phosphorylase can only hydrolize alpha 1,4 bonds so the debranching and transferase will hydrolyze the alpha 1,6 bond, move the rest to the free end of another branch and produce a free glucose.
Explain how phosphoglucomutase interconverts G-1-P and G-6-P
It is like an isomerase reaction. It moves the phosphate from carbon 1 to carbon 6.
Explain why the liver needs glucose-6-phosphatease and the skeletal muscles do not
The muscles are trying to regulate their own energy levels. They run glycolysis to produce ATP.
The liver is responsible for regulating the glucose levels in the blood to supply all vital organs. The liver has to be able to produce glucose by removing the phosphate on glucose-6-phosphate.
Describe the regulation of Glycogen phosphorylase
Glucose can bind to glycogen phosphorylase and push it toward the T form or inactive. AMP has no effect on glycogen phosphorylase in the liver.
In the muscle, AMP signals low energy so the reaction will be pushed toward the R (active) form and breakdown glycogen. This will form G-1-P and then G-6-P and then the muscle cell will run glycolysis to make ATP.
-high levels of ATP will push the reaction toward T (inactive) and glycogen will not be broken down. This only is an effector when the muscle glycogen phosphorylase is already in the inactive state (b).
-Muscle cells do not have glucagon receptors because they do not have glucose-6-phosphatase.
Explain the reaction catalyzed by glycogen synthase (not curved arrows)
Glutamate attacks the 1-position of UDP-glucose, which causes UDP to leave. The 4 position oxygen of the growing glucose chain attacks the glutamate at the 1 position. The glutamate will leave and the glycogen chain will be increased by one glucose molecule.
Know what makes the glycogen synthase reaction favorable
high insulin levels. Also when the glycogen, Gm subunit and PP1c complex is phosphorylated at site 1 it becomes more active and turns glycogen synthase on.
Explain the importance of glycogen being so highly branched
There are multiple sites available for glycogen to be broken down and more glucose can be produced.
Describe the regulation of Glycogen synthase
Allosteric Control: High levels of glucose and G-6-P will cause PP1 to activate glycogen synthase to the a form.
Hormonal Control: Glucagon and epinephrine are inhibitors. Insulin will activate glycogen synthase.
Describe the reciprocal regulation of glycogen metabolism
The reciprocal regulation of glycogen metabolism starts when glucagon binds to a receptor on the liver. The G protein will exchange GDP for GTP and then cAMP levels will increase. Through this increase, Protein kinase A is activated. This will allow phosphorylation of glycogen phosphorylase to the a form. In doing this though the PKA will phosphorylate glycogen synthase and make it the b form. This prevents glycogen from being broken down and built at the same time.
Be able to describe Type I glycogen storage disease (von Gierke’s disease) and one other
disease related to glycogen metabolism.
von Gierke disease: (LIVER)
-you do not have the enzyme glucose-6-phosphatase
-free glucose cannot be made so there is an increase in the amount of glucose-6-phosphate in the cell.
-hypoglycemia, hyperlipidemia
von GEEK disease
-high fat
-low muscle
-fatty liver
McArdle’s and Her’s:
Lack of glycogen phosphorylase
McArdle’s=Muscle
Her’s=Hepatic
McArdle’s-
-hypoglycemia and muscle cramps on exertion
-myoglobin in the urine
Her’s-
-Fasting hypoglycemia
-Hepatomegaly (big liver)
