Glycogen Metabolism Flashcards

1
Q

How is glycogen stored?

A

Large cytosolic granules

10% of liver mass and 1-2% of muscle mass

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2
Q

What is the structure of glycogen?

A

a1->4 linked glucose with a1->6 branches

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3
Q

Where does the addition and removal of glucose in glycogen occur?

A

The non-reducing ends

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4
Q

What is the function of glycogen phosphorylase and what cofactor does it require?

A

Removes glucose from non-reducing ends until it reaches four glucose molecules from a branch point

Requires pyridoxal phosphate

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5
Q

What is the function of glycogen debranching enzyme?

A

Has two activities:

Transferase activity moves 3 glucose moelcules from branch to non-reducing end

Glucosidase activity removes remaining glucose molecule

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6
Q

What is the function of phosphoglucomutase?

A

Converts glucose-1-phosphate converted to glucose-6-phosphate

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7
Q

What is the function of glucose 6-phosphatase?

A

Converts G6P to glucose.

Phosphatase is expressed in the liver and kidney, but not muscle and adipose tissue

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8
Q

Where does glycogen synthesis take place?

A

Liver and Skeletal muscle

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9
Q

What is the first step in glycogen synthesis?

A

G6P to G1P via phosphoglucomutase

Reversal of reaction in glycogen breakdown

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10
Q

What is the function of UDP-glucose pyrophosphorylase?

A

Converts G1P to UDP-glucose

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11
Q

What is the function of glycogen synthase?

A

Adds glucose residue of UDP-glucose to the non-reducing end of glycogen

Removes UDP

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12
Q

What is the function of glycogen branching enzyme?

A

Adds a1–>6 branches

Removes 7 residues, 3 residues down to form a branch

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13
Q

What two things does the initiation of a new glycogen chain require?

A

Primer and glycogenin

Primer is usually a preformed polyglucose chain

Glycogenin can act as both a primer and enzyme

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14
Q

What is the function of glycogenin?

A

Binds to UDP-glucose and removes UDP. This continues six more times through the addition of glucose and removal of UDP.

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15
Q

Describe the structure of glycogen phosphorylase

A

Made up of two subunits, each exists in an a and b form

a = catalyticall active
b = less active
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16
Q

How is glycogen phosphorylase regulated?

A

By phosphorylation.

Phosphorylated = a form, active

Not Phosphorylated = b form, less active

17
Q

What is the function of phosphorylase b kinase?

A

To phosphorylate glycogen phosphorylase b and activated it.

18
Q

How is phosphorylase b kinase activated in the muscle?

A

epinephrine, increased Ca, increased AMP

19
Q

How is phosphorylase b kinase activated in the liver?

20
Q

What inhibits phosphorylase b kinase?

A

ATP, G6P, and insulin

Stimulate dephosphorylation via phosphatase

21
Q

What is the function of Phosphoprotein phosphatase 1 (PP1)?

A

Dephosphorylates glycogen phosphorylase a

22
Q

What signaling pathway initiates glycogen breakdown?

23
Q

How does glucose concentration affect phosphorylase activity?

A

Phosphorylase can act as a glucose sensor. Glucose can allosterically inactivate phosphorylase a and promote dephosphorylation

24
Q

How is glycogen synthase regulated?

A

Phosphorylation

a form, active - not phosphorylated

b form, inactive, - phosphorylated

25
How is the dephosphorylation of glycogen synthase stimulated?
Insulin, G6P, glucose
26
What stimulates the phosphorylation of glycogen synthase?
Glucagon and epinephrine
27
What are three regulations of carbohydrate metabolsim specific to muscle?
Lack enzymatic machinery for gluconeogenesis Lack receptors for glucagon Isozyme of pyruvate kinase not phosphorylated by PKA -so glycolysis is not turned off when cAMP is high
28
How do glycogen storage diseases present?
Hypoglycemia Liver damage and enlargement Myopathy