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Biochem BLOCK 2 > glycogen > Flashcards

glycogen Flashcards

1
Q

What is the only organ that can make glucose?

A

liver

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2
Q

what is the preferred energy source for the brain and required for RBCs?

A

glucose

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3
Q

constant levels of blood glucose

A

absolute requirement

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4
Q

what is essential for exercising muscle?

A

glucose

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5
Q

Where is glucose obtained from?

A
  1. diet
  2. GNG (fasting)
  3. Glycogen stores (quick energy)
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6
Q

Acetyl CoA

A

CANNOT make glucose

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7
Q

dietary intake of glucose

A
  • sporadic
  • dependent on the diet content
  • not always reliable source of blood glucose
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8
Q

GNG and glucose

A
  • can provide sustained synthesis of glucose

- BUT it is somewhat slow in responding to a falling blood glucose levels

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9
Q

glycogen and glucose

A

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

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10
Q

glycogen tissue distribution

A

-virtually any cell in the body may contain glycogen (high levels can be damaging)

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11
Q

Main stores of glycogen

A

skeletal muscle

liver

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12
Q

glycogen in skeletal muscle

A
  • for its own use as a fuel source
  • Can never exit skeletal muscle
  • 400g
  • 1-2% fresh weight of resting muscle
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13
Q

glycogen in liver

A
  • maintain blood glucose during early fasting
  • goal is to be released to maintain blood glucose levels
  • 100g
  • 10% of fresh weight of an adult well-fed liver
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14
Q

hepatocytes

A

will contain more glycogen than skeletal muscle

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15
Q

glycogen storage diseases

A

amount of glycogen stored in liver and skeletal muscle can be significantly higher or lower

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16
Q

absence of glucose

A
  • glycogen is degraded to glucose and it rapidly released from liver and kidney glycogen
  • muscle glycogen is extensively degraded in exercising muscle to provide that tissue with an important energy source
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17
Q

as glycogen stores are depleted

A

the synthesis of glucose (gluconeogenesis) takes over, therefore glycogen serves as a glucose source in the gap between the fall of blood glucose hours after a meal and the onset of gluconeogenesis which needs time to kick in and occurs a few hours later

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18
Q

glycogen structure

A
  • branched chain polysaccharide made from a-D-glucose (linear)
  • 10-40x10^3 glucose molecules in 1 glycogen granule
  • primary glycosidic bond is a(1,4) linkage
  • branch containing a(1,6) linkage every 8-10 glucose residues
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19
Q

discrete cytoplasmic granules (B-particles)

A

large molecules of glycogen

-associated with the enzymes necessary for the synthesis and degradation

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20
Q

glycogenesis

A

synthesis of glycogen

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21
Q

step 1 of glycogensis

A

synthesis of uridine diphosphate glucose

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22
Q

step1: synthesis of uridine diphosphate glucose

A
  • a-D-glucose goes to glucose 6-P with enzyme hexo/glucokinase
  • glucose 6-P goes to glucose 1-P with enzyme phosphoglucomutase
  • UTP cleaves a phosphate and becomes UDP glucose
  • highly endergonic
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23
Q

What does hexo/glucokinase do to a-D glucose

A

turns it to glucose 6-P and traps it in the cell

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24
Q

What drives the synthesis of UDP?

A

that fact that it’s highly exergonic

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25
Q

step 2 of glycogenesis

A

synthesis of a primer to initiate glycogen synthesis

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26
Q

glycogen synthase

A
  • CAN NOT add UDP glucose to a single glucose molecules

- It can ONLY ELONGATE existing glycogen molecules (primers)

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27
Q

if no glycogen primers are available

A
  • protein glycogenin serves as a primer
  • specific Tyr residue serves as attachment point for glycogen synthase
  • glycogenin itself catalyzes this attachment reaction and the attachment of next few UDP-glucose molecules via a(1,4) glycosidic bond
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28
Q

step 3 of glycogenesis

A

elongation of glycogen chains

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29
Q

step 3: elongation of glycogen chains

A

-enzyme: glycogen synthase

30
Q

glycogen synthase

A
  • elongates the existing glycogen primers
  • transfer UDP-glucose to the non-reducing end of the primer
  • forms a(1,4) glycosidic bonds ONLY between C-1 of UDP glucose and C-4 from the primer
  • rate limiting enzyme
31
Q

What bonds does glycogen synthase form?

A

a(1,4) glycosidic bonds ONLY between C-1 of UDP glucose and C4 from the primer

32
Q

What kind of bonds does glycogenic form between the UDP glucose molecules?

A

a(1,4)glycosidic bonds

33
Q

What is the primary glycosidic bond of glycogen?

A

a(1,4) linkage

34
Q

What type of bond is there every 8-10 glucose residues on glycogen?

A

branch containing a(1,6) linkage

35
Q

step 4 of glycogenisis

A

formation of branches

36
Q

Step 4: formation of branches

A
  • enzyme: branching enzyme (amylo…)
  • removes a chain of 6-8 glucose residues from the end of the glycogen chain (breaks an a(1,4) bond)
  • attaches it to an non-terminal glucose residue by an a(1,6) bond
  • functions as a 4:6 transferase
37
Q

What does the branching enzyme function as?

A

4:6 transferase

38
Q

degradation of glycogen (glycogenolysis)

A
  • NOT a simple reversal of the glycogenesis
  • separate set of enzymes
  • 4-step pathway
39
Q

4 step pathway of glycogenolysis

A
  1. shortening of chains
  2. removal of branches
  3. conversion of glucose 1-P to glucose 6-P
  4. conversion of glucose-6-P to glucose (LIVER ONLY)
40
Q

step 1 of glycogenolysis

A

shortening of the chains

41
Q

shortening of the chains

A
  • step 1 of glycogenolysis

- enzyme glycogen phosphorylase

42
Q

glycogen phosphorylase

A
  • step 1 of glycogenolysis
  • rate limiting regulatory step
  • tissue specific isoforms: liver, brain, muscle
  • sequentially cleaves a(1,4) glycosidic bonds from the ends of the glycogen chains
  • uses inorganic Pi to cleave the bond and simultaneously attaches it to the glucose
  • yields glucose 1-P
  • stops when the chain has been shortened to 4 remaining glucosyl units from a branch point
  • Requires PLP (derived from it B6) as a coenzyme
43
Q

limit dextrins

A
  • shortening of chains for glycogenolysis stops here.
  • 4 remaining glucosyl units
  • cannot fit very well with the active center of enzyme at this point
44
Q

Step 2 of glycogenolysis

A

removal of branches

enyme is debranching enzyme

45
Q

debranching enzyme

A
  • removal of branches
  • single protein with 2 activities (2 different active sites)
  • 4:4 transferase activity
  • 1:6 glucosidase activity

(HAPPEN SIMULTANEOUSLY)

46
Q

4:4 transferase activity of debranching enzyme

A
  • removes 3 of the 4 glucosyl residues at the end of a chain breaking an a (1,4) bond
  • transfers them to the end of another chain creating an a(1,4) linkage
47
Q

1:6 glucosidase activity of debranching enzyme

A
  • removes the remaining single glucose residue attached via a(1,6) linkage at the branch point
  • yields free glucose (no Pi)
48
Q

Step 3 og glycogenolysis

A

conversion of glucose 1-P to glucose 6-P

enzyme is phosphoglucomutase

49
Q

phosphoglucomutase

A
  • enzyme for conversion of glucose 1-P to glucose 6-P
  • forms intermediate glucose-1,6-bisP
  • activated by glucose 1-6-bis-P
  • modified by Ser-phosphorylation at the catalytic site
50
Q

step 4 of glycogenolysis

A
  • dephosphorylation of glucose 6-P to glucose

- enzyme is glucose 6-phosphatase

51
Q

glucose 6-Phosphatase

A
  • dephosphorylation of glucose 6-P to glucose
  • expressed in liver and kidney cortex
  • expressed lesser in the pancreatic B-cells and intestinal mucosa
  • ER transmembrane protein with active sites facing ER lumen
  • a complex of multiple component proteins
52
Q

glucose 6-Phosphatase complex

A
  • complex of multiple component proteins
    • 3 transporters: G6PT1, 2, 3
    • catalytic subunit- G6Pase-a-B
53
Q

degradation of glycogen in the lysosomes

A
  • lysosomal a(1,4)-glucosidase
  • product of housekeeping gene
  • regulated at the level of protein expression
  • optimal pH=4.5
  • only about 1-3% of the glycogen
  • purpose of this pathway not well understood
54
Q

regulation of glycogenesis in tissues

A

liver- in well fed state

muscle- begins at rest

55
Q

regulation of glycogenolysis in tissues

A

liver-during fasting

muscle- during exercise

56
Q

What are regulatory enzymes of glycogenesis and glcogenolysis?

A

glycogen synthase and glycogen phosphorylase

57
Q

how is enzyme regulation in glycogenesis and glycogenolysis accomplished at 2 levels?

A
  1. hormonal regulation to meet the needs of the body as a whole
  2. Allosteric regulation to meet the needs of the particular tissue
58
Q

What is glycogen phosphorylase activated by in the liver?

A

hormonal

  • epinephrine
  • glucagon

Allosterically
-none

59
Q

what is glycogen phosphorylase activated by in the muscle?

A

homronal
-epinephrine

allosterically

  • AMP
  • Ca2+
60
Q

What is glycogen phosphorylase inhibited by in the liver?

A

hormonal
-insulin

allosterically

  • glucose 6-P
  • glucose
  • ATP
61
Q

What is glycogen phosphorylase inhibited by in the muscle?

A

hormonal
-Insulin

allosterically

  • Glucose 6-P
  • ATP
62
Q

What is glycogen synthase activated by in the liver?

A

hormona
-insulin

allosterically
-glucose 6-P

63
Q

What is glycogen synthase activated by in the muscle?

A

hormonal
-insulin

allosterically
-glucose 6-P

64
Q

What is glycogen synthase inhibited by in the liver?

A

hormonal

  • glucagon
  • epinephrine

allosterically
-none

65
Q

What is glycogen synthase inhibited by in the muscle?

A

hormonal
-epinephrine

allosterically
-none

66
Q

Why are branches important?

A
  • increases solubility
  • increases surface area
    • more enzymes
    • faster degradation
    • more nonreducing ends for faster synthesis and degradation
67
Q

Von Gierke

A

deficient enzyme
-glucose 6-phosphatase (liver, kidney)

clinical features

  • severe fasting hypoglycemia
  • lactic acidosis
  • hepatomegaly
  • hyperlipidemia
  • hyperurecemia
  • short stature

glycogen structure
-normal

68
Q

Pompe

A

deficient enzyme
-lysosomal a(1-4)-glucosidase

clinical features

  • cardiomegaly
  • muscle weakness
  • death by 2 years

Glycogen structure
-glycogen-like material in inclusions

69
Q

Cori

A

deficient enzyme
-debranching enzyme

clinical features

  • mild hypoglycemia
  • liver enlargement

glycogen structure
-short outer branches, single glucose residue at outer branch

70
Q

Anderson

A

deficient enzyme
-branching enzyme

clinical features

  • infantile hypotonia
  • cirrhosis
  • death by 2 years
  • rare

glycogen structure
-very few branches, especially towards periphery

71
Q

McArdle

A

deficient enzyme
-muscle glycogen phophorylase

clinical features

  • muscle cramps and weakness on exercise
  • myoglobinuria

glycogen structure
-normal

72
Q

Hers

A

deficient enzyme
-hepatic glycogen phophorylase

clinical features

  • mild fasting hypoglycemia
  • hepatomegaly
  • cirrhosis

glycogen structure
-normal

Biochem BLOCK 2 (14 decks)

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