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Biochemistry: Exam 3 > Glycogen Breakdown > Flashcards

Glycogen Breakdown Flashcards

1
Q

What is the composition of glycogen?

A

Alpha (1-4) linked D-glucose with alpha(1-6) linked branches every 8-14 residues.

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2
Q

What type of shape does a glycogen molecule take?

A

Spheroidal

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3
Q

In what types of cells is glycogen most prominent?

A

Muscle and liver

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4
Q

Where do the enzymes that catalyze glycogen synthesis and breakdown come from?

A

They are contained in glycogen.

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5
Q

From which ends of glycogen is glucose removed from?

A

Non-reducing ends

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6
Q

True or false: Glycogen contains many reducing ends, but only one non-reducing end.

A

False; glycogen contains many non-reducing ends, but only one reducing end.

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7
Q

What is glycogenolysis?

A

The breakdown of glycogen into glucose

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8
Q

The breakdown of glucose

A

glycogenolysis

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9
Q

What pathway converts glucose to pyruvate?

A

glycolysis

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10
Q

What pathway converts pyruvate to glucose?

A

gluconeogenesis

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11
Q

What is the end of a glycogen molecule with a free carbon #1 called? How many reducing ends does each molecule of glycogen have?

A

A reducing end; one

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12
Q

What does glycogen phosphorylase do?

A

Cleaves glycogen bonds by substituting a Pi to form glycogen and glucose-1-phosphate (G1P).

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13
Q

Which enzyme breaks glycogen branch bonds in the second step of glycogenolysis?

A

Glycogen debranching enzyme

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14
Q

Which enzyme converts G1P to G6P?

A

Phosphoglucomutase

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15
Q

What is the RDS of glycogenolysis?

A

The first step, glycogen phosphorylase

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16
Q

What controls glycogen phosphorylase?

A

1) Allosteric interactions (inhibitors=ATP, G6P, and glucose, activators=AMP)
2) Covalent modification (phosphorylation and dephosphorylation)

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17
Q

What is the phosphorylated form of glycogen phosphorylase?

A

Phosphorylase a

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18
Q

What is the dephosphorylated form of glycogen phosphorylase?

A

Phosphorylase b

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19
Q

Why can glycogen phosphorylase only get within 5 units from a branch?

A

The crevice on its surface that connects the glycogen binding site to the active site is too small.

20
Q

True or false: Several glucose units can be cleaved by glycogen phosphorylase without unbinding/rebinding glycogen between catalytic cycles?

A

True

21
Q

What is the cofactor for glycogen phosphorylase? What is it derived from?

A

pyridoxal-5-phosphate (PLP); from vitamin B6

22
Q

True or false: Glycogen phosporylase undergoes conformational changes from T to R and vice versa.

A

True

23
Q

What activator promotes a conformation change in glycogen phosphorylase from T to R?

A

AMP

24
Q

True or false: The T site is buried in glycogen phosphorylase and has low substrate affinity.

A

True

25
Q

What substrate binds at the T state of glycogen phosphorylase, inhibiting the shift from the T to the R state?

A

ATP

26
Q

Which form of glycogen phosphorylase requires AMP for activity?

A

Phosphorylase b

27
Q

Which form of glycogen phosphorylase is active without AMP?

A

Phosphorylase a

28
Q

Which glycolysis enzyme works to remove branches?

A

Glycogen debranching enzyme

29
Q

How does the glycogen debranching enzyme work?

A

It removes a trisaccharide from a branch to a non-reducing end, then hydrolyzes the remaining alpha (1-6) bond, yielding glucose and debranched glycogen.

30
Q

True or false: The debranching enzyme phosphorylizes the alpha(1-6) glucose bond.

A

False; it hydrolyzes it

31
Q

True or false: The glucose that is being removed by a debranching enzyme is G1P.

A

False; it is glucose

32
Q

True or false: The debranching enzyme works much faster than glycogen phosphorylase.

A

False; glycogen phosphorylase is much faster.

33
Q

Which branches of a glycogen molecule are degraded most quickly by a debranching enzyme?

A

The outer banches (in just a few seconds); the slower debranching occurs.

34
Q

Why does maximum muscle contraction last only a few seconds?

A

In part, because the debranching of inner branches of glycogen is slow.

35
Q

True or false: The debranching enzyme has only one active site.

A

False; it has two

36
Q

Which enzyme converts G1P to G6P in glycogenolysis?

A

Phosphoglucomutase

37
Q

True or false: Near equilibrium, the reaction of G1P to G1,6P to G6P by phosphoglucomutase is irreversible.

A

False

38
Q

What can happen to the G6P from glycogenolysis?

A

It can enter glycolysis or enter the Hexose Monophosphate Pathway (Pentose Phosphate Pathway).

39
Q

In the liver, what must happen to G6P for it to be put back into circulation from the cell?

A

It must be de-phosphorylated to glucose.

40
Q

What enzyme transports G6P to the ER to be be de-phosphorylated?

A

G6P translocase

41
Q

What enzyme dephosphorylates G6P to glucose in the ER?

A

G6Pase (Glucose-6-phosphatase)

42
Q

What transporter transports glucose out of the liver?

A

GLUT2

43
Q

Why do muscles and other tissues besides the liver not release glucose back into circulation?

A

They lack GTPase to break G16 back down to glucose.

44
Q

What is the result of any defect in the G6P hydrolysis system in liver/

A

Type 1 Glycogen Storage Disease

45
Q

What are the symptoms of Type 1 Glycogen Storage Disease?

A

1) Increased [G6P] and liver and kidney[glucose]
2) Inability to increase blood glucose concentration
3) Hepatomegaly and hypoglycemia
4) General failure to thrive

Biochemistry: Exam 3 (23 decks)

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