Biochemistry

Glycogen Structure and Synthesis

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ENDOGENOUS GLUCOSE POLYMERS

Glycogen

• Synthesized in liver & muscle (insulin: glucagon is high)
• Branched structure
• Linear segments: glucose monomers linked with alpha (1,4) glycosidic bonds
• Branch points: alpha (1,6) glycosidic bonds
• Branch point functions: i. solubilize glycogen ii. create terminal sugars for release
EXOGENOUS GLUCOSE POLYMERS
• Dietary

Amylopectin (starch)

• Fewer branches than glycogen
• Obtained from: potatoes, rice, etc.

Cellulose

• No branches
• Obtained from plants
• Glucose monomers linked with beta (1,4) glycosidic bonds
• Humans lack enzymes to break beta (1,4) glycosidic bonds

GLYCOGEN SYNTHESIS

  1. Glucose + ATP –> Glucose 6P + ADP
    • Hexokinase (M) and Glucokinse (L)
  2. Glucose 6P –> Glucose 1P (reversible)
    • Phosphoglucomutase
  3. Glucose 1P + UTP –> UDP-glucose + PPi
    • PPi + H2O –> 2Pi (drives reaction forward)
    • UDP-glucose = substrate for glycogen synthesis
  4. UDP-glucose + glycogen polymer –> glycogen polymer (+1 glucose residue) + UDP
    • Glycogen synthase: alpha (1,4) glycosidic bonds (adds 1 glucose-residue/rxn)
  5. Branching enzyme adds branches
    • Breaks off (at least) 6 terminal residues from linear portion to make branch
    • Catalyzes alpha (1,6) linkage

Glycogenin: primer for glycogen chain

• Catalyzes first 4-8 glucose residues
• First glucose binds tyrosine residue in glycogenin
• Glycogen synthase adds glucose residues to preexisting glucose polymer

CLINICAL CORRELATION

Type IV Glycogen Storage Disease: Anderson’s Disease

• Branching-enzyme deficiency
• Presents as long, linear polymers of glucose
• Visible at very young age, produces cell damage
• Aka amylopectosis (amylopectin ~ glycogen w/ less branching)

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