Amylases are a group of isoenzymes that are able to hydrolyse the glycosidic bonds in starch. The importance of starch as an energy source throughout life makes amylases important enzymes in both animals and plants. In humans and other mammals, α-amylase is produced by the salivary glands and the pancreas and is an important digestive enzyme. Salivary and pancreatic secretions contain α-amylase, which hydrolyses various parts of amylose chains (starch) releasing shorter polysaccharide chains such as the disaccharide maltose and isomaltose, as well as longer trisaccharides. As well as α-amylase, plants contain β-amylase, a 1,4-α-D-glucan maltohydrase which is able to hydrolyse only the non-reducing ends of starch, releasing maltose and limit dextrins. During grain development in cereals, β-amylase is synthesised and accumulated in the endosperm of the grain where it is one of the major proteins.
β-amylase is present in some grains such as barley in both an active form, and a partially active form that shows reduced catalytic ability. At the end of the development period, the grain dries and up to 75% of the β-amylase becomes attached to the starchy endosperm via the production of disulphide bridges. As the starch grain matures, a larger part of the β-amylase becomes partially inactivated due to covalent attachment to the periphery of the starch granules. During germination it is likely that the bound β-amylase is released by reducing agents such as disulphide reductase, or by a proteolytic enzyme. During germination of the grains, starch stored in the endosperm is hydrolysed by α-amylase, β-amylase and α-glucosidase (maltase). While β-amylase is synthesised by the growing grain and accumulates in the endosperm prior to germination, both α-amylase and α-glucosidase only accumulate during germination of the grain.
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