Gluconeogenesis, the Pentose Phosphate Pathway and Glycogen Metabolis

GLYCOGEN METABOLISM

• Glycogen stored in muscle and liver cells.
• Important in maintaining blood glucose levels.
• Glycogen structure: ? 1,4 glycosidic linkages with ? 1,6 branches.
• Branches give multiple free ends for quicker breakdown or for more places to add additional units.

Glycogen Degradation

• Glucose residues of starch and glycogen released through enzymes called starch phosphorylases and glycogen phosphorylases.
• Catalyze phosphorolosis:

polysaccharide +Pi ---> polysaccharide(n-1) + glucose 1-phosphate

• Pyridoxal phosphate (PLP) is prosthetic group in active site of enzyme; serves as a proton donor in active site.
• Allosterically inhibited by high [ATP] and high [glucose 6-phosphate].
• Allosterically activated by high [AMP].
• Sequentially removes glucose residues from nonreducing ends of glycogen, but stops 4 glucose residues from branch point --> leaves a limit dextran.
• Limit dextran further degraded by glycogen-debranching enzyme (glucanotransferase activity) which relocated the chain to a free hydroxyl end.
• Amylo-1,6-glucosidase activity of debranching enzyme removes remaining residues of chain.
• This leaves substrate for glycogen phosphorylase.
• Each glucose molecule released from glycogen by debranching enzyme will yield 3 ATPs in glycolysis.
• Each glucose molecule released by glycogen phosphorylase will yield 2 ATPs in glycolysis.
• Why?