Action of Exo-Acting 3-Amylases

The (3-amylases catalyze starch hydrolysis by a mechanism that gives inversion of configuration at the anomeric center. All known (3-amylases have an exo-mechanism and act on the non-reducing ends of starch polymer chains or starch polymer-derived chains. There are two general classes of (3-amylases, those that are classically known as (3-amylases and produce (3-maltose, and those that are known as glucoamylases and produce (3-D-glucose. 

Maltose (3-amylases are primarily found in plants and have been isolated from sweet potatoes,27 soybeans,28 barley29 and wheat.30 Maltose (3-amylases are also elaborated by bacteria, e.g. by Bacillus polymyxa,31 B. megaterium,32 B. cereus33 and Pseudomonas sp. BQ6.34 These (3-amylases all produce (3-maltose and a high molecular weight (3-limit dextrin. The limit dextrins result when the enzyme reaches an a-(l— -6) branch linkage, which it cannot pass. Approximately half of an amylopectin molecule is converted to (3-maltose the remaining half is the (3-limit dextrin. 

Glucoamylases have been found to have live to seven glucosyl unit-binding subsites with the catalytic groups located between the first and second subsites from the 

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