Starch Synthesis

The primary carbohydrate used for energy storage in plants is starch, the equivalent of glycogen in animals. Both starch and glycogen are polymers of glucose, with different glycosidic linkages between glucose monomers and different degrees of branching. Alpha-amylose, one form of starch, has glucose subunits joined by alpha(1-4) glycosidic bonds and does not have branches. These alpha-glycosidic bonds give the polymer a helical structure that aggregates, forming insoluble starch granules. Another form of starch, amylopectin, is also composed of glucose units joined largely by alpha(1-4) glycosidic bonds, with the addition of branches at regular intervals. Glycogen is similar to amylopectin, but with a greater degree of branching. Cellulose, the most abundant biological material on earth, is also composed of glucose monomers, but joined by beta glycosidic bonds, giving it a straighter shape that packs closely and provides mechanical strength in wood. Advantages of starch over glucose for energy storage are that large polymers take less space, have less water associated, and have a lower osmotic pressure, making storage in the cell easier. The biosynthesis of starch originates with glucose 1-phosphate, which is derived from photosynthetic products. Glucose 1-phosphate is activated by ADP-glucose phosphorylase to form ADP-glucose. The enzyme starch synthase catalyzes the addition of glucose monomers onto the end of the growing starch polymer, with one ADP as a leaving group for each new glucose unit added onto the chain. One reason for keen interest in starch is that plant starch is a common component of the human diet. Digestion of starch begins in the mouth with salivary amylase and continues in the small intestine, releasing glucose for absorption. Starch is also used industrially for many applications. The possibility of producing customized starches through manipulation of starch synthesis has contributed to the significant interest in the biochemistry of this process.