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Lecture32-2007 - Major fates of glucose in cells Glycogen...

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1 Glycogen Metabolism Lecture 32 Chapter 15, Sections 1-3 Major fates of glucose in cells: Fatty Acids Glycogen structure: Polymer of glucose: (Glc- ! 1,4-Glc) n ! 1,6 branches about every 10 ! 1,4-linked Glc. Glucoses added and removed from non-reducing ends. Reducing end attached in O-linkage to a Tyr residue on the protein glycogenin. Reducing Group Monosaccharides that can exist in open-chain form can reduce copper: Provides orientation to polysaccharides, much like N- and C-terminus of proteins.
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2 Glycogenin Tyr O Branching increases the number of non-reducing ends. The more non-reducing ends, the faster glucose can be added or removed. Glycogen exists as granules in the cytoplasm of cells: ! = granule " = one glycogen molecule Associated enzymes Glycogen Function • Sites: Liver: up to 10% of wet weight after large meal. Muscle: only 2% of wet weight, but largest store of glycogen in body. • Purpose: Liver: short-term (18-24 hours) storage form of glucose. Helps liver maintain blood glucose at 5 mM during short-term fasts. Muscle: required for short-term bursts of activity. When muscles perform lactic acid fermentation, large amounts of glucose are consumed. Separate pathways for glycogen synthesis and degradation:
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3 Glycogen Phosphorylase Catalyzes phosphorolysis of glycogen: Glycogen n + Pi <=> Glycogen n-1 + Glucose1P Removes single ! 1,4-Glc from non-reducing end. Does NOT cleave ! 1,6-Glc. Requires pyridoxal phosphate (PLP). # G o ’ = ~0, but [Pi] >> [G1P], so driven forward by mass action. Irreversible in cells.
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