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USMLE Biochemistry > Glycogen > Flashcards

Glycogen Flashcards

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USMLE® Step 1 flashcards
1
Q

What are the 4 steps for Glycogen synthesis from glucose-6-phosphate?

A 
G6P
-> Glucose-1-phosphate 
(UDP-glucose pyrophosphorylase) + UTP
-> UDP-Glucose 
(Glycogen Synthase)
-> Unbranched Glycogen 
(Branching enzyme)
-> Branched Glycogen
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2
Q

What enzyme causes branhced glycogen to become unbranched?

A

Debranching enzyme

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3
Q

What enzyme breaks down Unbranched Glycogen into Glucose-1-phosphate?

A

Glycogen phosphorylase

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4
Q

What enzyme converts Glucose-6-phosphate to glucose?

A

Glucose-6-Phosphatase

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5
Q

Where is Glucose-6-Phosphatase found?

A

Liver

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6
Q

What enzyme breaks down unbranched glycogen into glucose and is found in lysosomes?

A

alpha1,4 glucosidase

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7
Q

What does glycogen phosphorylase do?

A
  • Removes glucose molecules from glycogen polymer
  • Creates glucose-1-phosphate from unbranched glycogen
  • Stops when glycogen branches decreased to 2-4 linked glucose molecules (limit dextrins)
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8
Q

What vitamin stabilises glycogen phosphorylase?

A

Vitamin B6

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9
Q

What enzyme phosphorylates glycogen phosphorylase?

A

Glucagon (and epinephrine)

  • Becomes more active
  • More glucose
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10
Q

How do glucagon and insulin glycogen and glucose levels?

A

Through phosphorylating enzymes - glucagon

Through dephosphorylation (insulin)

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11
Q

What are the 2 enzymes which glucagon and epinephrine phosphorylate?

A
  • Glycogen phosphorylase (increasing its activity)

- Glycogen synthase (decreasing its activity)

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12
Q

What is the effect of phosphorylation on glycogen phosphorylase’s activity?

(Glucagon and epinephrine phosphorylate glycogen phosphorylase)

A

Increases activity, more glycogen -> Glucose

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13
Q

What is the effect of phosphorylation on glycogen synthase’s activity?

(Glucagon and epinephrine phosphorylate glycogen synthase)

A

Decreases its activity

  • less glycogen made from glucose
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14
Q

What enzymes are dephosphorylated by insulin?

A
  • Glycogen phosphorylase (decreased activity - less glucose produced)
  • Glycogen synthase (increased activity - more glycogen produced)
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15
Q

Describe the pathway inside the cell how epinephrine and glucagon work to activate glycogen phosphorylase?

Same process with glycogen synthase

A
  • Both bind to surface receptors
  • Activate Adenyl Cyclase
  • Raises cAMP
  • Activate protein kinase A
  • Phosphorylates intermediatw known as Glycogen phosphokinase A
  • Glycogen phosphokinase A then phosphorylates glycogen phosphorylase which breaks down glycogen
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16
Q

How does insulin work inside the cell to affect glycogen phosphorylase?

Same process with glycogen synthase

A
  • Binds to surface receptor
  • Activates tyrosine kinase
  • TK phosphorylates an enzyme called protein phosphatase 1
  • Protein phosphatase 1 (p) can then remove a phosphate group from GPKinase A, glycogen phosphorylase is then not active so you can then get glycogen breakdown
17
Q

How can glycogen phosphorylase be activated in the absence of hormones?

A

In muscles

  • Calcium/calmodulin complex activated by muscles can directly activate GPKinase A, can then phosphorylate glycogen phosphorylase
  • > Glycogen breakdown
18
Q

What is AMPs effect on Glucose and glycogen enzymes?

A

Activates Glycogen phosphorylase

-> Glucose

19
Q

What is the effect of ATP and Glucose on the glucose and glycogen enzymes?

A

Decreases the activity of glycogen phosphorylase

-> Glucose

20
Q

What is the effect of Glucose 6-P on the glucose and glycogen enzymes?

A

Activates/ increases activity of Glycogen synthase

- Glycogen

21
Q

What enzyme is defective in Von Gierke’s disease (1a)(Glycogen Storage Disease Type 1a)?

A

Glucose-6-phosphate deficiency (Type 1a)

22
Q

What is defective in Von Gierke’s disease (1b)(Glycogen Storage Disease Type 1b)?

A

Glucose transporter deficiency

23
Q

When and how does Von Gierke’s disease present?

A

Infancy (2-6 months of age)

  • Severe hypoglycemia between meals (lethargy, seizures, lactic acidosis)
  • Enlarged liver (excess glycogen) -> Liver failure
24
Q

What cycle becomes dysfunctional in those with Von Gierke’s disease?

A

Cori cycle (lactate cycle)

  • G6P cannot be converted to glucose in liver
  • Lactate builds up -> acidosis
25
Q

How is Von Gierke’s disease diagnosed?

A
  • DNA testing (preferred)

- Liver biopsy (historical test)

26
Q

How is Von Gierke’s disease treated?

A
  • Consumption of cornstarch between meals (glucose polymer)

- Avoidance of sucrose, lactose, fructose, galactose (worsen accumulation of G6P)

27
Q

What is the deficiency in Pompe’s disease (glycogen storage disease type II)?

A

Acid alpha-glucosidase deficiency (also lysosomal acid maltase)
- Accumulation of glycogen in lysosomes

28
Q

How does Pompe’s disease (glycogen storage disease type II) present?

A
  • Accumulation of glycogen in lysosomes
  • Classic form presents in infancy -> severe disease -> death in child/infant
  • Enlarged muscles
  • Cardiomegaly -> Death from HF
  • Enlarged tongue
  • Hypotonia
  • Hepatomegaly (from HF)
  • NO hypoglycemia / metabolic problems
29
Q

What is the deficiency in Cori’s disease (glycogen storage disease type III)?

A

Debranching enzyme

30
Q

How does Cori’s disease (glycogen storage disease type III) present?

A
  • Hypoglycemia/hepatomegaly in child/infant (not as severe as Von Gierke)
  • No lactic acidosis (Cori’s cycle intact)
  • Hypotonia/weakness - Muscle involvement (glycogen accumulation) (no
  • Gluconeogenesis intact
  • Possible cardiomyopathy w. hypertrophy
31
Q

What is the deficiency in McArdle’s disease (glycogen storage disease type V)?

A

Muscle glycogen phosphorylase deficiency

  • Myophosphorylase deficiency
  • Skeletal muscle has unique isoform of G-phosphorylase
  • Glycogen not properly broken down in muscle cells
32
Q

How does McArdle’s disease (glycogen storage disease type V) present?

A

Adolescence/ early childhood

  • Exercise intolerance, fatigue, cramps
  • Poor endurance, muscle swelling, and weakness
  • Myoglobinuria and C release (especially with exercise)
  • Urine may turn dark/red after exercise
  • Second wind in exercise
  • Flat venous lactate curve w. normal rise in ammonia levels during exercise
33
Q

What is the deficient enzyme in Andersen disease (type IV)?

A

Branching enzyme

- Neuromuscular form can present at any age

34
Q

How does Andersen disease (glycogen storage disease type IV) present?

A
  • Hepatosplenomegaly and failure to thrive in early infancy
  • Infantile cirrhosis
  • Musscle weakness
  • Hypotonia
  • Cardiomyopathy -> early childhood death

USMLE Biochemistry (44 decks)

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