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Glycogen Metabolism
Glycogenesis and Glycogenolysis
| Question | Answer |
|---|---|
| How is glycogen metabolism regulated? | Tightly via insulin, glucagon, and epinephrine or via allosteric regulations with intermediates |
| What form of glucose forms glycogen? | Alpha-D-Glucose |
| How is glycogen degradation regulated in muscle specifically? | Glycogen degradation is stimulated physically by muscle contraction and allosterically by Ca+ and AMP |
| At what rate does glycogen degradation/synthesis occur (in relation to G6P availability)? | Fast synthesis when G6P is abundant, Fast degradation when G6P is needed |
| What are glycogen stores used for in the muscle and the liver? | Glycogen degraded in the liver is used to increase blood glucose levels whereas glycogen degraded in the muscle is used for glycoloysis |
| Where is the largest *quantity/amount* of glycogen found in the body? | Skeletal muscle |
| Where is the highest *concentration* of glycogen in the body? | Liver |
| What parts of the body are effected by glycogen storage disorders? | Liver and/or muscle |
| Where in the cell are the enzymes for glycogen degradation located? | Cytosol |
| How and where in the cell is glycogen stored? | As granules in the cytosol |
| Why is glycogen degraded so quickly? | The branched structure of glycogen allows many enzymes to act simultaneously (This is tightly regulated however, and only synthases or phosphorylases are active at a given time) |
| Which 2 compounds are used in both glycogen synthesis and degradation? | G6P and G1P, they are reversibly formed from each other via the enzyme Phosphoglucomutase |
| Describe glycogen synthesis (in terms of compounds). | G -> G6P <-> G1P (+UTP) -> UDP-G (glycogen fragment OR glycogenin primer act to allow glycogen synthase to add length while branching enzyme form branches) -> Glycogen |
| Describe glycogen degradation (in terms of compounds). | G1P -> G6P |
| How is glycogen activated/inhibited in BOTH liver and muscle? | G6P allosterically activates glycogen synthase and allosterically inhibits glycogen phosphorylase |
| What type of bonds does glycogen synthase form? | A1->4glycosidic bonds |
| What type of bonds does the ‘branching enzyme’ form? | A1->6glycosidic bond |
| Explain why a glycogen fragment primer may be needed during glycogenesis. | When there is preexisting glycogen present in a cell undergoing glycogenesis, a fragment of glycogen will serve as a primer to elongate the preexisting chains of glucose |
| Explain why a glycogenin primer may be needed during glycogenesis. | When the glycogen stores of a cell have been completely depleted, there are no glycogen fragments to act as a primer for synthesis so glycogenin is needed to accept glucose residues from UDP-G to form glucosyl chains |
| Describe the action of glycogenin. | Glycogenin has a tyrosine residue (-OH) that reacts with UDP-G to link the first few Gs in a new glucosyl chain while releasing UDP. This reaction is catalyzed by the enzyme activity of glycogenin itself via autoglucosylation. |
| What happens to the newly formed glucosyl chain once Glycogenin adds the first few Gs? | The newly formed chain is very short but it can then act as a primer for glycogen synthase which will continue the elongation process. Glycogenin remains anchored to the beginning of the glucosyl chain and forms the core of the glycogen granule. |
| What is the name for the growing ends of the glucosyl chain of glycogen? | The nonreducing ends |
| Describe the covalent regulation of glycogenesis. | Glycogen synthase = active = dephosphorylated = done by protein phosphatase at high insulin levels, Glycogen synthase = inactive = phosphorylated = done by protein kinase A at high glucagon/epinephrine levels |
| Are the enzymatic reactions of covalent regulation of glycogen reversible? | No, the enzymes only work in one direction but the regulation itself is reversible via other enzymes. |
| Describe the degradation of glycogen via glycogen phosphorylase. | Glycogen phosphorylase performs phosphoryltic cleavage of A1->4 without using ATP, releasing G1P until there are 4 glucose molecules (the limit dextrin) |
| What is the coenzyme of glycogen phosphorylase? | pyroxidal phosphate which is formed from vitamin B6 |
| Describe the degradation of glycogen via bifunctional debranching enzyme. | Bifunctional debranching enzyme breaks A1->4 bonds of 3 of the 4 limit dextrin molecules, transferring them to a nonreducing end of another strand (elongating it), then breaks A1->6 bond releasing free glucose. |
| Describe what happens to G1P once released by glycogen phosporylase. | It is converted to G6P via phosphoglucomutase |
| What occurs to the G6P created by conversion through phosphoglucomutase? | It is used for glycolysis to produce 2 ATP |
| How much ATP does one molecule of G1P produce? | 3 ATP |
| Describe the how glycogen phosphorylase is covalently activated. | Glucagon and Epinephrine activate cAMP which activates Protein kinase A which activates glycogen phosphorylase kinase which activates glycogen phosphorylase from the b (inactive) to the a (active) form |
| Describe how glycogen phosphorylase is alllosterically activated. | glycogen phosphorylase kinase in activated by Ca+ ions and glycogen phosphorylase is activated by AMP (and Ca2+ indirectly) |
| What is the function of calmodulin? | Calmodulin is a protein that binds Ca2+ ions and modulated enzyme activation by Ca2+ ions. It can be a separate protein or a subunit of a protein, as is the case with glycogen phosphorylase kinase. |
| Aside from covalent and allosteric regulation of glycogen degradation, what other cellular structure is required for the breakdown of glycogen? | lysosomes, which use acid 1->4 glucosidase to degrade glycogen |
| Do glycogen synthesis and glycolysis occur at the same time? | Yes |
| Do gluconeogenesis and glycogen degradation occur at the same time? | Yes |
Created by:
sprater16