Glycogen Metabolism

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Glycogen Breakdown, Glycogen Synthesis and regulation of glycogen metabolism

What are the two major sites of glycogen storage? Which has the highest concentration of glycogen? Which has the most glycogen?

Liver and skeletal muscle. Liver has higher concentration, but skeletal muscle has the most because of greater mass.

What are the three stages of Glycog degradation?

(1) Release of Glucose 1-phosphate from glycogen. (2) Remodeling of the glycogen substrate to permit further degradation. (3) Conversion of glucose 1-phosphate into glucose 6-phosphate.

What are the 3 possible fates for glucose 6-phosphate?

(1) Glycolysis (2) Converted into free glucose and released into the bloodstream. (3) Processed by pentose phosphate pathway to yield NADPH and ribose derivatives.

What catalyzes the phosphorolytic cleavage of glycogen? What does the reaction release?

Glycogen Phosphorylase. It releases one Glucose 1-Phosphate and Glycogen (n-1 residues) by adding Pi to cleave a bond.

What points are not cleavable by phosphorylase? Where will it stop cleaving?

α-1,6-glycosidic bonds at branch points. When it reaches a terminal residue 4 residues away from a branch point.

What enzymes remodel the glycogen substrate for further degradation? How do they do this?

Transferase and α-1,6-Glucosidase. Transferase shifts a block of 3 glucosy lresidues from one outer branch to the other. α-1,6-Glucosidase hydrolyzes the α-1,6-glycosidic bond.

When the debranching enzyme hydrolyzes the α-1,6-glycosidic bond, what molecules are released? What happense to them?

One free glucose molecule and Glycogen (n-1 residues). The Glucose is phosphorylated by hexokinase. Glycogen(n-1 residues) is further degraded.

What enzyme converts Glucose 1-Phosphate to Glucose 6-Phosphate?

Phosphoglucomutase

What does the catalytic site of an active mutase molecule contain? How does this help form Glucose 6-Phosphate?

A phosphorylated serine residue. The phosphoryl group is transfered from the serine to the C-6 hydroxyl group of Glucose 1-Phosphate. The C-1 phosphoryl group is then transfered to the serine.

What enzyme does the liver contain that the muscle doesn't? What does it form and why would this be in the liver only?

Glucose 6-Phosphatase. This enzyme cleaves the phosphoryl group to form free glucose and Pi. This enables the glucose to leave the organ and be shuttled into cytoplasm because the major function of liver is to main the blood-glucose level in the blood.

What are the two forms of Glycogen Phosphorylase? Which form is more active than the other?

Phosphorylase b and Phosphorylase a. Phosphorylase b (unphosphorylated) is less active than Phosphorylase a (phosphorylated).

When is Phosphorylase b active? What inhibits it?

When AMP levels are high. ATP inhibits by competing with AMP. Glucose 6-P gives feedback inhibition because it favors the less active state.

Which Phosphorylase is normally active and why?

Phosphorylase a is usually active because the equilibrium favors the R state. Phosphorylase b is usually inactive because it favors the T state.

What converts Phosphorylase b into Phosphorylase a? What initiates this conversion?

A phosphorylation of a serine residue in each subunit. This is initiated by hormones created by fear or exercise.

For phosphorylation, exercise results in...

Elevated AMP which activates Phosphorylase b and hormone release that generates Phosphorylase a.

What enzyme activates Glycogen Phosphorylase by attaching a phosphoryl group? What activates it?

Phosphorylase kinase which is activated by Protein Kinase A (PKA) at the βsubunit and increases in Ca2+ levels at the δsubunit.

What hormones trigger the breakdown of glycogen? Which hormones are the liver and muscle more responsive to?

Glucagon and epinephrine. Epinephrine stimulates breakdown in the muscles (and somewhat in the liver). Glucagon primarily stimulates the liver.

How do hormones trigger the breakdown glycogen?

They initiate a cyclic AMP signal-transduction cascade.

What are the steps of the cyclic AMP signal-transduction cascade?

Hormones bind to 7TM receptors --> Activates GTP-bound subunit of Gs --> Activates Adenylase cyclase --> ATP converts to AMP --> Activates PKA--> Activates Phosphorylase Kinase --> Phosphorylates Phosphorylase b to Phosphorylase a.

In liver, stimulation by both glucagon and epinephrine leads to maximal mobilization of liver glucagon by...

Stimulating Phosphoinositide cascade that induces the release of Ca2+ which further activates phosphorylase kinase.

Phosphorylase kinase has four subunit types. Identify them.

ϒ=subunit possesses the protein kinase activity.
β=site of phosphorylation
α=also phosphorylated by PKA (makes better substrate for dephosphorylation/inactivation)
δ=site that binds and senses Ca2+

What does the enzyme Protein Phosphatase 1 (PP1) do in Glycogen Breakdown?

Removes a phosphoryl group from phosphorylase kinase (deactivates) and glycogen phosphorylase (converts into usually inactive b form).

Glucogen is Synthesized using UDP-glucose. What form of glucose is this?

Activated because its hydroxyl group is esterfied to the diphosphate moiety of UDP.

What is the key regulatory enzyme in Glycogen Synthesis? What reaction does it catalyze?

Glycogen synthase. Catalyzes the transfer of lucose from UDP-glucose to a growing chain. (Synthase of α-1,4-linkages)
"UDP-glucose + Glycogen (n residues) --> UDP + Glycogen (n+1 residues)"

Why is the branching step important in glycogen symthesis?

Branching increases the solubility of glycogen and creates a large number of terminal residues (sites for glycogen phosphorylase and synthase). AKA: branching increases the rate of glycoen synthesis and degradation.

Glycogen branching requires a transferase activity and an α-1,6-glucosidase.

False. Glycogen branching only requires a transferase activity. Debranching requires two enzyme activities.

Phosphorylation has opposite effects on the enzymatic activities of glycogen synthase and phosphorylase.

True. Phosphorylation converts the active a form of glycogen synthase into a usually inactive form b. The b form is only active if a high level of Glucose 6-phosphate is present.

How are Glycogen Breakdown and Synthesis reciprically regulated?

Glucagon and Epinephrine control both through PKA. PKA activates glycogen degradation and inactivates glycogen synthesis.

How does PP1 reverse the regulatory effects of kinases?

PP1 inactivates phosphorylase a and phosphorylase kinase by dephosphorylating them. Activates glycogen synthase through dephosphorylation.

What is PP1's regulatory subunit?

In the skeletal muscle: GM
In the liver: GL

When does Insulin stimulate the synthesis of glycogen? How does it do this?

When blood-glucose levels are high. Inactivates glycogen synthase kinase, PP1 then dephosphorylates/activates glycogen synthase. Glucose is stored as glycogen which inhibits phosphorylase a.

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