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Biochem Test 2 > Glycogen Metabolism > Flashcards

Glycogen Metabolism Flashcards

1
Q

What are the bonds that link together units of glucose molecules?

A

Alpha 1,4 glycosidic bonds

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2
Q

What is the bond found in the branch points of glucose monomers of separate chains?

A

Alpha 1,6 glycosidic bonds

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3
Q

What is the end that contains a terminal glucose with a free hydroxyl group at C4?

A

The non-reducing end

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4
Q

What does the reducing end have connected to it?

A

Glycogen in

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5
Q

What does glycogenin make?

A

A primer which is crucial for the glycogen synthesis

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6
Q

Where is glycogen stored?

A

Mostly found in the liver and the muscle and present as granules

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7
Q

What is the function of glycogen in the liver?

A

Liver glycogen regulates blood glucose levels

- sensor and storage

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8
Q

What is the function of glycogen in the muscle?

A

Provides a reservoir of fuel glucose for physical activity in the muscle

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9
Q

What are the three ways that glycogen metabolism is regulated?

A
  1. Allosteric control: molecules will bind to enzymes and turn them off
  2. Covalent modifications through reversible phosphorylation of key enzymes
    - kinase and phosphatase
  3. Hormonal control
    - insulin modulates
    - glucagon
    - epinephrine
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10
Q

List the steps of glycogen degradation

A
  1. Glycogen broken down to release glucose-1-phosphate
    - glycogen phosphorylase (rate-limiting)
  2. Glycogen remnant remodeled to permit further degradation
  3. Glucose-1-phosphate is converted to glucose-6-phosphate by phosphogluconutase
  4. Glucose-6-phosphate can go to the muscle or brain and then go into glycolysis
    - in glycolysis makes pyruvate, lactate, CO2 and H2O
    - can be converted to glucose to go into the blood stream for other tissues to use
    - can go to the pentose phosphate pathway to make NADPH and ribose derivative
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11
Q

What are the four key enzymes in glycogenolysis?

A
  • One to degrade glycogen (chain shortening)
  • 2 to remodel glycogen remnants
  • one to convert glycogen breakdown product suitable for further metabolism
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12
Q

What is the rate limiting step of glycogenolysis?

A

Glycogen phosphorylase (GP) catalyze so the cleavage of glycogen

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13
Q

Where does chain shortening occur?

A

The non-reducing end of the polymer

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14
Q

What is the mechanism of GP?

A

GP adds an orthophosphate and releases a glucose residue as glucose-1-phosphate

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15
Q

When does phosphorolysis of glucose end?

A

Ends when GP gets within 4 residues of the alpha 1,6 linkage of a branch point

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16
Q

What cofactor does GP use in chain shortening?

A

Pyridoxal phosphate (vitamin B6)

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17
Q

What is the role of transferase in chain shortening?

A

Transferase transfers a block of 3 of the remaining 4 glucose to the non-reducing end of the main chain forming a alpha 1,4 bond

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18
Q

What enzyme cleaves the alpha 1,6 bond of the glucose residue in order to release free glucose?

A

Debranching enzyme or alpha 1,6 glucosidase

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19
Q

What enzymes are responsible for converting branched glycogen to a linear structure?

A

Transferase and alpha 1,6 glucosidase

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20
Q

What does phosphoglucomutase do?

A

Converts Glucose-1-phosphate to glucose-6-phosphate by transferring a phosphoryl group from the enzyme to the substrate and transferring a different phosphoryl group back tot restore the enzyme to its initial state

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21
Q

Where is glucose-6-phosphate ONLY found?

A

In the liver where it can get converted to glucose

22
Q

What regulates Glycogen phosphorylase (GP)?

A
  1. Allosteric effectors

2. Reversible phosphorylation (responsive to hormones)

23
Q

What are the two forms of Glycogen phosphorylase (GP)?

A
  • Active form “a” form (R relaxed state) found in the liver

- inactive form “b” form (T tense state) found in the muscle

24
Q

How does glucose inactivate the active form of GP?

A

By binding to the active site and stabilizing the conformation in the inactive T state, when glucose levels are high there is no need to breakdown glycogen to make glucose- we will just activate glycogen synthesis to make more glycogen to store

25
Q

Describe how muscle GP is regulated?

A
  • in the muscle GP is found in the b form or inactive form
  • can be activated by AMP
  • AMP binds to the active site and converts b to the a state and the R state
  • when muscle contract ATP is converted to AMP and signals GP to breakdown glycogen
  • ATP and Glucose-6-phosphate are negative Allosteric regulators
26
Q

What is responsible for converting B form to A form of GP?

A

Phosphorylation of a single serine residue initiated by hormones and carried out by phosphorylase kinase (PK)

27
Q

Describe the activation of phosphorylase kinase

A
  • Phosphorylase kinase is present in the active B form
  • 4 Ca bind during a muscle contraction, nerve impulse, or hormone signal
  • PK becomes partially active
  • PKA and hormones phosphorylate PK
  • PK is fully active
  • Fully active form can then convert Phosphorylase B to phosphorylase A by phosphorylation using ATP
28
Q

When is glucagon released and what does it act on?

A

Glucagon is released when blood sugar levels are low

Acts on the liver

29
Q

When is epinephrine released and what does it act on?

A

Released during muscle activity

Effects on the muscle

30
Q

What regulates glucagon and epinephrine release?

A

G protein coupled receptors (GPCR)

31
Q

What signals glycogen breakdown?

A

Epinephrine and glucagon

32
Q

Liver and muscle forms of GP are an example of

A

Isozyme

33
Q

Hers Disease is caused by

A

A mutation in liver GP

34
Q

McArdle syndrome is caused by

A

A mutation in muscle GP

35
Q

What are the three key events in glycogenesis?

A
  1. Trapping and Activation of Glucose
  2. Elongation of glycogen primer
  3. Branching of glycogen chains
36
Q

What is the role of glucokinase/ hexokinase?

A
  • Found in the cytosol of hepatocytes and muscle cells
  • Catalyze phosphorylation of glucose to glucose-6-phosphate
  • This traps the glucose in these cells
37
Q

What is the role of phosphoglucomutase in glycogenesis?

A

It reversible isomerizes glucose-6-phosphate to glucose-1-phosphate

38
Q

What is the role of Uridine diphosphate (UDP)-glucose pyrophosphorylase in glycogenesis?

A
  • UDP transfers the Glucose-1-phosphate to uridine triphosphate (UTP)
  • This action generates UDP-glucose which is the active form of glucose
  • The breakdown of pyrophosphate to Pi generates energy and drives the reaction forward
39
Q

What is the rate limiting enzyme in glycogenesis?

A

Glycogen synthase

  • catalyze so the transfer of glucose from UDP-glucose to the non-reducing end of glycogen chain
  • forms the alpha 1,4 glycosidic bonds between glucose molecules
40
Q

What happens when glycogen chain reaches 11 residues?

A

Chain fragments (about 7 residues long) at the alpha 1,4 glycosidic link and reattached somewhere else through an alpha 1,6 link by enzyme glucosyl (4:6) transferase

41
Q

How can you increase solubility of glycogen?

A

Branching increases which also increases number of terminal non-reducing ends

Branching increases rate at which glycogen can be synthesized and degraded

42
Q

What is the key enzyme of glycogen synthesis regulation?

A

Glycogen synthase

43
Q

What regulates Glycogen synthase kinase (GSK)?

A

Insulin and PKA

44
Q

What are the two key enzymes in the regulation of glycogen metabolism?

A

Glycogen phosphorylase and glycogen synthase - the rate limiting steps of degradation and synthesis

45
Q

What controls both glycogen breakdown and synthesis via PKA?

A

Glucagon and epinephrine controls both

46
Q

What happens in the fed state?

A

Glycogenesis is favored- glucose and insulin are both high and ATP is high

  • when glycogen synthesis favored, the dephosphorylates form of glycogen synthase (active) and glycogen phosphorylase (inactive) are predominant
47
Q

What occurs in the fasting state?

A

Blood glucose levels are low
Glucagon levels are high
Cellular calcium and AMP are elevated (in exercising muscle)
Glycogenolysis is favored

-when glycogen degradation is favored phosphorylated forms of glycogen synthase (inactive) and glycogen phosphorylase (active) are predominant

48
Q

Describe how insulin regulates glycogen metabolism

A 
High blood glucose 
Release of insulin by beta cells of pancreas
Binding of insulin to its receptor tyrosine kinase 
Activation of signaling cascade 
-4 key proteins:
GLUT 4 (glucose transporter)
Protein kinase B (PKB)
Protein phosphatase 1 (PP1)
Glycogen synthase kinase 3 (GSK3)

Formation of the insulin receptor complex
Activation of PKB
Translocation of GLUT to membrane
PKB phosphorylated PP1 (activate) and GSK3 (inactivate)
Active PP1 dephosphorylates glycogen synthase (active) and dephosphorylates glycogen phsophorylase (inactivate)
NET RESULT: activation of glycogen synthase and inactivation of glycogen phosphorylase

49
Q

Describe Type 2 diabetes.

A

Reduced sensitivity to insulin
Insulin resistant
Mutations in insulin receptor and/or downstream signaling proteins
Down-regulation in receptor levels triggered by elevated insulin (leading to endocytosis and degradation of the insulin receptor). Not replaced by translation

50
Q

What is the glucose sensor in liver cells?

A

Glycogen phosphorylase

51
Q

Describe the affects of McArdle syndrome?

A

Limited ability to perform strenuous exercise because of painful muscle cramps; otherwise patient is normal and well developed

Defective phosphorylase in the muscle

52
Q

What is the clinical affect of Hers Disease?

A

Enlargement of the liver; hypoglycemia, ketosis, hyperuricemia, and hyperlipidemia

Due to defective phosphorylase in the liver

Biochem Test 2 (5 decks)

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