Glycogen Degradation

What is most of glucose in glycogen linked by?

a-1,4 glycosidic bonds

About every 12 residues of glucose in glycogen is what?

An a-1,6 glylcosidic branch

Where are the largest stores of glycogen in?

Liver and muscle

First step of glycogen degradation

Release of glucose 1-phosphate

Second step of glycogen degradation

Remodeling of glycogen substrate

Third step of glycogen degradation

Conversion of glucose 1-phosphate into glucose 6-phosphate

What 3 fates does glucose-6 phosphate have?

• Glycolysis

• Pentose phosphate pathway

  • Conversion to glucose and released to the bloodstream

What cleaves glycogen to release glucose 1-phosphate?

Glycogen phosphoylase

What does glycogen phosphoylase hydrolyze?

a-1,4-glycoisidic bonds

Glycogen phosphorylase catalyzes what:

Phosphorolysis reaction that yields glucose 1-phosphate

What remodels glycogen for complete degradation

Transferase and a-1,6-glucosidase

What does transferase do?

Shift a block of 3 glucosyl residues from one outer branch to the other

What does a-1,6-glucosidase do

Cleave the a-1,6 glycosidic bond at the branch point, releasing free glucose

Debranching enzymes:

Transferase

a-1,6 glucosidase

What does the debranching enzyme yield?

Free glucose

What does phosphoglucomutase do?

Convert glucose 1-phosphate into glucose 6-phosphate

What intermediate is between glucose 1-phosphate and glucose 6-phosphate

Glucose 1,6-bisphosphate

What does glucose 6-phosphatase do in the liver?

Generate free glucose from glucose 6-phosphate in the liver

A pyridoxal phosphate participates in what?

Phosphorolytic cleavage of glycogen

What is the prosthetic group of glycogen phosphorylase?

Pyridoxal 5’ phosphate (PLP)

What does the PLP serve as?

Proton donor and acceptor

What is the Tense form of the phosphorylase?

Less active

What is the relaxed phase of the phosphorylase?

More active

Unphosphorylated form of the phosphorylase?

Less active

What is the phosphorylated form of the phosphorylase?

More active

In the b form, what is favored?

T form

In the a form, what is favored?

R form

b,T =

unphosphorylated, tense, and least active

a, R=

phosphorylated, relaxed, most active

What is the default state of the liver phosphorylase?

R,a form

Liver phosphorylase is prepared to generate glucose unless?

High levels of glucose is present

What is the negative allosteric regulator of liver phosphorylase?

Glucose, causing the transition from R to T state

What is the default state of muscle phosphorylase?

b,T = least active

What happens to muscle phoshorylase when energy is needed?

Increase in AMP levels, phosphorylase binds to AMP, stabilizing the R state

What is the T state of phosphorylase stabilized by in muscle phosphorylase?

glucose 6-phosphate

What activates muscle phosphorylase?

High levels of AMP

What inhibits muscle phosphorylase?

High levels of ATP and glucose-6-phosphate

Phosphorylase kinase is activated by what

Phosphorylation and calcium ions

What stimulates glycogen breakdown in muscle during exercise?

Epinephrine

What hormone is secreted in response to low blood sugar and stimulates glycogen breakdown ONLY in the liver?

Glucagon

What proteins transmit the singal for initation of glycogen breakdown?

G proteins

What does intrinsic GTPase activity of G protein converting the bound GTP to GDP do?

Turn off glycogen breakdown

What does phosphodiesterase converting cyclic AMP to AMP do?

Turn off glycogen breakdown

What does protein phosphatase 1 (PP1) do?

Inactivate both phosphorylase kinase and glycogen phosphorylase via dephosphorylation, turning off glycogen breakdown