Chapter 24 Glycogen Degradation

Flashcards about Glycogen Degradation

What is glycogen?

A highly branched homopolymer of glucose present in all tissues.

Where are the largest stores of glycogen found?

Liver and muscle.

What is the role of liver glycogen?

To break down glycogen and release glucose to the blood to provide energy, especially for the brain and red blood cells.

What is the role of muscle glycogen?

To mobilize glycogen stores to provide energy for muscle contraction.

Where does degradation of Glycogen take place?

At the surface of the glycogen granule

What enzyme degrades glycogen from the nonreducing ends of the glycogen molecule?

Glycogen phosphorylase.

What reaction does phosphorylase catalyze?

A phosphorolysis reaction that yields glucose 1-phosphate.

What enzyme converts glucose 1-phosphate into glucose 6-phosphate?

Phosphoglucomutase.

What type of glycosidic bonds can glycogen phosphorylase cleave?

α-1,4-glycosidic bonds.

What is the role of transferase in glycogen breakdown?

It shifts a small oligosaccharide near the branch point to a nearby chain, making glucose moieties accessible to phosphorylase.

What enzyme cleaves the α-1,6 bond at the branch point in glycogen?

A debranching enzyme (α-1,6-glucosidase).

What is released when α-1,6-glucosidase cleaves the α-1,6 bond at the branch point?

A free glucose.

What intermediate does phosphoglucomutase use to form glucose 6-phosphate from glucose 1-phosphate?

Glucose 1,6-bisphosphate.

What enzyme generates free glucose from glucose 6-phosphate in the liver?

Glucose 6-phosphatase.

Why is glucose 6-phosphatase important?

The free glucose is released into the blood for use by other tissues such as the brain and red blood cells.

Which tissues is Glucose-6-Phosphatase absent from?

most tissues other than the liver

What two enzymes are required for the liver to release glucose into the blood when an organism is asleep and fasting?

Glycogen Phosphorylase and Glucose-6-Phosphatase

What is the key regulatory enzyme for glycogen degradation?

Glycogen phosphorylase.

What are the two forms of phosphorylase?

A less active b form and a more active a form.

How do the a and b forms of phosphorylase differ?

The a form has a phosphorylated serine residue, while the b form does not.

What states do both the a and b forms of phosphorylase display?

R⬄T equilibrium.

In phosphorylase b form, which state is favored?

The T state.

In phosphorylase a form, which state is favored?

The R state.

What is the default state of liver phosphorylase?

The a form in the R state.

What is a negative regulator of liver phosphorylase?

Glucose.

What isozyme forms exist with liver and muscle phosphorylase?

Liver phosphorylase and muscle phosphorylase are isozymes

In muscle, what is the default form of phosphorylase?

The b form in the T state.

What stabilizes the R state of muscle phosphorylase?

AMP.

What stabilizes the T state of muscle phosphorylase?

ATP and glucose 6-phosphate.

What activates Muscle Phosphorylase (Phosphorylase b)?

Activated by AMP (signals low energy)

What inhibits Muscle Phosphorylase (Phosphorylase b)?

inhibited by ATP and glucose-6-phosphate (signals high energy)

What activates Liver Phosphorylase (Phosphorylase a)?

none

What inhibits Liver Phosphorylase (Phosphorylase a)?

Inhibited by glucose (not glucose-6-phosphate) and ATP.

Which type of muscle fiber is rich in glycogen phosphorylase?

Type IIb fibers (fast-twitch fibers).

What stimulates the phosphorylation of phosphorylase?

The hormones glucagon and epinephrine (adrenaline).

What enzyme converts glycogen phosphorylase from the b state to the a state?

Phosphorylase kinase.

What activates phosphorylase kinase?

Phosphorylation and Ca2+ binding.

What hormones initiate G-protein cascades that result in the production of cAMP?

Glucagon (in liver) and epinephrine (in muscle).

What converts cAMP into AMP?

Phosphodiesterase.

What removes phosphoryl groups from phosphorylase kinase and glycogen phosphorylase, thereby inactivating the enzymes?

Protein phosphatase 1.