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Flashcards by sophiakostich, updated more than 1 year ago
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Created by sophiakostich
almost 11 years ago
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| Question | Answer |
| What are the four enzymes required for glycogen breakdown? | Glycogen phosphorylase (key regulatory step), transferase, a-1,6-glucosidase, phosphoglucomutase |
| What kind of reaction is glycogen phosphorylase? What happens in this reaction. | A phospholysis reaction. The glycogen chain is phosphorylated at a glycosidic bond to break glucose off result in glucose 1-phosphate. |
| True or false: glycogen phosphorylase breaks off glucose from glycogen at the reducing end of the chain. | FALSE. Glycogen phosphorylase breaks off glucose from the non-reducing end of the chain. |
| What is the function of the transferase enzyme in glycogen breakdown? | It works to move 3 of the remaining four residues near the end of branches to the end of the chain. This is because glycogen phosphorylase cannot work near a branch point (it stops 4 residues away from a branch point). |
| What is the function of the enzyme a-1,6-glucosidase? | It breaks the 1,6-glycosidic bond in the one residue that remains on the branch of glycogen and produces GLUCOSE (not glu-1,6 phosphate. |
| What is the function of the final enzyme in glycogen breakdown - phosphoglucomutase? | It converts glucose 1-phosphate produced to glucose 6-phosphate so it can be used by cells. |
| What is the key regulatory point in glycogen breakdown in muscle? | Glycogen phosphorylase. |
| Which form of glycogen phosphorylase is active - a or b? | a is the active form - it is the phosphorylated form. b is the inactive form - it is dephosphorylated. |
| What are the other forms of glycogen phosphorylase (other than a and b) and which one is active? | The other two forms are R and T state. R is active and T is inactive. |
| If glycogen phosphorylase is in its a form then will the equilibrium be pushed to R state or T state? | a form is active therefore it will be pushed towards R state as that is the active form. |
| What enzyme performs the phosphorylation/activation reaction of glycogen phosphorylase? | Phosphorylase kinase. |
| What enzyme performs the dephosphorylation reaction for glycogen phosphorylase and glycogen synthase? | Protein Phosphatase 1. |
| Does phosphorylation of glycogen phosphorylase use or make ATP? | It uses 2 ATP. Dephosphorylation (inactivation) uses 2ADP to make ATP. |
| How is the b form of glycogen phosphorylase allosterically regulated in muscle? | It is regulated by AMP and ATP. Increase AMP shifts phosphorylase b into the R state to force the enzyme into the active state (as high AMP means low ATP). High ATP shifts the b form of glycogen phosphorylase to T state. |
| What is the importance of the override mechanism where AMP activates glycogen phosphorylase b? | It forces the enzyme on which may be crucial in certain situations in muscles for example in the fight or flight response where breakdown of glycogen is needed very rapidly and suddenly. |
| Why is regulation of glycogen phosphorylase different in the liver? | It differs as its role is to maintain blood glucose levels and is not involved with ATP production and it has different isoform of the enzyme. |
| Which form of glycogen phosphorylase is allosterically regulated by glucose in the liver? | The a form. |
| True or false: the b form of glycogen phosphorylase still has adenylate control and override in the liver. | FALSE. there is no adenylate control and no override to switch on the enzyme/pathway as it is not needed in the liver. |
| What is the effect of increased glucose on the b form of glycogen phosphorylase in the liver? | Increase glucose allosterically binds and switches off the enzyme in the liver as no more is required in the blood so no glycogen breakdown is needed. |
| What is the enzyme called that activates glycogen phosphorylase? | Phosphorylase kinase - it activates glycogen phosphorylase by phosphorylating it |
| What are the two key methods used to regulate the phosphorylase kinase enzyme? | Hormones and Ca2+ levels. |
| What is the effect of either hormones or Ca2+ binding to phosphorylase kinase enzyme? | They both partially activate the enzyme and therefore there is partial activation of glycogen phosphorylase. |
| What is the significance of Ca2+ being used to regulate phosphorylase kinase enzyme and therefore breakdown of glycogen? | If Ca2+ levels are high then it will switch on the enzyme. Therefore this means that when muscles contract the enzyme can be switched on by increasing levels of Ca2+ to produce ATP for contraction from glucose (from glycogen breakdown) |
| What is the effect on phosphorylase kinase enzymes if both Ca2+ and hormones bind? | It results in a fully active enzyme which can very rapidly activate glycogen phosphorylase. |
| What are the three enzymes required for glycogen synthesis? | UDP - glucose pyrophosphorylase, Glycogen synthase/glycogenin, glycogen transferase |
| Which of the three enzymes involved in glycogen synthesis is a key regulatory step? | Glycogen synthase. |
| What is the purpose of the first step in glycogen synthesis? Where is energy released from to drive the process? | To join glucose to the carrier molecule UDP. It is driven by energy released by hydrolysis of the pyrophosphate bond. |
| What is the function of the second reaction in glycogen synthesis? | It adds UDP-glucose produced in the first step to glycogen chains. |
| True or false: glycogen synthase only adds UDP-glucose to chains that are over 6 units long. | FALSE. It only adds to chains over 4 units long. |
| What is the function of the third enzyme in glycogen synthesis? | It is the branching enzyme. It breaks 7 residues at a time and forms a branch out of them. |
| What are the three specific requirements of the enzyme glycogen transferase to allow it to branch? | 1. the part that it breaks off must be from the non-reducing end 2. it must come from a chain greater than 11 subunits 3. The branch must be at least 4 residues from another branch. |
| What is the purpose of branching in glycogen? | It increases the solubility of the molecule. It increases the number of ends and non-reducing termini that are available therefore glycogen can be broken down or synthesised more quickly. |
| What is the active form of glycogen synthase: a or b? | The active form of glycogen synthase is b. |
| What is the enzyme that phosphorylates glycogen synthase? Does phosphorylating it activate or inactivate it? | The enzyme is PKA. Phosphorylating glycogen synthase inactivates it. |
| What is the pathway that allosterically activates the b form of glycogen synthase? | If there are high levels of glucose 6-phosphate then the b form is activated |
| What inhibits and activates the enzyme protein phosphatase 1 which is responsible for dephosphorylates of glycogen phosphorylase and synthase? | Inhibited by phosphorylation by PKA. Activated by insulin (insulin sensitive protein kinase). |
| True or false: the enzyme protein phosphatase 1 switches on glycogen breakdown and switches off glycogen synthesis. | FALSE. Protein phosphatase 1 dephosphorylates the enzymes glycogen synthase and phosphorylase. Glycogen phosphorylase is inactive in its dephosphorylated form therefore the enzyme switches off glycogen breakdown. |
| What is the role of fructose 2,6-bisphosphate with regard to glycolysis and gluconeogenesis? | It upregulates glycolysis by stimulating PFK and downregulates gluconeogenesis by inhibiting Fructose 1,6-bisphosphate. |
| What are the two enzymes that regulate Fructose 2,6-bisphosphate? Why is it significant that it is these two enzymes? | Phosphofructokinase 2 (PFK2) and Fructobisphosphatase 2 (PBPase2). They are significant as they are on the same polypeptide chain. |
| True or false: PFK2 and PBPase2 can never be on at the same time. | TRUE. |
| When PFK2 and PBPase2 are dephosphorylated which one is on? Does this result in upregulation or downregulation of glycolysis? | PFK2 is on. This results in production of Fru 2,6 BP therefore upregulation of glycolysis (and downregulation of glyconeogensis) to decrease blood glucose. |
| Does glucagon stimulate phosphorylation or dephosphorylation of PFK2 and PBPase2? Explain your answer. | Glucagon stimulations phosphorylation and switches on PBPase2 (and PFK2 off). This results in less Fru 2,6 BP as it is converted to Fru 6P. Therefore glycolysis decreases and gluconeogenesis increases and blood glucose goes up. |
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