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Updated: May 1 2020

Glycogen

  • Overview
    • Structure
      • glycogen
        • polymer of glucose
          • straight chain with α-1,4-bond
          • branches with α-1,6-bond
      • glycogen granule
        • core
          • glycogenin
    • Function
      • energy reserves that can provide glucose during a fast or ↑ energy demand
        • supplies exhausted in < 24 hrs
      • stored mainly in the liver and muscle
        • muscle does not have glucose-6-phosphatase so it cannot release free glucose
          • stores for its own consumption
        • liver does have glucose-6-phosphatase so it can release free glucose
          • can use supplies to maintain blood glucose levels
  • Glycogenesis
    • Glycogen synthesis
    • Pathway
      • glucose-6-phosphate converted to glucose-1-phosphate
      • UDP group added to form UDP-glucose
      • UDP-glucose added to polymer in an α-1,4 linkage
        • catalyzed by glycogen synthase
          • rate limiting step of glycogen synthesis
      • polymer rearranged to create α-1,6 linked branches
        • catalyzed by branching enzyme
          • deficiency = Anderson disease
    • Regulation
      • glycogen synthase
        • in liver
          • activated by insulin
          • inhibited by glucagon, epinephrine
        • in muscle
          • activated by insulin
          • inhibited by epinephrine
  • Glycogenolysis
    • Glycogen catabolism
    • Pathway
      • glucose-glucose bond broken by addition of a phosphate
        • catalyzed by glycogen phosphorylase
          • rate limiting step of glycogenolysis
            • hepatic deficiency = Hers disease (type VI)
            • muscle deficiency = McArdle disease (type V)
      • glucose-1-phosphate freed
        • converted to glucose-6-phosphate
      • debranching enzymes removes α-1,6 linked branches
        • deficiency = Cori's disease (type III)
      • liver converts glucose-6-phosphate to glucose
        • catalyzed by glucose-6-phosphatase
          • deficiency = von Gierke disease (type I)
      • muscle puts glucose-6-phosphate into glycolysis
    • Regulation
      • glycogen phosphorylase
        • in liver
          • activated by epinephrine, glucagon
            • via cAMP/protein kinase A
          • inhibited by insulin
          • remember: exact opposite of glycogen synthase
            • hepatic glycogen regulatory processes both turn on forward direction and turn off reverse
        • in skeletal muscle
          • activated by epinephrine, AMP, Ca2+
          • inhibited by insulin, ATP
          • remember: since muscular glycogen can only supply itself, it is regulated by its own energy supply (AMP/ATP ratio); while liver must supply energy to many other tissues, it functions independently of AMP/ATP ratio in hepatocytes
  • Glycogen Storage Diseases (Glycogenolyses)
    • Overview
      • all disorders have abnormal glycogen metabolism
        • leads to an accumulation of glycogen within cells
        • organ dysfunction
      • remember: disorders numbered in order of pathway from end (glucose release) to beginning (breakdown of glycogen polymer)
    • Glucose release
      • Type I: von Gierke
        • lacks glucose-6-phosphatase
        • presentation
          • liver cannot release stored glucose
            • hepatomegaly
            • severe hypoglycemia
          • body must rely on fat/protein catabolism for energy
            • hyperlipidemia
            • hyperuricemia
            • lactic acidosis
          • normal glycogen structure
        • tests
          • stimulation test with glucagon, fructose, galactose
          • does not ↑ serum glucose
    • Lysosomal pathway
      • Type II: Pompe "trashes the Pump (heart)"
        • lacks lysosomal α1,4-glucosidase
          • degrades glycogen-resembling material in endosomes
        • presentation
          • buildup of glycogen in cardiac muscle
            • electron dense granules inside lysosomes
            • cardiomegaly
            • hypertrophic cardiomyopathy
    • Branching/debranching
      • Type III: Cori
        • lacks debranching enzyme
          • remember:
            • Cori = can't Catabolize branches
            • 6-pack core - alpha 1,6 glucosidase defective
        • presentation
          • liver cannot break down glycogen past a branch point
            • hepatomegaly
            • hypoglycemia
            • abnormal glycogen structure
              • short outer glycogen chains
      • Type IV: Anderson
        • lacks branching enzyme
          • remember: Anderson = can't Add branches
        • presentation
          • liver cannot form branched glycogen granules
            • hypotonia
            • cirrhosis
    • Phosphorylase
      • Type V: McArdles
        • lacks muscle phosphorylase
          • remember: McArdles = Muscle
          • can't breakdown glycogen to glucose-1-phosphate
          • accumulation of glycogen in muscle tissue
        • presentation
          • muscle weakness/cramps upon exertion
          • myoglobinuria
          • normal glycogen structure
      • Type VI: Hers
        • lacks hepatic phosphorylase
          • remember: Hers = Hepatic
        • presentation
          • hepatomegaly
          • fasting hypoglycemia
            • can be mild due to gluconeogenic compensation
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