Starch and Sucrose Metabolism


Carbohydrates are a major component of the human diet, and include starch (amylose and amylopectin) and disaccharides such as sucrose, lactose, maltose and, in small amounts, trehalose. The digestion of starch begins with the action of amylase enzymes secreted in the saliva and small intestine, which convert it to maltotriose, maltose, limit dextrins, and some glucose. Digestion of the limit dextrins and disaccharides, both dietary and starch-derived, to monosaccharides – glucose, galactose, and fructose – is accomplished by enzymes located on the luminal surfaces of enterocytes lining the microvilli of the small intestine. 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, which can then be converted to UDP-xylose or UDP-glucuronate and, eventually to glucuronate. UDP-glucose can also serve as a precursor to the synthesis of glycogen via glycogen synthase. More specifically, glycogen is synthesized from monomers of UDP-glucose by the enzyme glycogen synthase, which progressively lengthens the glycogen chain with (α1→4) bonded glucose. As glycogen synthase can only lengthen an existing chain, the protein glycogenin is needed to initiate the synthesis of glycogen. The glycogen-branching enzyme, amylo (α1→4) to (α1→6) transglycosylase, catalyzes the transfer of a terminal fragment of 6-7 glucose residues from a nonreducing end to the C-6 hydroxyl group of a glucose residue deeper into the interior of the glycogen molecule. The branching enzyme can only act upon a branch having at least 11 residues, and the enzyme may transfer to the same glucose chain or adjacent glucose chains. Another enzyme known as starch phosphorylase or glycogen phosphorylase can also convert starch into glycogen. Glycogen functions as the secondary short term energy storage in animal cells. It is made primarily by the liver and the muscles, but can also be made by glycogenesis within the brain and stomach. Glycogen is the analogue of starch, a less branched glucose polymer in plants, and is commonly referred to as animal starch, having a similar structure to amylopectin. Glycogen is found in the form of granules in the cytosol in many cell types, and plays an important role in the glucose cycle. Glycogen is cleaved from the nonreducing ends of the chain by the enzyme glycogen phosphorylase to produce monomers of glucose-1-phosphate that is then converted to glucose 6-phosphate (G6P). G6P can continue on the glycolysis pathway and be used as fuel or G6P can enter the pentose phosphate pathway via the enzyme glucose-6-phosphate dehydrogenase to produce NADPH and 5-carbon sugars or, in the liver and kidney, G6P can be dephosphorylated back to glucose by the enzyme glucose 6-phosphatase. This is the final step in the gluconeogenesis pathway.

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References

  1. Lehninger, A.L. (2005) Lehninger principles of biochemistry (4 th ed.). New York: W.H Freeman.
  2. Salway, J.G. (2004) Metabolism at a glance (3 rd ed.). Alden, Mass. : Blackwell Pub.