Starch is a homopolysaccharide formed from two different types of glucose polymers, amylose and amylopectin. Plants use starch as a means to organize and store glucose(energy) molecules within plant cells. Amylose is a polymer of glucose molecules linked by alpha 1-4 bonds and is unbranched. The molecular weight of amylose ranges from thousands to millions. Amylopectin is also linked by alpha 1-4 bonds but it is a branched polymer, with branches occurring at every 24-30 glucose molecules by alpha 1-6 linkages. Starch is often more abundant in the roots of plants. Because of the abundance of available OH functional groups, starch has a lot of water molecules bound to it within the plant cells. Amylose and amylopectin bind together in a double helical structure forming starch granules. The double helical structure formed in starch is stabilized by hydrogen bonds, and has six glucose molecules per turn and there is a 60 degree turn between each residue. Because amylose is unbranched, the structure is tightly formed so that x ray diffraction/crystallization can be used for structure determination, and iodine coloring tests can be used to identify it.

Carbohydrates are produced from photosynthesis. Because more carbs are produced than the plant requires for energy usage, an excess amount is produced. The plants store this excess of carbohydrates as sucrose and starch. For temporary storage, the storage site is the chloroplasts, and for longer term storage, the storage site is the amyloplasts. Amyloplasts are found in roots and seeds of plants, therefore these parts of plants hold large amounts of starch.

Synthesis

Inorganic pyrophosphatase binds glucose 1- phosphate to ATP to form glucose-ADP in a condensation reaction.

Starch synthase binds the glucose ADP molecule to the reducing end of the existing starch molecule. Starch synthase has two active sites.

Chloroplasts contain another enzyme that creates branches in the starch molecules.

The enzymes within chloroplasts (aldolase, fructose 1,6 bisphosphatase, phosphohexose isomerase, and phosphoglucomutase) convert the trios phosphates into starch. Aldolase produces fructose 1, 6 bisphosphate, fructose 1,6 bisphosphatase creates fructose 6 phosphate, phosphohexose isomerase produces glucose 6 phosphate, and phosphoglucomutase creates glucose 1 phosphate, in that order.

Starch synthase forms a covalent bond to the reducing end of the starch molecule, and the second active site binds to the glucose-ADP molecule. The enzyme catalyzes the nucleophilic attack of the C-4 hydroxyl (OH) group on the glucose molecule in the first active site on the C-1 of the glucose in active site two. When this occurs, the second active site remains attached to the now attached glucose molecules, and the first active site releases its glucose, and accepts another glucose from another glucose-ADP molecule. Then, the hydroxyl group of this newly accepted molecule in the first active site takes place in a nucleophilic attack, bonding to and displacing the other, now growing glucose polymer from the second active site, with the growing polymer now remaining bound to the first active site. As you can see, the active sites rotate and this process repeats to synthesize starch.

Degradation in plants

Starch phosphorylase breaks down starch by attaching a phosphate group to the non reducing end of the glucose chains that make up starch. This creates a glucose-1-phosphate molecule and the remaining, now shorter, starch polymer. This process repeats until a branch is encountered, at which point a debranching enzyme breaks the alpha 1-6 glucose branching bonds. The glucose-1-phosphate produced from this process is then converted to glucose-6-phosphate by phosphoglucomutase.

Digestion in Humans

Humans break down starch with enzymes called amylases. Amylases use hydrolysis to cleave the alpha 1-4 bonds that hold the starch together. Amylases are found in human saliva and are secreted by the pancreas into the gastrointestinal tract to breakdown complex sugars such as starch and glycogen enabling them to be absorbed into the bloodstream.

Industry

When starch is in the presence of warm water, the granules dissociate and it forms a vicious fluid mixture. This mixture is used in a variety of industrial processes including as a thickening and stiffening agent in liquids that lack the appropriate texture. Upon cooling or standing, the starch begins to reform itself, expelling water from its networks. This is known as syneresis.

In the pharmaceutical industry starch is used as a disintegrant, a diluent, and a binder. It helps to aid in the break up of the tablet, it adds bulk to the tablet, and to increase cohesion in a mixture holding it together.

Starch is used in alcohol fermentation, sugar production, food production, and paper production for its adhesive properties.


Sources

1. Brown, William Henry, Brent L. Iverson, Eric V. Anslyn, and Christopher S. Foote. Organic Chemistry. Australia: Cengage Learning, 2009. Print.

2. Nelson, David L., and Michael M. Cox. Lehninger Principles of Biochemistry. New York: W.H. Freeman, 2008. Print.

Allen, Loyd V., and Howard C. Ansel. Ansel's Pharmaceutical Dosage Forms and Drug Delivery Systems (10th Edition). Baltimore, MD: Wolters Kluwer Health, 2014. Print.