The Enzymatic Digestion Of Starch (part I) And The Biogenesis Of Muscle Glycogen (part Ii)

Date of Award




Degree Name

Doctor of Philosophy (Ph.D.)


Biochemistry and Molecular Biology


In the first part of this thesis, the characterization of the pig intestinal sucrase-isomaltase complex and its separate subunits was performed using highly purified (alpha)-limit dextrins. A pathway for the complete digestion of starch to glucose was discovered using the combination of pancreatic (alpha)-amylase, sucrase-isomaltase and limit dextrinase (another intestinal enzyme). The (alpha)-amylase hydrolyzes the starch chains to the vicinity of the branch points ((alpha)-1,6 links) forming (alpha)-limit dextrins and maltose. The (alpha)-limit dextrins are hydrolyzed by the limit dextrinase to the 6('3)-(alpha)-D-glucosylmaltotriose (a tetrasaccharide) and glucose. The isomaltase activity then hydrolyzes the (alpha)-1,6 linkage in this tetrasaccharide and converts it to glucose and maltotriose. The maltose and maltotriose formed are completely hydrolyzed to glucose by the sucrose and/or limit dextrinase activities.In the second part of this thesis, a covalently bound protein of Mr 40,000 (+OR-) 2,000 was found attached to rabbit muscle glycogen via a novel tyrosine-glucose linkage. This protein has been named glycogenin and there is one glycogenin molecule per glycogen molecule. A protein thought to be free glycogenin was found to become glucosylated when muscle extract was incubated with UDP ('14)C glucose. The presence of glycogenin in glycogen and the fact that the linkage is via tyrosine raises the possibility that the action of insulin in stimulating glycogen synthesis may be exerted via the synthesis or "activation" of glycogenin. This is inferred from the fact that insulin promotes tyrosine phosphorylation by causing its cell-surface receptor to become an active tyrosine kinase. Given that muscle glycogen is constructed on glycogenin, then the amount of the latter may determine the amount of glycogen that the cell can lay down, introducing a new dimension in the regulation of glycogen metabolism.A novel phosphodiester group has been found attached to muscle glycogen via its glucose residues. The structure and significance of this phosphodiester is still under investigation.


Chemistry, Biochemistry

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