Module 5 - Glycogen Metabolism

Question 1

Calmodulin

Answer 1

An intracellular calcium-binding protein found in eukaryotes.

Serves to regulate the activities of many calcium-responsive proteins and processes.

Question 2

Epinephrine (adrenaline)

Answer 2

A hormone secreted by the adrenal glands in response to stress and/or fear. Initiates the “fight or flight” response.

Question 3

Glucagon

Answer 3

A peptide hormone released by the pancreas in response to low blood glucose levels.

Serves to mobilize glucose from glycogen stores

Question 4

Glycogen

Answer 4

A branched, homopolysaccharide energy storage form of glucose residues.

Main-chain residues are joined through α-1,4 linkages with branch points occurring through α-1,6 linkages.

glucose residues joined together one after the other

Question 5

Glycogen Phosphorylase

Answer 5

A catabolic enzyme involved in the breakdown of glycogen

Question 6

Glycogen Synthase

Answer 6

The rate-limiting enzyme of glycogen synthesis

Question 7

Glycogenesis

Answer 7

The process of glycogen synthesis

Question 8

Glycogenin

Answer 8

A protein which initiates synthesis of a glycogen granule.

It catalyzes the synthesis of an oligosaccharide chain which is attached to one of its own amino acid residues

Question 9

Glycogenolysis

Answer 9

The process of glycogen breakdown

Question 10

Insulin

Answer 10

A hormone released from the pancreas in response to ingested carbohydrate.

It stimulates glycogen synthesis

Question 11

Phosphorolysis

Answer 11

The cleavage of a compound in which the attacking group is inorganic phosphate

Question 12

Uridine Diphosphate Glucose (UDP-Glucose)

Answer 12

A nucleotide sugar involved in glycosyltransferase reactions in metabolism.

Question 13

Glycogen synthesis and breakdown

Answer 13

both events occur at the non-reducing ends of glycogen

due to the branched structure, glycogen has many sites where synthesis and degradation can occur

The branched structure of glycogen increases the rate of both glycogen synthesis and degradation

Question 14

Glycogen Is an Efficient Storage Form of Glucose

Answer 14

there is an input of energy required to synthesize glycogen, 2 ATP for every glucose incorporated.

However, the energy yield from the breakdown of glycogen yields far more ATP, in fact 31 ATP for every glucose-6-P produced when completely oxidized.

Thus, the overall efficiency of storage is close to 94%, making glycogen a very efficient storage form of glucose.

Question 15

Glycogen Is Synthesized and Degraded by Different Pathways

Answer 15

the biosynthesis of glycogen uses different enzymes and intermediates than the degradative pathway

while they share some intermediates, the biosynthetic pathway (glycogenesis) uses an activated form of glucose, UDP-glucose, to add glucose units to glycogen.

This intermediate is not part of the degradative pathway (glycogenolysis)

both the synthesis and degradation of glycogen have glucose-6-P as a common intermediate.

Question 16

mutase

Answer 16

isomerases that change the position of a phosphate group in a molecule

Question 17

Glucose-6-P to Glucose-1-P

Answer 17

When glycogen synthesis is needed, some of the glucose-6-P is converted to glucose-1-P by phosphoglucomutase

a reversible reaction which is necessary for glycogenolysis to proceed

Question 18

Glucose-1-P to UDP-Glucose

Answer 18

glucose-1-P is converted to UDP-glucose, the activated form of glucose, by the enzyme UDP-glucose pyrophosphorylase

Question 19

Why does the hydrolysis of a pyrophosphatase make a reaction irreversible?

Answer 19

in this reaction, the terminal two phosphate groups on UTP are released as pyrophosphate (PPi).

PPi is quickly degraded into two inorganic phosphates by a pyrophosphatase, which is an irreversible reaction.

Question 20

glycogen synthase

Answer 20

The enzyme that uses UDP-glucose to transfer the glucose portion to an existing glycogen molecule

Glycogen synthase adds a glucose to carbon 4 of a terminal glucose moiety (non-reducing end) on a glycogen molecule, forming an α-1,4 glycosidic bond

glycogen synthase can only add glucose units onto an oligosaccharide that consists of at least 4 glucose units.

Question 20

how are the α-1,6 glycosidic bonds formed that produce branching?

Answer 20

there is a branching enzyme that specifically catalyzes the α-1,6 linkage

Branching enzyme then catalyzes the formation of an α-1,6 linkage and thus a new branch point.

This new branch point must be at least 4 residues away from an existing one, and averages between 8-12 residues apart.

Once a branch point has been formed, both non-reducing ends can be extended by glycogen synthase, and more branches catalyzed by branching enzyme.

Question 21

for glycogen synthesis, the key regulatory enzyme is

Answer 21

glycogen synthase

The enzyme is most sensitive to glucose-6-P, which is a strong allosteric activator of glycogen synthase

Question 22

glycogen synthase is involved in another type of regulation

Answer 22

involves a posttranslational modification, specifically phosphorylation

can be phosphorylated at multiple sites

The unphosphorylated form, called glycogen synthase a, is the active form

while the inactive form is called glycogen synthase b

The role of the various phosphorylations has not been completely figured out, but in general the state of phosphorylation plays a fine-tuning role

Question 23

The principle enzyme that degrades glycogen is

Answer 23

glycogen phosphorylase

However, because of the branch points, two additional enzymes are required to help degrade the highly-branched glycogen particles

Question 24

How does Glycogen Phosphorylase work?

Answer 24

This enzyme cleaves off glucose units one at a time from the non-reducing end of glycogen strands, by catalyzing a phosphorolysis reaction.

instead of using water, the enzyme uses inorganic phosphate

will continue to degrade glycogen until the needs of the cell change and signal the enzyme to slow down

Question 25

why is glycogen degraded by phosphorolysis instead of hydrolysis?

Answer 25

by using phosphorolysis, the released sugar (glucose-1-P) is phosphorylated, and doesn’t have to be phosphorylated at the expense of an ATP to enter glycolysis.

So there is an energy advantage in using a phosphorolysis reaction to degrade glycogen.

Question 26

debranching

Answer 26

glycogen phosphorylase can only cleave α-1,4 glycosidic bonds, the branch points represent an obstacle

glycogen phosphorylase stops working when it reaches a residue that is 4 glucose units away from a branch point

Question 27

How does the glycogen degrading deal with the branch points?

Answer 27

1) a transferase enzyme removes the terminal three glucose residues as a block, and transfers them to another branch where they can be degraded by glycogen phosphorylase. leaving a single glucose residue at the branch point

2) Debranching enzyme (also known as α-1,6-glucosidase) hydrolyzes the α-1,6 linkage, in this case releasing a free glucose molecule rather than glucose-1-P

Question 28

What Happens to the Glucose-1-P That Is Released by Glycogen Degradation?

In order for it to enter other metabolic pathways, it must first be converted to glucose-6-P

Answer 28

This is catalyzed by phosphoglucomutase, the same enzyme discussed in glycogen synthesis.

The reaction catalyzed by this enzyme is reversible in the cell, and the direction of the net flux is dependent on the needs of the cell.

Question 29

What happens to the glucose-6-P that’s released y glycogenolysis?

Answer 29

depends on the tissue

In liver, the glucose-6-P produced from glycogenolysis is acted on by glucose-6-phosphatase, which cleaves the phosphate group off, producing free glucose which leaves the liver and enters the blood.

In muscle, glucose-6-phosphatase is absent in muscle because the gene coding for it is silenced. The glucose-6-P produced in muscle from glycogenolysis is therefore metabolized through glycolysis for ATP production.

Question 30

Regulation of Liver Glycogen Phosphorylase

Answer 30

There are two layers of regulation of this enzyme; phosphorylation and allosteric regulation.

1) Glycogen phosphorylase exists as a dimer of identical subunits, and each subunit can be phosphorylated on a single serine residue to convert the enzyme to a more active “a” form.

This phosphorylation is catalyzed by phosphorylase kinase.

Almost all phosphorylations in the cell are reversible, and in this case the dephosphorylation is carried out by phosphorylase phosphatase which converts glycogen phosphorylase to the less active “b” form.

2) Glycogen phosphorylase is also allosterically regulated by glucose, which inhibits the enzyme

When glucose binds to the enzyme (and it can bind to either the “a” or “b” form), it causes a conformational change in the enzyme such that the active site in the enzyme is partially blocked.

Question 31

Regulation of Muscle Glycogen Phosphorylase

Answer 31

The isoform in muscle is subject to regulation by covalent phosphorylation, as well as allosteric regulation.

During contraction, as ATP gets consumed, levels of AMP increase.

AMP reflects a low energy state in a cell; it turns out that it is a potent allosteric activator of muscle phosphorylase b

ATP is an inhibitor of muscle phosphorylase b since it competes with AMP for binding to the enzyme

Glucose-6-P also inhibits the enzyme, which makes sense as well since if its levels are high, it reflects a condition where adequate amounts of glucose are present for energy purposes

only glycogen phosphorylase b in muscle that is sensitive to allosteric modifiers; the phosphorylated form, glycogen phosphorylase a, is fully active regardless of the levels of AMP, ATP, and glucose-6-P present.

Question 32

What two hormones trigger glycogen breakdown

Answer 32

epinephrine and glucagon

Question 33

What causes the release of epinephrine?

Answer 33

Epinephrine is released when a quick burst of energy is needed (a flight or fight response), which most likely is going to require sudden and vigorous muscle contraction and thus fuel to support it.

Question 34

What causes the release of glucagon?

Answer 34

Glucagon, is secreted by the pancreas in response to low blood sugar, and thus would be expected to signal to the liver to release glucose.

Question 35

How do epinephine and glucagon activate glycogen phosphorylase by the regulatory cascade?

Answer 35

When either glucagon or epinephrine bind to their respective receptor on the surface of a liver or muscle cell, respectively, an enzyme called adenylate cyclase becomes activated.

Adenylate cyclase catalyzes the conversion of ATP to cyclic AMP (cAMP)

cAMP binds to a protein kinase called protein kinase A, and in doing so causes a conformational change such that the catalytic subunit is able to carry out phosphoryations on target proteins.

One target protein is phosphorylase kinase, which upon phosphorylation becomes activated. It in turn phosphorylates phosphorylase b, which converts it to the active form phosphorylase a.

Question 36

Why is it called a cascade?

Answer 36

One hormone-binding event leads to many cAMP molecules produced, which activates a protein kinase A molecule.

Each kinase molecule can phosphorylate and activate many phosphorylase kinase molecules, and each of these can activate many glycogen phosphorylase molecules.

So the degree of signal amplification is enormous.

Question 37

How does calmodulin work in muscle contraction?

Answer 37

Phosphorylase kinase is activated when Ca2+ binds to a subunit of this enzyme;

Muscle contraction is triggered by the release of calcium from the sarcoplasmic reticulum.

Thus, at the same time that muscle contraction is stimulated, glycogen breakdown is also stimulated which provides the fuel for ATP generation.

Question 38

what happens when protein kinase A is activated by cAMP?

Answer 38

It doesn’t only act on phosphorylase kinase

it also phosphorylates glycogen synthase a, the more active form.

When phosphorylated, it is converted to the less active glycogen synthase b

This leads to a situation where at the same time that glycogen degradation is being stimulated, glycogen synthesis is being inhibited

Question 39

Since activation of glycogen breakdown and inhibition of glycogen synthesis both involved the phosphorylation of enzymes, it makes sense that in order to reverse these processes, the dephosphorylation or removal of phosphate groups from these enzymes must have to occur.

What are the specific protein phosphatase involved in glycogen metabolism?

Answer 39

protein phosphatase 1 (PP1)

removes phosphate groups from both phosphorylase kinase and glycogen phosphorylase a, which inactivates both and thus inhibits glycogenolysis

it also removes phosphates from glycogen synthase, which increases its activity and therefore stimulates glycogen synthesis.

Question 40

How does Insulin stimulate glycogen synthesis?

Answer 40

1) It increases the number of glucose transporters (GLUT4) in the cell membrane
**This increases glucose uptake into cells, which is then converted to glucose-6-P

2) it leads to the inactivation of glycogen synthase kinase
**This reduces the phosphorylation state of glycogen synthase and thus increases its activity

3) The binding of insulin to its receptor leads to the phosphorylation and formation of a second messenger, insulin receptor substrate (IRS-P)

Question 41

What happens to blood glucose after we eat a meal?

Answer 41

blood glucose levels go up and insulin is released from the pancreas to help cells take up the glucose and to store much of it as glycogen.