Carbohydrates are a major component of the diet, and include starch (amylose and amylopectin) and disaccharides such as sucrose, lactose, maltose and, in small amounts, trehalose. Once released from starch or once ingested, sucrose can be degraded into beta-D-fructose and alpha-D-glucose via lysosomal alpha-glucosidase or sucrose-isomaltase. Beta-D-Fructose can be converted to beta-D-fructose-6-phosphate by glucokinase and then to alpha-D-glucose-6-phosphate by the action of glucose phosphate isomerase. Phosphoglucomutase 1 can then act on alpha-D-glucose-6-phosphate (G6P) to generate alpha-D-glucose-1-phosphate. alpha-D-Glucose-1-phosphate (G6P) has several possible fates. It can enter into gluconeogenesis, glycolysis, or the nucleotide sugar metabolism pathway. UDP-glucose pyrophosphorylase 2 can convert alpha-D-glucose-1-phosphate into UDP-glucose, UDP-glucose can then be used to produce D-glucose via trehalose. UDP-glucose can also serve as a precursor to the synthesis of glycogen via glycogen synthase. Glycogen is a starch analogue commonly called an animal starch. Glycogen is found in the cytosol in granules. Glycogen is cleaved and converted to glucose-6-phosphate (G6P) which undergoes glycolysis or can enter the pentose phosphate pathway.
References
Starch and Sucrose Metabolism References
Farkas I, Hardy TA, Goebl MG, Roach PJ: Two glycogen synthase isoforms in Saccharomyces cerevisiae are coded by distinct genes that are differentially controlled. J Biol Chem. 1991 Aug 25;266(24):15602-7.
Henrissat B, Deleury E, Coutinho PM: Glycogen metabolism loss: a common marker of parasitic behaviour in bacteria? Trends Genet. 2002 Sep;18(9):437-40.
Lazarowski ER, Shea DA, Boucher RC, Harden TK: Release of cellular UDP-glucose as a potential extracellular signaling molecule. Mol Pharmacol. 2003 May;63(5):1190-7.
Mu J, Cheng C, Roach PJ: Initiation of glycogen synthesis in yeast. Requirement of multiple tyrosine residues for function of the self-glucosylating Glg proteins in vivo. J Biol Chem. 1996 Oct 25;271(43):26554-60.
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