Term
| Why is glycogen branched? |
|
Definition
| To allow for more rapid synthesis and degradation, and to increase glycogen solubility |
|
|
Term
| Which linkages are used in glycogen |
|
Definition
| alpha1-4 connect glucoses in a line, while alpha1-6 is used to start a new branch |
|
|
Term
| On which end of glycogen do synthesis and degradation occur? |
|
Definition
| Opposite the reducing end |
|
|
Term
| Action of glycogen phosphorylase |
|
Definition
|
|
Term
| Action of debranching enzyme |
|
Definition
| degrades branch points in glycogen |
|
|
Term
| action of glycogen synthase |
|
Definition
|
|
Term
| action of Phosphorylase kinase |
|
Definition
| regulates glycogen phosphorylase activity |
|
|
Term
| action of cAMP dependent protein kinase |
|
Definition
| regulates glycogen synthase and phosphorylase |
|
|
Term
| action of Protein Phosphatase I |
|
Definition
| dephosphorylates proteins |
|
|
Term
| action of Protein inhibitor I |
|
Definition
| regulates Protein Phosphatase I |
|
|
Term
| How is UDP-glucose synthesized |
|
Definition
| Glucose is converted to G6P by Hexokinase/Glucokinase. G6P then converted to G1P by Phosphoglucomutase. G1P then converted to UDP-Glucose by UDP-Glucose pyrophosphorylase or G1P uridylyltransferase |
|
|
Term
| How is glycogen synthesized from UDP-glucose? |
|
Definition
| UDP-glucose + glycogen primer (length n) --> UDP + glycogen (length n+1). |
|
|
Term
| How is a glycogen chain initiated? |
|
Definition
| Glycogen Initiator Synthase, an activity of glycogenin, adds the first six glucose residues to itself (glycogenin). Glycogen synthase and branching enzyme can then use this as a primer to fully synthesize glycogen. |
|
|
Term
| How is glycogen degraded? |
|
Definition
Glycogen Phosphorylase catalyzes
a phosphorolysis reaction of alpha(1,4) bonds, releasing G1P. The enzyme can only work up to four residues from an
a (1,6) branch point, leading to the formation of a limit dextrin. Requires the cofactor vitamin B6, pyridoxal phosphate |
|
|
Term
| Quick summary glycogen synthesis |
|
Definition
| Glycogen synthase adds one glucose residue from UDP-glu to non-reducing end of existing glycogen chain. Branching enzyme transfers residues from one chain to create an alpha(1,6) bond and a new non-reducing end. Increases glycogen solubility and enzyme access rate to the ends. |
|
|
Term
| Quick summary glycogen degradation |
|
Definition
| Glycogen phosphorylase degrades glycogen at non-reducing ends one residue at a time via a phosphorolysis reaction. Will only work up to four residues of a branch point. Debranching enzyme removes 3 glucosyl residues from a non-reducing end and adds to another chain. Removes a (1,6) linkages by a hydrolysis reaction |
|
|
Term
| How pancreas regulates glycogen metabolism |
|
Definition
| Pancreas alpha cells release glucagon, which travels to the liver and stimulates the conversion of glycogen to glucose, increasing Blood Glucose Levels. Conversely, pancreas beta cells release insulin, which travels to the liver and stimulates the conversion of glucose to glycogen, decreasing BGL. |
|
|
Term
| Action of cAMP dependent kinase |
|
Definition
| It activates Phosphorylase kinase and inactivates Glycogen synthase |
|
|
Term
| status of PFK-2, PK, glycogen synthase, and Phosphorylase when cAMP dependent protein kinase is active |
|
Definition
Liver PFK-2 is phosphorylated, the kinase is inactivated, leading to the loss of the major activator of PFK-1, so PFK-1 activity decreases.
Liver pyruvate kinase is phosphorylated, and inactivated, leading to a slowdown of liver glycolysis.
Glycogen synthase is phosphorylated, inactivated, such that glycogen is not synthesized (both liver and muscle).
Phosphorylase is phosphorylated, activated, such that glycogen degradation can occur (both liver and muscle). |
|
|
Term
| Different forms of glycogen synthase |
|
Definition
| Glycogen synthase a is the active form. It is the I form. Glyocgen synthase b is the inactive form. it is the D form and is phosphorylated and is allosterically activated by G6P |
|
|
Term
| Allosteric activation of Glycogen Synthase D |
|
Definition
1. G6P activates the phosphorylated form of glycogen synthase (b form)
2. Synthase b is the same as the “D” form: dependant on G6P for activity
3. If G6P levels are high: adequate energy, store excess as glycogen
4. G6P decreases the Km of the D form for UDP-glucose, as well as increasing the Vmax |
|
|
