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Alpha-amylase (α-Amylase, E.C.3.2.1.1) is an enzyme that catalyzes the hydrolysis of internal α-1,4-glycosidic linkages in starch and glycogen, yielding such glucose, maltose and maltotriose units. It is the major form of amylase found in humans and other mammals. It is also secreted by many fungi, and present in seeds containing starch as a food reserve. Amylases are among the most important enzymes and are of great significance for biotechnology, constituting a class of industrial enzymes having approximately 25% of the world enzyme market, which have potential application in a wide number of industrial processes such as food, fermentation, textile, paper, detergent, and pharmaceutical industries.
Sources
Amylases are widespread in animals, fungi, plants, and are also found in the unicellular eukaryotes, bacteria and archaea. Ptyalin, a salivary α-amylase (α-1,4-α-D-glucan-4-glucanohydrolase) is one of the most important enzymes in saliva. It is known to be mainly involved in the initiation of the digestion of starch in the oral cavity. Fungal sources of α-Amylase are confined to terrestrial isolates, mostly to Aspergillus species and to only few species of Penicillium, P. brunneum being one of them. The fungal source used predominantly for commercial production of α-Amylase are the strains of Aspergillus spp. Aspergillus oryzae, A. niger and A. awamori are most commonly used species for commercial production among several others. α-Amylase can be produced by different species of bacteria, but for commercial applications α-amylase is mainly derived from the genus Bacillus. α-Amylases produced from Bacillus licheniformis, Bacillus stearothermophilus, and Bacillus amyloliquefaciens find potential application in a number of industrial processes such as in food, fermentation, textiles and paper industries. Plant sources had not been considered with enough significance as the source of these enzymes yet.
Structure
Despite wide difference of microbial α-amylases characters, their molecular weights are usually in the same range 40-70 kDa while the highest molecular weight of α-amylases, 210 kDa, for Chloroflexus aurantiacus. Whereas, 10 kDa of Bacillus caldolyticus α-amylase was reported to be the lowest value. α-amylases from different organisms share about 30% amino acid sequence identity and all belong to the same Glycosyl Hydrolase family 13 (GH-13 family of protein).
The amylase has a three-dimensional structure capable of binding to substrate and, by the action of highly specific catalytic groups, promote the breakage of the glycoside links. The human α-amylase is a classical calcium-containing enzyme composed of 512 amino acids in a single oligosaccharide chain with a molecular weight of 57.6 kDa. The protein contains 3 domains: A, B, and C. These domains are generally found on all α-amylase enzymes. The A domain constitutes the core structure, with a (β/α)8-barrel.The B domain consists of a sheet of four anti-parallel β-strands with a pair of anti-parallel β-strands. Long loops are observed between the β-strands. Located within the B domain is the binding site for Ca2+-Na+-Ca2+. Domain C consisting of eight β-strands is assembled into a globular unit forming a Greek key motif. It also holds the third Ca2+ binding site in association with domain A. Positioned on the C-terminal side of the β-strands of the (β/α)8-barrel in domain A is the active site.
Production
Submerged fermentation (SmF) and solid state fermentation (SSF) are two main methods used for production of α-Amylase on a commercial scale. SmF employs free flowing liquid substrates, such as molasses and broths. The products yielded in fermentation are secreted into the fermentation broth. The substrates need to be constantly replenished because the substrates are utilized quite rapidly. This fermentation technique is suitable for microorganisms that require high moisture content for their growth, such as bacteria. SmF is primarily used for the extraction of secondary metabolites that need to be used in liquid form. SSF is a method used for microbes which require less moisture content for their growth. The solid substrates commonly used in this method are, bran, bagasse, and paper pulp. The main advantage is that nutrient-rich waste materials can be easily recycled and used as substrates in this method. Therefore, SSF is considered as a promising method for commercial production of enzymes.
Industrial Application
The most widespread applications of α-amylases are in the starch industry, which are used for starch hydrolysis in the starch liquefaction process that converts starch into fructose and glucose syrups. The use of enzymes in detergents formulations enhances the detergents ability to remove tough stains and making the detergent environmentally safe. Amylases are the second type of enzymes used in the formulation of enzymatic detergent, and 90% of all liquid detergents contain these enzymes. Textile industries are extensively using alpha amylases to hydrolyze and solubilize the starch, which then wash out of the cloth for increasing the stiffness of the finished products. More than 70% bread in U.S.A, Russia and European countries contain alpha amylase. Amylases play important role in bakery products. These enzymes can be added to the dough of bread to degrade the starch in the flour into smaller dextrins, which are subsequently fermented by the yeast. The addition of α-amylase to the dough results in enhancing the rate of fermentation and the reduction of the viscosity of dough, resulting in improvements in the volume and texture of the product.
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