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Glycogen is the body’s storage form of glucose, and glycogenesis is the genesis of glycogen. Glycogen is synthesized when the body has excess glucose, allowing the glucose to be saved for later.

Glycogen is stored as granules in the cytoplasm of cells. It is a branched polymer, and it’s composed of straight chains and branches. The straight chains are formed by glycogen synthase, which links glucose molecules together via alpha-1,4 bonds. The branch points are formed by glycogen branching enzymes, which links glucose molecules via alph-1,6 bonds.

There are two main places where glycogen is stored: 1) in the liver, which stores glycogen to maintain blood-glucose levels during periods of fasting and (2) in muscle, which uses stored glycogen to meet the energy demands of exercise.

Key Points

  • Glycogenesis
    • Etymology
      • Glycogenesis = glycose + genesis
        • Glycose = old term for glucose
        • Genesis = the generation / making of
    • Summary
      • Glycogenesis stores excess sugar as glycogen
        • Allows the organism to store up fuel to prepare for future energy needs
      • Glycogen is a storage form of glucose
        • Glycogen is a branched polymer
          • Glucose molecules are linked in straight chains by alpha-1,4 bonds
          • Branch points off of those straight chains are linked by alpha-1,6 bonds
    • Location
      • Glycogen is primarily found in the liver and in skeletal muscle
        • Liver: to maintain blood glucose levels
        • Skeletal muscle: to meet exercise demands
      • Glycogen is stored in the cytoplasm of cells as granules
    • Reaction steps
      •  glucose-6-phosphate →[phosphoglucomutase] glucose-1-phosphate
        • Phosphoglucomutase is an isomerase (Isomerization reaction)
        • starting substrate glucose-6-phosphate is produced by the first step of glycolysis
      • Glucose-1-phosphate + UTP → [UDP-glucose pyrophosphorylase] UDP-glucose + PPi
        • 1 UTP = 1 ATP (energetically)
          • This energy is used to produce high energy UDP-glucose
        • PPi (pyrophosphate) is quickly metabolized to form two Pi (inorganic phosphates)
          • This drives the reaction forward (Le Chatlier’s principle)
      • UDP-glucose → [glycogen synthase] glycogen alpha-1,4-bonds
        • Glycogen synthase ‘synthesizes’ straight chain alpha-1,4 bonds
        • Glycogen is synthesized from UDP-glucose--glucose that has been ‘charged’ with energy from UTP
        • Irreversible (main regulatory step)
      • Glycogen alpha-1,4-bond → [branching enzyme] glycogen alpha-1,6-bond
        • Branching enzyme catalyzes the formation of glycogen branches
          • Breaks alpha-1,4 straight chain bond
            • By hydrolysis
          • Attaches the fragment to the side of the chain with an alpha-1,6, bond
            • This creates a branch
    • Regulation
      • Regulated based on energy need
        • Glycogenesis occurs when there is excess energy/high blood glucose levels
      • Glycogen synthase reaction is the primary regulatory step
      • Glycogenesis speeds up (activation) 
        • low energy need
          • Glucose-6-phosphate, Insulin
            • Present when the body has plenty of high energy molecules
      • Glycogenesis slows down (inhibition) 
        • high energy need
          • Epinephrine, glucagon
            • present when the body is low on energy or needs more