Thursday, April 23, 2015

Salivary amylase and starch

Salivary amylase is a digestive enzyme secreted by the salivary glands, and about 1.7 liters of saliva is produced everyday by a healthy human. It is responsible for breakdown of the polysaccharide “starch” into dextrins (oligosaccharides), maltose (disaccharide) and glucose (monosaccharide) in the mouth.

Starch consists of two types of molecules, “amylose” (20-30%) and “amylopectin” (70-80%) and both are polymers of glucose. Amylose is a linear chain of glucose units that are linked by α-1,4 bonds, whereas amylopectin is a branched molecule in which glucose units are linked by α-1,4 bonds in the straight chain region in addition to α-1,6 bonds at the branching points. The relative proportions of amylose to amylopectin depend on the source of the starch.

Branch of amylopectin

Salivary amylase hydrolyses only the α-1,4 bonds that link glucose units. Therefore, starch and glycogen, which consist of α-1,4 bonds, are hydrolyzed by amylase while. The cellulose consisting of β-1,4 bonds, cannot be hydrolyzed by salivary amylase. The α -amylase constitute a family of endo-amylases that catalyze the initial hydrolysis of starch into shorter oligosaccharides through the cleavage of α-D-(1-4) glycosidic bonds. Neither terminal glucose residues nor α-1,6-linkages can be cleaved by α-amylase.

The conformation of (α-1→4) linkages in amylose, amylopectin causes these polymers to assume coiled helical structures. The iodine ion will be trapped in the coiled helical structures of starch and gives blue color. After digestion of starch by salivary amylase, no more helical structures available, and no color will be produced in addition of iodine. Therefore iodine can be taken as indicator to measure the activity of amylase, and sometimes, to find the inhibitor of amylase.

Nelson DDL, Cox MM (2005) Lehninger Principles of Biochemistry, 5th edition, W.H. Freeman and Co. Publisher, New York, USA, 248-252.

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