Glycogenolysis lec 8
- 2. DEGRADATION OF GLYCOGEN (GLYCOGENOLYSIS) • The degradative pathway that mobilizes stored glycogen in liver and skeletal muscle is not a reversal of the synthetic reactions. Instead, a separate set of cytosolic enzymes is required. When glycogen is degraded, the primary product is glucose 1-phosphate, obtained by breaking α(1→4) glycosidic bonds. In addition, free glucose is released from each α(1→6)-linked glucosyl residue.
- 3. A. Shortening of chains Glycogen phosphorylase sequentially cleaves the α(1→4) glycosidic bonds between the glucosyl residues at the nonreducing ends of the glycogen chains by simple phosphorolysis (producing glucose 1 phosphate) until four glucosyl units remain on each chain before a branch point
- 6. B. Removal of branches • Branches are removed by the two enzymic activities of a single bifunctional protein, the debranching enzyme. • First, oligo-α(1→4)→α(1→4)-glucan transferase activity removes the outer three of the four glucosyl residues attached at a branch. • It next transfers them to the nonreducing end of another chain, lengthening it accordingly. Thus, an α(1→4) bond is broken and an α(1→4) bond is made, and the enzyme functions as a 4:4 transferase. Next, the remaining single glucose residue attached in an α(1→6) linkage is removed hydrolytically by amylo-α(1→6)-glucosidase activity, releasing free glucose.
- 7. C. Conversion of glucose 1-phosphate to glucose 6-phosphate • Glucose 1-phosphate, produced by glycogen phosphorylase, is converted in the cytosol to glucose 6-phosphate. In the liver, glucose 6-phosphate is transported into the endoplasmic reticulum (ER) by glucose 6-phosphate translocase. There it is converted to glucose by glucose 6-phosphatase—the same enzyme used in the last step of gluconeogenesis. The glucose then moves from the ER to the cytosol.
- 8. D. Lysosomal degradation of glycogen A small amount (1–3%) of glycogen is continuously degraded by the lysosomal enzyme, α(1→4)-glucosidase (acid maltase). The purpose of this pathway is unknown. However, a deficiency of this enzyme causes accumulation of glycogen in vacuoles in the lysosomes, resulting in the serious glycogen storage disease Type II: Pompe disease
- 9. Assignment • Define glycogenesis and glycogenolysis? • What is the importance of glycogenesis and glycogenolysis?