C430 Glycogen metabolism
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1. What are the major storage forms of glucose in animals & plants?
2. What is the purpose of storing glucose in the manner above?
1. Starch, cellular and GLYCOGEN
2. Serves as primary short-term energy reserve for organisms
1. Where is glycogen mostly stored?
2. Is it a short-term or long-term energy supply?
3. What is the basic structure of glycogen (3)
1. Muscle & liver
3. (1) Thousands of a-d-glucose monomers that are (2) highly branched (3) Linked with a 1,4 (amylose) and 1,6 glycosidic bonds (amylopectin)
- 1. B-d-glucose
- 2. A-d-glucose
- 3. Fructose
1. Draw a 1,4 and a 1,6 linkages
2. What type of reaction is required to create a 1,4 linkages? Draw it
3. Define gluconeogenesis
3. Gluconeogenesis - synthesis of glucose from pyruvate (end product of glycolysis) and other glycolytic intermediates.
What is the mechanism for gluconeogenesis (pyruvate --> glucose)? pay special attention to three steps that are different.
Where does it occur?
1. Pyruvate carboxylase: (pyruvate
+ATP+biotin --> OAA
2. PEPCK (OAA
+ GTP --> PEP
Fructose 1,6 bisphosphatase 1
Glucose 6 phosphatase
: (Glucose 6 P
+ H2O --> glucose
Mostly in liver, but also in muscle.
1. What allosterically inhibits (2) and activates (1) PFK-1?
2. What allosterically inhibits pyruvate kinase? (2)
3. What is RDS for gluconeogenesis? What activates that enzyme? (2) What inhibits its second activator (2)
4. How does system limit substrate cycling between glycolysis and GNG?
5. What does glucagon/epinephrine do? What does insulin do?
1. PFK-1 is inhibited by ATP and citrate, activated by 2,6bisphosphate.
2. Pyruvate kinase is inhibited by ATP & cAMP dep phosphorylation.
3. RDS is pyruvate carboxylase - activated by acetyl CoA and FBPase-1 (inhibited by F2,6BP and AMP)
4. Reciprocal allosteric control mainly achieved by opposing effects of fructose 2,6 bisphosphate on PFK-1 and FBPase 1.
5. Glucagon/epinephrine decrease F2,6BP --> increase cAMP --> phosphorylating PFK-1/FBPase-2) --> activates FBPase 2.
Insulin increases F2,6BP --> decrases cAMP and activates PFK-1
1. Where does pyruvate carboxylase take place? PEPCK?
2. What does pyruvate carboxylase need to work? (3)
3. What does PEPCK need to work? (2)
4. What reactions do the three exceptions to glycolysis for GNG correspond with?
5. What does pyruvate carboxylase need to begin working.
6. What would the result be if glycolysis and gluconeogenesis were allowed to run simultaneously?
- 1. Mitochondria, cytoplasm
- 2. HCO3-, biotin, and ATP
- 3. Mg2+ and GTP
- 4. Because those 3 rxns are highly exergonic and are essentially reversible, so we need roundabout method.
5. Acetyl CoA as effector
6. Consumption of ATP and production of heat.
1. In the exceptions of glycolysis for GNG, what is used? What is it based on?
2. Why does pyruvate first convert to OAA (4C) then to PEP (3C)?
3. Why is it important to limit the number of exceptions to parallel pathways?
4. How many ATPs does it take to go from pyruvate to 1,3 bisphoshpoglycerate? in Total rxn?
5. Does muscle have G6Ptase?
1. Reciprocal regulation. Based on ratio of reactants:products
2. To set up attack to form PEP --> because 4C OAA readily decarboxylate into PEP.
3. Keeping other rxns/enzymes identical conserve energy.
4. 2 ATP and 1 GTP; 4 ATP and 2 GTP
5. NO, cannot release glucose into blood
1. Define glycogenolysis.
2. What are the enzymes used? (3)
3. What vitamin is used? For what enzyme?
4. What does the first enzyme use to break bonds?
5. Why can't the first enzyme go past 4 glucoses?
- 1. Degradation of glycogen
- 2. Glycogen phosphorylase, glyc debranching enzyme, and phosphoglucomutase (phosphoglycerate mutase is in 2nd phase of glycolysis)
3. Vitamin B6 is essential for glycogen phosphorylase
4. Pi, not H2O!
Describe mechanism of glycogenolysis (5)
- 1. Glycogen phosphorylase uses Pi (NOT H2O) attacking nonreducing ends, removing terminal glucose as G1P.
- - some energy is preserved in high energy phosphate ester (G1P
2. Glycogen phosphorylase acts repetitively until it reaches a point four glucose residues away from branching point (a,1,6) b/c of sterics!
3. Glycogen debranching enzyme: transfers 3 glucose units from limit branch
to main chain (dG = 0 b/c energies cancel out)
4. Last glucose unit is hydrolyzed and released as free glucose.
5. Phosphoglucomutase (has Ser residue w/ P group) G1P <---> G1,6BP <--> G6P. G6P can enter glycolysis or be released by liver as free glucose.
1. What enzyme is shared between glycogen synthesis & breakdown? How is it regulated?
2. Describe mechanism of glycogenesis
1. Phosphoglucomutase - reciprocal regulation based on ratio of reactants:products
- 1. Phosphoglucomutase: G6P<--> G1P
- 2. UDP glucose pyrophosphorylase: G1P+UTP --> UDP glucose (activated) + PPi (PPi --> 2Pi quickly b/c of thermodynamics)
- dG = -35 kJ/mol; released energy is used to drive addition of glucose to glycogen chani
3. Glycogen synthase - adds UDP-glucose to growing polymer via a 1,4 linkage.
4. Branching enzyme - transfers 7-9 glucose units (breaking a1,4 bond) from main chain and tacks it on as a1,6 bond.
Branching is sterically spaced.
1. Why does glucose have to be transformed into UDP-glucose? (2)
2. What happens to the PPi that are also in the product?
3. What is dG for UDpglucose pyrophosphorylase? What is energy released used for?
4. What dictates how branches are added?
1. To keep G6P to converting to G1P based on LeChatlier's
2. Increase weak interactions between substrate and enzyme (decrease activation barrier)
- 2. They quickly convert to 2Pi because of negative charges that repel each other. 2 Pi is thermodynamically more favorable vs. inorganic pyrophosphate.
3. -35 kJ/mol. Used to add activated glucose to growing glycogen polymer.
4. Sterics - whenever chain gets too long, branching enzyme will cut it off and create a branch.
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