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Biochem Block 2 > Glycogen > Flashcards

Glycogen Flashcards

1
Q

Glucose is preferred energy source for…

A

Brain and RBC

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2
Q

Where is glucose obtained from?

A

Diet
GNG
Glycogen storage

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3
Q

Dietary intake of glucose

A
  • sporadic
  • dependent on diet content
  • not always a reliable source
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4
Q

GNG glucose supply

A
  • can provide sustained synthesis of glucose

- is somewhat slow in responding to falling blood glucose levels

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5
Q

Glycogen storage supply of glucose

A

-mechanisms for storing a supply of glucose in a rapidly metabolized form

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6
Q

Places you can find glycogen

A
  • virtually any cell is capable of containing glycogen, but it VERY SMALL AMOUNTS
  • Skeletal muscle(uses it as own fuel source, cannot exit into the blood stream)
  • Liver(releases it into the blood to maintain blood glucose levels)
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7
Q

Glycogen distribution in liver

A
  • 100g

- 10% of the fresh weight of an adult well fed liver

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8
Q

Glycogen distribution in skeletal muscle

A
  • 400g

- 1-2% of the fresh weight of resting muscle

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9
Q

Water and glycogen

A
  • glycogen storage is associated with water storage
  • water has 5X the weight of glycogen
  • your weight can vary significantly based on the amount of glycogen you have stored
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10
Q

degradation of glucose (liver and muscle cells)

A
  • liver: when not getting glucose from diet, glycogen is degraded to glucose and released from liver and kidney(very small amount from kidney)
  • muscle:in exercising muscle, glycogen is degraded to provide that tissue with energy
  • glycogen serves as a glucose source in the gap between the fall of glucose levels after a meal and the onset of GNG(few hours)
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11
Q

Glycogen structure

A
  • HIGHLY branched polysaccharide made EXCLUSIVELY from alpha D glucose
  • 10-40,000 glucose molecules per one glycogen
  • primary bond is alpha 1-4 linkage(linear)
  • every 8-10 glucosyl residues, there is a branch with alpha 1-6 linkage
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12
Q

Discrete cytoplasmic granules

A
  • beta particles
  • associated with ALL the enzymes necessary for degradation and synthesis of glycogen
  • all enzymes are associated all the time
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13
Q

Step 1: Synthesis of Uridine diphosphate glucose

A
  1. alpha D glucose —>glucose 6P SUPER FAST
  2. glucose 6P—>Glucose 1P
  3. UTP—->UDP
  4. UDP glucose (highly energetic carrier to transport glucose)
    - if sufficient energy, it will save glucose for later
    - highly exergonic(drives the rest of synthesis)
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14
Q

Step 2: Synthesis of a primer to initiate glycogen Synthesis

A
  1. glycogen synthase CANNOT add UDP-glucose to a single glucose molecule, it can only ELONGATE primers
  2. no primers? glycogenin serves as primer
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15
Q

Glycogenin

A
  • protein that can serve as primer if there are no primers available
  • specific tyrosine residue serves as attachment point for glycogen synthesis
  • catalyzes(acts as enzyme) the attachment reaction and the attachment for the next few UDP-glucose molecules via 1-4 glycosidic bonds
  • after it binds 4, its a primer so glycogen synthase can take over
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16
Q

Step 3: elongation of glycogen chains

A

Enzyme: glycogen synthase

  • elongates existing primers
  • transfers UDP-glucose to NON REDUCING end of primer
  • forms alpha 1-4 bonds ONLY between UDP and primer
  • rate limiting
17
Q

Step 4: Formation of branches

A

Enzyme: branching enzyme

  • removes chain of 6-8 glucosyl residues from the end of glycogen chain(breaks 1-4 bond)
  • attaches it to non terminal glucosyl residue by 1-6 bond
  • functions as 4:6 transferase
18
Q

Why are branches important?

A
  1. increases solubility of glycogen

2. increases surface area to make more reducing ends for faster degradation and synthesis

19
Q

Glycogenolysis

A

-degradation of glycogen
-NOT A REVERSAL OF GLYCOGENESIS
-gets its own set of enzymes(still on beta particles)
-4 step pathway
Shortening of chain
removal of branches
glucose1P—->glucose 6P
glucose 6p—->glucose(ONLY IN LIVER)

20
Q

Step 1 of degradation: shortening of chain

A

Enzyme: glycogen phosphorylase

  • rate limiting step
  • tissue specific isoforms for liver, muscle, and brain
  • cleaves alpha 1-4 bonds from ends of chains
  • uses INORGANIC PI to cleave AND attach itself to glucose
  • yields glucose 1P
  • stops when chain is only 4 glucosyl units from branch point
  • requires PLP(vitamin B6) as coenzyme
21
Q

Limit dextrins

A
  • branch point

- once it is down to 4 units, it does not fit in active site of enzyme anymore, so shortening of chains done

22
Q

Step 2 of degradation: removal of branches

A

Enzyme: debranching enzyme

  • single protein, 2 activities
    4: 4 transferase activity: removes 3 of the 4 glucosyl residues by breaking 1-4 bond and transfers the to end of another chain by 1-4 bond

1:6 glucosidase activity: removes remaining residue attached by 1-6 bond and creates free glucose!

23
Q

Step 3 of degradation: Conversion of Glucose 1P to glucose 6P

A

Enzyme: phosphoglucomutase

  • forms intermediate glucose 1-6 bisphosphate
  • activated by glucose 1-6 bisphosphate
  • ser-phosphorylation allows to put another on, then release ser and you only have one P
24
Q

Step 4 of degradation: glucose 6P—> glucose

A

Enzyme: Glucose 6-phosphatase

  • liver and kidney
  • sometimes in beta pancreatic cells, but much less
  • ER transmembrane with active site facing ER lumen
  • complex (3 transporters and catalytic subunit)
  • G6PT1,2,3 and G6Pase alpha or beta
25
Q

degradation in lysosomes

A

Enzyme: lysosomal alpha 1-4 glucosidase

  • housekeeping gene(always expressed)
  • regulated at the level of protein expression
  • optimal pH=4.5(acidic)
  • only 1-3% of glycogen
  • purpose of this pathway is not understood, but when it is missing, it is no bueno
26
Q

regulation of glycogenesis and glycogenolysis in liver

A
  • glycogenesis is up in well fed state

- glycogenolysis is up during fasting

27
Q

Regulation of glycogenesis and glycogenolysis in muscles

A
  • glycogenesis is up at rest

- glycogenolysis is up during exercise

28
Q

Hormonal regulation

A

to meet the needs of the body as a whole

29
Q

Allosteric regulation

A

to meed the needs of a particular tissue only

30
Q

glycogen phosphorylase in liver

A
  • activated by epinephrine and glucagon
  • inactivated by insulin
  • inactivated by glucose6P,glucose, and ATP
31
Q

glycogen phosphorylase in muscle

A
  • activated by epinephrine, high AMP and Ca2+

- inhibited by insulin and high ATP and Glucose 6P

32
Q

Glycogen synthase in liver and muscles

A
  • both activated by insulin and Glucose 6P
  • muscle inhibited by epinephrine
  • liver inhibited by epinephrine and glucagon
33
Q

Von Gierke

A

-deficient: glucose 6-phosphatase
-Location: liver and kidnet
-Features: fasting, lactic acidosis, hypoglycemia, glucose cannot leave cell(goes to synthesize fats)
Structure:normal

34
Q

Pompe

A

Deficient: Lysosomal alpha 1-4 glucosidase

  • Location: can effect all cells
  • Features: muscle weakness, death at 2, cardiomegaly
  • Structure: glycogen like material in inclusions
35
Q

Cori

A
  • Deficient: Branching enzyme
  • location: yea idk
  • Features: mild hyperglycemia, liver enlargement
  • Structure: short outer branches, single glucose residue at outer branch
36
Q

Andersen

A

Deficient: Branching enzyme (super rare)

  • Location: synthesis of glycogen
  • Features: infantile hypotonia, cirrhosis, death at 2
  • Structure: very few branches, less soluble
37
Q

Mcardle

A
  • Deficiency: Muscle glycogen phosphorylase
  • Location:muscles only.(liver isoform is still gucci)
  • Features: muscle cramps, weakness, myoglobinuria
  • Structure: normal
38
Q

Hers

A
  • Deficiency: hepatic glycogen phosphorylase
  • Location: liver only(muscle isoform is still gucci)
  • Structure: Normal

Biochem Block 2 (11 decks)

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