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Cellular Biochemistry and Metabolism ( CLS 333 ). Dr. Samah Kotb Nasr Eldeen. Glycogen Catabolism. introduction. DIGESTION & ABSORPTION OF CARBOHYDRATES The digestion of complex carbohydrates is by hydrolysis to liberate oligosaccharides, then free mono- and disaccharides.
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Cellular Biochemistry and Metabolism(CLS 333 ) Dr. Samah Kotb Nasr Eldeen
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.
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
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.
Glycogen phosphorylase catalyzes the breakdown of glycogen. Glycogen + Pi Glucose 1- phosphate + glycogen (n residues) (n-1 residues)
Glycogen phosphorylase uses pyridoxal phosphate (PLP) a derivative of pyridixine (vitamine B6) as a coenzyme. B6 is required for the mobilization of glucose from glycogen. It is also required for other biochemical reactions such as transamination.
Phosphorylase is specific for the α-1,4 linkage. Two additional enzymes are required
Glycogen Catabolism • Glucose from storage (or diet) • α-Amylase is an endoglycosidase, It cleaves amylopectin or glycogen tomaltose, maltotriose and other smalloligosaccharides • It is active on either side of a branch point,but activity is reduced near the branchpoints • Debranching enzyme cleaves limit dextrins
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.
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.
Glucagon A few hormone molecules cause the release of large amounts of glucose, a cascade.