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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
PLP participates in...
The phosphorolytic cleavage of glycogen
26
Phosphorylase is regulated by...
Allosteric interactions and Reversible phosphorylation
27
What is involved in allosteric interactions?
ATP, glucose 6-phosphate, glucose and AMP
28
What hormones are involved in reversible phosphorylation?
Hormones: Glucagon, epinephrine and insulin
29
In allosteric regulation, what does T stand for?
T = tense ## Footnote *Less active*
30
In allosteric regulation, what does R stand for?
R = relaxed ## Footnote *More active*
31
In reversible phosphorylation, what does b stand for?
b = unphosphorylated ## Footnote *Less active*
32
In reversible phosphorylation, what does a stand for?
a = phosphorylated ## Footnote *More active*
33
In the *b* form, the _____ state is favored.
*b* - T (tense)
34
In the *a* form, the _____ state is favored.
*a* - R (relaxed) state
35
What is the most active phosphorylase?
*a* form in R state
36
What is the least active phosphorylase?
*b* form in T state
37
How is glycogen phosphorylase (glycogen degradation) controlled in skeletal muscle?
Skeletal Muscle: ATP production for contraction **\*Energy status** is important Muscle uses glucose to produce energy for itself
38
How is glycogen phosphorylase (glycogen degradation) controlled in liver?
Liver: Regulation of blood glucose levels \***Glucose levels** are important Liver maintains glucose homeostasis of the organism as a whole
39
In _muscle_, the default form is the \_\_\_\_\_.
*b* form in the T state (When ATP and glucose needs are minimal)
40
When energy is needed, as signaled by an increase in AMP levels, the phosphorylase binds \_\_\_\_\_, which \_\_\_\_\_\_.
Binds AMP, which stabilizes the R state
41
The T state of the phosphorylase is stabilized by...
ATP and glucose 6-phosphate (Controlled by the energy charge of the muscle cell)
42
Increase AMP activates and transitions to the \_\_\_\_\_.
R state
43
When ATP and Glucose 6-phosphate (Inhibitors) are increased, then...
The transition back from R --\> T occurs
44
The default state of liver phosphorylase is the \_\_\_\_\_.
*a* form in the R state (most active form)
45
Liver phosphorylase is prepared to generate blood glucose unless _____ of glucose are present.
High levels
46
Glucose is a _____ of liver phosphorylase, facilitating the transition from R to T state.
Negative allosteric regulator
47
Liver degrades glycogen until there are _____ glucose levels, then glucose will transition the enzyme from \_\_\_\_\_.
Until high glucose levels Enzyme from R --\> T (less active)
48
Allosterically regulated form of Muscle? Liver?
Muscle: *b* Liver: *a*
49
High levels of AMP (low energy) _____ muscle.
Activate
50
High levels of ATP (high energy) _____ muscle.
Inhibit
51
High levels of glucose 6-phosphate (high energy) _____ muscle.
Inhibit
52
High blood glucose levels _____ liver.
Inhibit
53
Hormonal Regulation: Phosphorylase kinase is activated by...
Phosphorylation and Calcium ions
54
Phosphorylase *b* is transitioned to phosphorylase *a* by...
Phosphorylase kinase | (Unphosphorylated --\> Phosphorylated)
55
Epinephrine is released during _____ and stimulates glycogen breakdown mainly in \_\_\_\_\_.
During _exercise_ mainly in _muscle_
56
Glucagon is secreted in response to ___ blood sugar levels and stimulates glycogen breakdown in the \_\_\_\_\_.
Response to _low_ blood sugar levels in the _liver_
57
What transmits the signal for initiation of glycogen breakdown?
G proteins
58
Glycogen breakdown is turned off by 3 mechanisms:
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
Which ONE of the following hormones stimulates glycogen degradation in the muscle? epinephrine, glucagon, insulin, leptin, secretin
Epinephrine
60
The branching of glycogen occurs by a(n):
a-1,6 Glycosidic bond
61
A deficiency in the glycogen phosphorylase in liver would result in:
Hypoglycemia
62
The glycogen phosphorylase b in muscle is allosterically inhibited by high levels of:
Glucose 6-phosphate
63
Glucagon and epinephrine bind to their 7TM receptors and activate \_\_\_\_\_.
G proteins
64
G-proteins activate _____ leading to increased _____ levels.
Activate: Adenylate cyclase Increased: cAMP levels
65
cAMP activates \_\_\_\_\_.
Protein kinase A
66
PKA activates _____ leading to phosphorylation and activation of \_\_\_\_\_.
Activates: Phosphorylase kinase Activation of: Glycogen phosphorylase

Biochemistry Exam 3 (10 decks)

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