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Gastrointestinal System > Glycogen/GSD - Abali 3/8/16 > Flashcards

Glycogen/GSD - Abali 3/8/16 Flashcards

1
Q

glycogen basics

A

storage form of glucose

glucose molecules attached in numerous chains to a core protein, glycogenin - base for elongating/degrading chains

glycogenolysis: first means of maintaining blood sugar

next step: gluconeogenesis (*remember: always running in the background to clear lactate from RBCs)

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2
Q

fuel sources within the body

  • where do you find glycogen?
A

RBCs (exclusive) and brain (gl + ketones in starvation) NEED GLUCOSE as fuel at all times

1. proteins : breakdown of sk muscle → fuel gluconeogenesis

2. fats : useful for ATP production, not for gluconeogenesis

3. glycogen (small store)

  • liver - used to regulate blood glucose levels
  • skeletal muscle - used as fuel for muscles only (no G6Pase = can’t form, secrete glucose for use by other tissues)
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3
Q

maintenance of blood glucose

A

carried out by hepatic glycogen

  • as post-prandial glucose levels drop, glycogenolysis and background gluconeogenesis pick up to compensate and maintain blood glucose
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4
Q

cardiac muscle and glycogen

A

cardiac muscle has v little glycogen

  • uses primarily FA beta ox for energy
  • doesn’t get much benefit from glycolysis

consequence: anything that interrupts flow of oxygenated blood to heart leads to serious damage/death

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5
Q

lactic acidosis and uric acid

A

in lactic acidosis, body will try to get rid of buildup via URAT1 transporter (organic anions - lactate - out, urate in) in kidneys

consequence: buildup of uric acid

*can also happen in ketoacidosis!

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6
Q

glucose polymers

A

starch: least branched, alpha 1:4 glycosidic bonds

cellulose: beta 1:4 glycosidic bonds

glycogen: most branched, alpha 1:4 glycosidic bonds

*glycogen and amylopectin (form of starch) also contain occasional alpha 1:6 bonds at branch points holding two chains together

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7
Q

why is glycogen a good source of energy?

why is it a better fuel reserve than fats?

A
  • can be rapidly broken down into glucose
    • enzymes of glycogen metab are bound to the surface
    • many terminal glucoses particles = lots of potential to release glucose
  • don’t need oxygen to generate energy (can take glucose through anaerobic resp)

why glycogen > fats for rapid energy?

  • fats need oxygen for beta ox
  • fats have to be mobilized from adipose tissue to wherever you need energy
  • brain needs glucose → animals can’t turn fats into glucose
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8
Q

steps of glycogenesis

A
  1. trap glucose: glucose → glucose-6-P
    * glucokinase [liver], hexokinase [everywhere else]
  2. transitions: G6P → G1P → UDP-glucose
    * phosphoglucomutase; G1P uridyl transferase
  3. glycogen synthase goes to work: strips glucose from UDP-glucose, adds it to an alpha 1:4 chain on glycogen (seeded on glycogenin core)
    * glycogen synthase
  4. branching: if alpha1:4 chain is at least 11 residues long, at least 6 of those residues can be shifted and made into a new branch
    * branching enzyme
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9
Q

steps of glycogenolysis

A
  1. 4trimming: glycogen branches stripped down to branches of 4 units each. released as G1P → isomerized into G6P → glucose [via G6Pase]
    * glycogen phosphorylase
  2. debranching-step1: blocks of 3 residues are shifted from one terminal branch to another
    * debranching enzyme
  3. debranching-step2: the single remaining residue on the cut branch (the alpha 1:6 moiety) is removed → linear chain of alpha 1:4 linkages. released as glucose
    * debranching enzyme

implication: wayyyy more alpha 1:4 linkages than alpha 1:6s → much more glucose will be released through glycogen phosphorylase activity (90%) than debranching enzyme activity (10%)

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10
Q

glycogenolysis: fate of glycogen in the liver

A

the liver expresses G6Pase

  • can modify G6P → glucose for release into bloodstream for brain/RBCs
  • allows liver to regulate blood sugar levels
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11
Q

glycogenolysis: fate of glycogen in the skeletal muscle

A

skeletal muscle DOES NOT EXPRESS G6Pase

  • G6P is stuck in cells and sent into glycolysis → pyruvate
    • if anaerobic: pyruvate → lactate [lactate DH]
    • if aerobic: pyruvate → acetyl CoA [PDH] → TCA cycle
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12
Q

glycogen metabolism regulation: basics

A

1. allosteric regulation

2. hormonal regulation

  • glucagon, epinephrine : activate glycogenolysis
  • insulin
    • shuts down glycogenolysis (dephosphorylates/deactivates glycogen phosphorylase)
    • triggers glycogenesis (dephosphorylates/activates glycogen synthase)

3. neuronal regulation

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13
Q

allosteric regulation of glycogen metabolism

A

+ = glycogenesis: G1P → glycogen

[via glycogen synthase]

  • = glycogenolysis: glycogen → G1P

[via glycogen phosphorylase]

LIVER

+ : G6P [buildup from import, leftover from glycolysis]

- : glucose, ATP, G6P [“endpdts” of glycogen phosphorylase]

*general rule: high energy inhibits buildup

MUSCLE

+ : G6P [buildup from import, leftover from glycolysis]

- : ATP, G6P; Ca, AMP

*general rule: high energy inhibits buildup

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14
Q

covalent regulation of glycogen metabolism: epinephrine

A

receptors in liver and muscle

activates cAMP → PKA pathway → phosphorylates target

  • P’s glycogen synthase: deactivates it
  • P’s glycogen phosphorylase: activates it
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15
Q

covalent regulation of glycogen metabolism: glucagon

A

receptors in liver ONLY

activates cAMP → PKA pathway → phosphorylates target

  • P’s glycogen synthase: deactivates it
  • P’s glycogen phosphorylase: activates it
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16
Q

regulation of glycogen metabolism: Ca

A

Ca is elevated in active muscle

activates phosphorylase kinase

  • P’s glycogen phosphorylase: activates it

takeaway: high Ca leads to glycogenolysis

17
Q

what causes upregulation of glycogenolysis?

A

acute and chronic stress trigger glycogenolysis

  1. physiologic (increased use of blood glucose during exercise)
  2. pathologic (blood loss/shock)
  3. psychological (acute/chronic threats)
18
Q

4 hormones [sources] : triggers → effect on glycogenolysis

A

acute and chronic stress trigger glycogenolysis

  1. glucagon [pancreatic alpha cells] : hypoglycemia → rapid activation
  2. epinephrine [adrenal medulla] : acute stress, hypoglycemia → rapid activation
  3. cortisol [adrenal cortex] : chronic stress → chronic activation
  4. insulin [pancreatic beta cells] : hyperglycemia → inhibition
19
Q

LIVER

fasting state

[blood] → tissue response

A

[glucagon high, insulin low]

→

glycogenolysis high,

glycogenesis low

20
Q

LIVER

carb meal

[blood] → tissue response

A

[glucose high: insulin high, glucagon low]

→

glycogenolysis low,

glycogenesis high

21
Q

LIVER

exercise/stress

[blood] → tissue response

A

[glucose high, epi high]

→

glycogenolysis high

22
Q

MUSCLE

fasting

[blood] → tissue response

A

[insulin low]

→

glucose transport low

glycogenesis low

23
Q

MUSCLE

carb meal

[blood] → tissue response

A

[insulin high]

→

glucose transport high

glycogenesis high

24
Q

MUSCLE

exercise/stress

[blood] → tissue response

A

[epi high; tissue levels of AMP high]

→

glycogenesis low

glycogenolysis high

glycolysis high

25
Q

key enzymes in glycogen metabolism

A
  1. glucose 6 phosphatase : converts G6P → glucose for export
  2. alpha 1,6 glucosidase (aka debranching enzyme)
  3. glycogen phosphorylase : breaking down alpha 1:4 glycosidic linkages
  4. branching enzyme
26
Q

PAS-D staining

A

periodic acid-Schiff-diastase

PAS stains glycogen

diastase = alpha amylase, which breaks glycogen down

PAS-diastase staining allows you to see glycogen stores by showing you a before/after of degradation

27
Q

von Gierke disease

type I

A

glucose 6 phosphatase deficiency

enzyme affected : G6Pase

glycogen : normal

tissues affected : liver, kidney

  • inability to export glucose from gluconeogenesis or glycogenolysis → severe hypoglycemia

tx : frequent feeding with carbs (uncooked starch), possibly nasogastric tube to feed while sleeping

28
Q

Pompes disease

type II

A

alpha 1,4 glucosidase deficiency

enzyme affected : alpha 1,4 glucosidase (aka acid maltase)

glycogen : accumulates in lysosome due to inability to degrade

tissues affected : heart

  • accumulation of glycogen in cardiac tissue leads to LVH, cardiomegaly
  • typically, death before 2
  • might also see accumulation/weakness in muscle

tx : recombo alpha 1,4 glucosidase given to pt might help some symptoms

29
Q

Coris disease

type III

A

alpha 1,6 glucosidase deficiency

[debranching enzyme]

enzyme affected : alpha 1,6 glucosidase (aka debranching enzyme)

glycogen : shorter branches, impeded glycogenolysis

tissues affected : liver

  • hepatomegaly
  • hypoglycemia
30
Q

Andersens disease

type IV

A

alpha 4,6 glucosidase deficiency

[branching enzyme]

enzyme affected : alpha 4, 6 glucosidase (aka branching enzyme)

glycogen : no branches : long, insoluble chains

tissues affected : liver

  • hepatomegaly
  • cirrhosis (presents as infantile cirrhosis → failure to thrive, death)
31
Q

McArdles disease

type V

A

glycogen phosphorylase (muscle isozyme) deficiency

enzyme affected : glycogen phosphorylase (muscle)

glycogen : normal, but accumulates

  • can’t be broken down for use in anaerobic glycolysis; dependent on circulating glucose or eventual FA beta ox

tissues affected : muscle

  • decreased exercise tolerance w/ muscle cramps
  • myoglobinuria
32
Q

Hers’ disease

type VI

A

glycogen phosphorylase (liver isozyme) deficiency

enzyme affected : glycogen phosphorylase (liver)

glycogen : normal, but accumulates

  • can’t be broken down for blood glucose regulation

tissues affected : liver

  • hepatomegaly
  • fasting hypoglycemia
    • why only fasting? you still have gluconeogenesis runnning in background!
33
Q

Taruis disease

type VII

A

muscle PFK1 deficiency

enzyme affected : phosphofructokinase (muscle)

glycogen : normal

tissues affected : muscle

  • decreased exercise tolerance
  • myoglobinuria
  • hemolytic anemia
34
Q

causes of hyperuricemia

A
  1. increased lactate/other anion being secreted → antiport-ing of urate through URAT-1
  2. increased G6P can lead to increase in purine synth through HMP pathway → uric acid
  3. decreased hepatic concentration of P (inhibitor of AMP deaminase) → increased purine catabolism, producing uric acid
  • P would typically be released by G6Pase, of which there is none in von Gierkes disease
  • no G6Pase → no P → no inhibition of AMP deaminase (rate limiting enzyme for A nt → uric acid) → more uric acid
35
Q

von Gierke’s disease : buildup of TAGs and cholesterol

A

lack of G6Pase forces the G6P released through glycogenolysis to go through glycolysis

→ buildup of acetyl CoA

→ increase TAG synth

→ increase synth/secretion of VLDL

Gastrointestinal System (35 decks)

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