1. Dr. Samah Kotb Nasr Eldeen

2. GLYCOGEN CATABOLISM CHAPTER 4

3. INTRODUCTION

4. DIGESTION & ABSORPTION OF CARBOHYDRATES The digestion of complex carbohydrates is by hydrolysis to liberate oligosaccharides, then free mono- and disaccharides.  Amylases Catalyze the hydrolysis of Starch The hydrolysis of starch by salivary and pancreatic amylases catalyze random hydrolysis of α(1→4) glycoside bonds, yielding dextrins, then a mixture of glucose, maltose, and isomaltose.  Disaccharidases (Brush Border Enzymes) The disaccharidases—maltase, sucrase-isomaltase, lactase, —are located on the brush border of the intestinal mucosal cells where the resultant monosaccharides and others arising from the diet are absorbed.

7. Dr Samah Kotb7

8. GLYCOGEN CATABOLISM

9. Glycogen Mobilization: Glycogenolysis Glycogenolysis is a catabolic process; the breakdown of glycogen to glucose units. Glycogen is principally stored in the cytosol granules of :- –Liver –Muscle

10. Liver Cell

11. Glycogen Function In liver – The synthesis and breakdown of glycogen is regulated to maintain blood glucose levels. In muscle - The synthesis and breakdown of glycogen is regulated to meet the energy requirements of the muscle cell.

12. Glucose 6-phosphate has 3 fates.

13. Glycogen phosphorylase catalyzes the breakdown of glycogen. Glycogen + Pi  Glucose 1- phosphate + glycogen ( n residues ) (n-1 residues)

14. Glycogen phosphorylase uses pyridoxal phosphate (PLP) a derivative of pyridixine (vitamine B 6 ) as a coenzyme. B 6 is required for the mobilization of glucose from glycogen. It is also required for other biochemical reactions such as transamination.

15. Phosphorylase is specific for the α-1,4 linkage. Two additional enzymes are required

16. Glycogen Catabolism Glucose from storage (or diet) α-Amylase is an endoglycosidase, It cleaves amylopectin or glycogen to maltose, maltotriose and other small oligosaccharides It is active on either side of a branch point, but activity is reduced near the branch points Debranching enzyme cleaves limit dextrins

18. Metabolism of Tissue Glycogen Tissue glycogen is an important energy reservoir - its breakdown is carefully controlled. Glycogen consists of high molecular weight granules. Glycogen phosphorylase cleaves glucose from the nonreducing ends of glycogen molecules. This is a phosphorolysis, not a hydrolysis. Metabolic advantage: product is a sugar phosphate The glucose 1-phosphate is converted to glucose 6- phosphate by phosphoglucomutase.

20. Hormonal Regulation of Glycogen Catabolism

21. Glucagon Stimulate Glycogen breakdown Liver is responsive to glucagon Glucagon signal, a cascade of molecular events leading to glycogen breakdown. It utilizes a G-protein-dependent signal-transduction pathway.

22. A few hormone molecules cause the release of large amounts of glucose, a cascade. Glucagon