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Biochemistry Exam 3 > Chapter 24: Glycogen Degradation > Flashcards

Chapter 24: Glycogen Degradation Flashcards

1
Q

Glycogen is a highly branched…

A

Homopolymer of glucose

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2
Q

Most of the glucose residues in glycogen are linked by ______ bonds; about every 10 residues there is a ______.

A

Linked by a-1,4 glycosidic bonds

a-1,6 glycosidic branch

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3
Q

Largest stores of glycogen are in…

A

Liver and muscle

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4
Q

Liver breaks down glycogen and releases glucose to the bloodstream to provide energy for…

A

Brain and RBCs

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5
Q

Muscle glycogen stores are mobilized to provide energy for…

A

Muscle contraction (Requires ATP)

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6
Q

Glycogen degradation consists of 3 steps:

A
  1. Release of glucose 1-phosphate
  2. Remodeling of the glycogen substrate
  3. Conversion of glucose 1-phosphate into glucose 6-phosphate
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7
Q

Glucose 6-phosphate has 3 fates:

A
  1. Glycolysis
  2. Pentose phosphate pathway
  3. Conversion to glucose - released into bloodstream
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8
Q

Glycogen releases Glucose 1-phosphate via

A

Glycogen phosphorylase (Catalyst of this phosphorolysis reaction)

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9
Q

Glycogen phosphorylase hydrolyzes _____.

A

a-1,4 glycosidic bonds

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10
Q

Phosphorolysis of glycogen is _____ by releasing a phosphorylated sugar.

A

Energetically favorable (Whereas if you use a free glucose, ATP is required)

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11
Q

What else is needed for the breakdown of glycogen?

A

A Debranching Enzyme:

a-1,6 Glucosidase

&

Transferase

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12
Q

How does transferase remodel glycogen?

A

Shifts a block of 3 glycosyl residues from one outer branch to the other

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13
Q

How does a-1,6 Glucosidase remodel glycogen?

A

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

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14
Q

In summary, Glycogen phosphorylase cleaves _____ which releases _____.

A

Cleaves a-1,4 glycosidic bonds

Releases glucose 1-phosphate

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15
Q

In summary, Debranching enzyme cleaves _____ which releases _____.

A

Cleaves a-1,6 glycosidic bonds

Releases glucose

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16
Q

Glucose in Glycogen is released as:

A

90% Glucose 1-phosphate (a-1,4)

10% glucose (a-1,6)

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17
Q

What converts Glucose 1-Phosphate into Glucose 6-Phosphate?

A

Phosphoglucomutase

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18
Q

What is the intermediate from Glucose 1-Phosphate to Glucose 6-Phosphate?

A

Glucose 1,6-bisphosphate

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19
Q

The conversion of Glucose 1-Phosphate to Glucose 6-Phosphate is an important conversion because…

A

It is needed so that glycogen can enter the metabolic mainstream

*Phosphoryl shift reaction that does not require ATP

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20
Q

What hydrolytic enzyme is absent from muscle, but present in the liver?

A

Glucose 6-Phosphatase

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21
Q

Glucose 6-Phosphatase generates _____ from _____ in the liver.

A

Free glucose from Glucose 6-Phosphate

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

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22
Q

Glycogen phosphorylase has _____ which is a derivative of _____ as prosthetic group.

A

Has pyridoxal 5’ phosphate (PLP)

Derivative of pyridoxine (vitamin B6)

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23
Q

The 5’ phosphate of PLP serves as _____ and as _____ (acid-base catalysis).

A

Proton donor and as proton acceptor

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24
Q

A deficiency in _____ can affect glycogen degradation.

A

Vitamin B6

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25
Q

PLP participates in…

A

The phosphorolytic cleavage of glycogen

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26
Q

Phosphorylase is regulated by…

A

Allosteric interactions and Reversible phosphorylation

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27
Q

What is involved in allosteric interactions?

A

ATP, glucose 6-phosphate, glucose and AMP

28
Q

What hormones are involved in reversible phosphorylation?

A

Hormones: Glucagon, epinephrine and insulin

29
Q

In allosteric regulation, what does T stand for?

A

T = tense

Less active

30
Q

In allosteric regulation, what does R stand for?

A

R = relaxed

More active

31
Q

In reversible phosphorylation, what does b stand for?

A

b = unphosphorylated

Less active

32
Q

In reversible phosphorylation, what does a stand for?

A

a = phosphorylated

More active

33
Q

In the b form, the _____ state is favored.

A

b - T (tense)

34
Q

In the a form, the _____ state is favored.

A

a - R (relaxed) state

35
Q

What is the most active phosphorylase?

A

a form in R state

36
Q

What is the least active phosphorylase?

A

b form in T state

37
Q

How is glycogen phosphorylase (glycogen degradation) controlled in skeletal muscle?

A

Skeletal Muscle: ATP production for contraction

*Energy status is important

Muscle uses glucose to produce energy for itself

38
Q

How is glycogen phosphorylase (glycogen degradation) controlled in liver?

A

Liver: Regulation of blood glucose levels

*Glucose levels are important

Liver maintains glucose homeostasis of the organism as a whole

39
Q

In muscle, the default form is the _____.

A

b form in the T state

(When ATP and glucose needs are minimal)

40
Q

When energy is needed, as signaled by an increase in AMP levels, the phosphorylase binds _____, which ______.

A

Binds AMP, which stabilizes the R state

41
Q

The T state of the phosphorylase is stabilized by…

A

ATP and glucose 6-phosphate

(Controlled by the energy charge of the muscle cell)

42
Q

Increase AMP activates and transitions to the _____.

43
Q

When ATP and Glucose 6-phosphate (Inhibitors) are increased, then…

A

The transition back from R –> T occurs

44
Q

The default state of liver phosphorylase is the _____.

A

a form in the R state (most active form)

45
Q

Liver phosphorylase is prepared to generate blood glucose unless _____ of glucose are present.

A

High levels

46
Q

Glucose is a _____ of liver phosphorylase, facilitating the transition from R to T state.

A

Negative allosteric regulator

47
Q

Liver degrades glycogen until there are _____ glucose levels, then glucose will transition the enzyme from _____.

A

Until high glucose levels

Enzyme from R –> T (less active)

48
Q

Allosterically regulated form of Muscle? Liver?

A

Muscle: b

Liver: a

49
Q

High levels of AMP (low energy) _____ muscle.

50
Q

High levels of ATP (high energy) _____ muscle.

51
Q

High levels of glucose 6-phosphate (high energy) _____ muscle.

52
Q

High blood glucose levels _____ liver.

53
Q

Hormonal Regulation: Phosphorylase kinase is activated by…

A

Phosphorylation and Calcium ions

54
Q

Phosphorylase b is transitioned to phosphorylase a by…

A

Phosphorylase kinase

(Unphosphorylated –> Phosphorylated)

55
Q

Epinephrine is released during _____ and stimulates glycogen breakdown mainly in _____.

A

During exercise mainly in muscle

56
Q

Glucagon is secreted in response to ___ blood sugar levels and stimulates glycogen breakdown in the _____.

A

Response to low blood sugar levels in the liver

57
Q

What transmits the signal for initiation of glycogen breakdown?

A

G proteins

58
Q

Glycogen breakdown is turned off by 3 mechanisms:

A
  1. ) Intrinsic GTPase activity of G proteins converts bound GTP to GDP
  2. ) Phosphodiesterase converts cyclic AMP to AMP
  3. ) Protein phosphatase (PP1) inactivates both phosphorylase kinase and glycogen phosphorylase by dephosphorylation
59
Q

Which ONE of the following hormones stimulates glycogen degradation in the muscle?

epinephrine, glucagon, insulin, leptin, secretin

A

Epinephrine

60
Q

The branching of glycogen occurs by a(n):

A

a-1,6 Glycosidic bond

61
Q

A deficiency in the glycogen phosphorylase in liver would result in:

A

Hypoglycemia

62
Q

The glycogen phosphorylase b in muscle is allosterically inhibited by high levels of:

A

Glucose 6-phosphate

63
Q

Glucagon and epinephrine bind to their 7TM receptors and activate _____.

A

G proteins

64
Q

G-proteins activate _____ leading to increased _____ levels.

A

Activate: Adenylate cyclase

Increased: cAMP levels

65
Q

cAMP activates _____.

A

Protein kinase A

66
Q

PKA activates _____ leading to phosphorylation and activation of _____.

A

Activates: Phosphorylase kinase

Activation of: Glycogen phosphorylase

Biochemistry Exam 3 (10 decks)

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