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1024 Glycogen Metabolism 1-2 Dr Lieberman
UC MED 2015 1024 Glycogen Metabolism 1-2 Dr Lieberman
21
Medical
Graduate
10/24/2011

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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
degrades glycogen
Term
Action of debranching enzyme
Definition
degrades branch points in glycogen
Term
action of glycogen synthase
Definition
makes glycogen
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 ā€œDā€ 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
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