glycogen – Chemistry – Flashcards
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| what are the 2 ways glycogen synthesis can be initiated? |
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| glycogen synthesis can be initiated using a pre-existing glycogen molecule as a primer, or a protein called glycogenin can be used to create a glycosidic bond via a tyrosine hydroxyly group, (connects the reducing carbon to glycogenin) |
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| what is glucose's transporter in the liver and muscle respectively? |
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| liver: Glut2, muscle: Glut4 |
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| what is the benefit of branching in glycogen via alpha 1,6 bonds? |
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| this allows several glucose molecules to be added or removed simultaneously |
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| what happens when glucose is brought into into a liver or muscle cell? |
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| glucokinase and hexokinase facilitate phosphorylation of glucose to glucose-6-phosphate. |
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| what is the difference between glucokinase and hexokinase in terms of inhibition? why? |
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| hexokinase is inhibited by G6P, while glucokinase is inhibited by F6P. (next intermediate). glucokinase is found in the liver, where any extra glucose coming into the cell can simply be converted to glycogen, the synthesis of which also starts with G6P. glucokinase also has a lower affinity for glucose, which means it is only active at high levels of blood glucose. hexokinase however is in cells that do not create glycogen, (glycolysis is the only glucose pathway) and its inhibition by G6P ensures that cells are not overloaded with glucose |
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| what is the enzyme that converts G6P to the G1P, (next product in the formation of glycogen)? is this rxn reversible? |
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| phosphoglucomutase, yes it is reversible - based on substrate availability, (higher levels of G6P = more glycogen) |
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| what is the enzyme that catalyzes the conversion of G1P to UDP-glucose, what other molecule is needed for this rxn? is there anything given off? |
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| UDP-glucose pyrophosphorylase, and a UTP molecule is needed. 2 phosphates are given off. |
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| how does UDP-glucose become part of glycogen? what enzyme catalyzes this rxn? |
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| glycogen synthase transfers the glucose to the pre-existing glycogen molecule,(or tyrosine OH group off of glycogenin), releasing the UDP, (which can be reconverted to UTP with a P from ATP) |
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| what happens after about 10-11 alpha 1-4 bonds are created within glycogen? |
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| 4:6 transferase breaks an alpha 1-4 bond, removing about 6-8 glucose molecules from the glycogen string and creates a 1-6 alpha branching bond about 4 more glucoses down the remaining glycogen strand |
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| what are the 2 kinds of hormonal regulation for glycogen synthesis? |
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| covalent modification, (phosphorylation/dephosphorylation), of an enzyme or induction of enzymes that up-regulate mRNA, increasing a key enzyme |
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| how does insulin turn off glycogen breakdown? |
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| insulin activates phosphatases to remove phosphate groups from glycogen phosphorylase, in this case dephosphorylating the enzyme INACTIVATES IT. (Glycogen phosphorylase catalyzes the rate-limiting step in the degradation of glycogen in animals by releasing glucose-1-phosphate from the terminal alpha-1,4-glycosidic bond.) |
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| how does insulin activate glycogen synthesis? (2 ways) |
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| insulin activates phosphatases to remove phosphate groups from glycogen synthase, in this case removing phosphate groups ACTIVATES it. insulin also stimulates the uptake of glucose by muscle via Glut4 |
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| what is the insulin effect, what is the glucagon effect? what is the benefit of these effects? |
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| insulin effect: causes enzymes to lose their phosphate groups, glycogen synthesis occurs. glucagon effect: causes enzymes to be phosphorylated,so glycogen breakdown occurs. this is beneficial b/c you don't want glycogen synthesis and breakdown occurring simultaneously, (futile cycle). |
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| what is the action of Glut4 dependent on? where is it located? |
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| Glut4 action is dependent on insulin, it is located in the muscle |
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| before a meal, is glycogen synthase phosphorylated or dephosphorylated? how does G6P affect this state? |
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| it is phosphorylated, inactive. G6P allosterically activates the phosphorylated glycogen synthase. this allows PARTIAL but IMMEDIATE activation of the phosphorylated (~inactive) glycogen synthase |
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| where does allosteric activation of glycogen synthase take place? how long does covalent modification take? |
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| liver and muscle. covalent modification can take several minutes |
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| if glycolysis/TCA are going at "top speed" but not keeping up with levels of glucose, high G6P levels stimulate what? |
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| allosteric activation of glycogen synthase, which begins the process for storing glucose as glycogen |
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| of glycogen, fatty acids, and ATP what is the order of use in the body? |
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| ATP is used first, then glycogen, then fatty acids |
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| what enzyme breaks down glycogen? what does phosphorylation affect in terms of its activity? |
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| glycogen phosphorylase breaks the alpha 1,4 linkages, giving off a G1P |
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| how close to a branching point can glycogen phosphorylase remove glucose molecules? are there energy requirements? |
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| glycogen phosphorylase can remove G1P up to 4 molecule away from a branching point. there is no energy requirements, the product is phosphorylated, (causes it to be polar, unable to leave the cell - marked for intracellular catabolism) |
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| what is the benefit of glucose being released as G1P? |
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| G1P is more reactive and it doesn't take energy to break the the bond |
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| what is the difference between 4:6 transferase, 4:4 transferase, and alpha 1-6 glucosidase? |
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| 4:6 transferase breaks a 1-4 alpha bond and creates a 1-6 alpha (branched) bond. 4:4 transferase is a debranching enzyme that takes the 3 glucoses right after a 1-6 bond and attaches them to the end of the adjacent 1-4 bonds. alpha 1-6 glucosidase takes the remaining 1-6 bonded glucose and releases it, (NOT as G1P - does need some energy). |
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| what is the most common form of glycogen storage disease? |
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| type 5, McArdle's. |
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| what is McArdle's? |
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| it is a deficicency in myophosphorylase, the enzyme that removes G1P from glycogen |
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| what are symptoms of McArdle's? |
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| symptoms include myalgia, early fatigue, painful cramps, and myoglobinuria/rhabdomyolosis, (myoglobin/muscle cells in urine resulting from serious damage to muscle cells). pts are usually asymptomatic unless they have been through extreme physical extertion |
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| what happens to G1P after it is released from glycogen IN THE LIVER? |
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| phosphoglucomutase converts it to G6P, and glucose 6 phosphatase, (liver enzyme that is a corollary to glucokinase), converts this to glucose, which can then go into the bloodstream |
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| what enzyme converts G1P to G6P? is this reversible? |
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| phosphoglucomutase, yes it is reversible. (G1P has a higher likelihood of accumulating - though this is still dependent on substrate levels) |
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| what enzyme in the liver performs the opposite conversion, (G6P -> glucose) to glucokinase's, (glucose -> G6P)? |
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| G6P phosphatase |
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| where is G6 phosphatase located in the cell, how does this inform the location of glycogen and G6P? (in reference to the conversion of glucose to G6P) |
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| G6 phosphatase is located in the SER membrane, and therefore glycogen is located in the cytosol near the SER, and G6P is translocated to the SER |
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| what could potentially happen to G6P in the liver once converted back from G1P if insulin levels suddenly rose? why does this not happen? |
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| G6P could enter glycolysis. insulin and glucagon levels should not be high at the same time, due to a series of regulatory steps: 1)insulin/glucagon control of PFK2/F26bisPase 2)glucokinase is translocated to the nucleus if levels of glycogen/F6P are high 3)low insulin, causes pyruvate kinase to be phosphorylated, and thus less active |
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| what happens to G1P after it is released from glycogen IN THE MUSCLE? |
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| phosphoglucomutase converts it to G6P, but there is no G6 phosphatase to convert G6P back to glucose, so it has to enter glycolysis. muscle cannot contribute directly to blood glucose levels |
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| what hormones/neurotransmitters regulate glycogen levels in liver and muscle? |
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| insulin and epinephrine regulate glycogen levels in all tissues, while glycogen only acts in the liver. |
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| how does epinephrine signal cells as compared to glucagon? |
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| similarly, it binds to its receptor which activates a G protein (GDP->GTP), which and activates the adenylate cyclase, which converts ATP to cAMP. cAMP then binds to protein kinase A, (releasing the catalytic subunits). cAMP uses an ATP to phosphorylate glycogen synthase, INHIBITING IT. |
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| how does insulin counteract glucagons inhibitive effect on glycogen synthase? |
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| insulin activates phosphatases to remove phosphate groups from glycogen synthase, in this case removing phosphate groups ACTIVATES it. |
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| what is the glucagon effect? |
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| inactivation of glycogen synthase and activation of glycogen phosphorylase |
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| what is the net effect of PKA phosphorylation? |
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| the breakdown of glycogen |
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| what is the pathway for breakdown of glycogen in the liver? |
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| PKA is activated, (from cAMP...glucagon or epinephrine), which activates glycogen phosphorylase kinase, which activates glycogen phosphorylase, which breaks down glycogen |
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| what is the signaling molecule to initiate glycogen breakdown in the muscle? |
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| epinephrine, via cAMP |
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| what 2 allosteric activators function in the muscle to help jumpstart glycogen breakdown that are not used in the liver? why are these additional activators needed? |
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| Ca++ allosterically activates glycogen phosphorylase kinase, and AMP allosterically activates glycogen phosphorylase. this is to set in motion immediate response to epinephrine's signal, where covalent modification via phosphorylation can take up to several minutes. the allosteric activation occurs just until phosphorylation kicks in |
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| does hexokinase, (in muscle), always have some action, even when glucagon levels are high? |
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| yes, or we would die |
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| what glycolytic regulatory step related to PFK in the liver is not found in the muscle? |
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| PFK-2 cannot be switched off in the muscle, (therefore F2,6,bisP levels are always high) |
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| how do high epinephrine levels affect PFK in the liver? |
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| epinephrine may INACTIVATE, (via cAMP and PKA), PFK-2, inhibiting production of F2,6 bisP, slowing PFK 1 |
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| what is different about pyruvate kinase in the muscle? |
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| it cannot be phosphorylated, and thus inactivated |
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| define glycogenesis |
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| formation of glycogen |
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| define glycogenolysis |
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| breakdown of glycogen |
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| what is an allosteric activator for glycogen synthase in bother the liver and the muscle? |
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| G6P |
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| what are |
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| asdsa |
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