Amylase, Beta

1-4-α-D-Glucan maltohydrolase

β-Amylase is an exoenzyme that releases successive maltose units from the nonreducing end of a polysaccharide chain by hydrolysis of α-1,4-glucan linkages. The shortest normal saccharide attacked is maltotetraose (Myrback and Neumuller 1950). Since it is unable to bypass branch linkages in branched polysaccharides such as glycogen or amylopectin, the hydrolysis is incomplete and a macromolecular limit dextrin remains.

β-Amylase is found primarily in the seeds of higher plants and sweet potatoes. It yields a single product: maltose. The enzyme is useful in structural studies of starch and glycogen. Marshal and Whelan (1973) report on the removal of any contaminating β-glucosidase.

Characteristics of β-amylase from Sweet Potato:

Molecular weight: 206,000 ± 1,000 (Colman and Matthews 1971).

Composition: The enzyme is tetrameric with molecular symmetry (Colman and Matthews 1971). Thoma et al. (1965) report on the amino acid composition. See also Uehara et al. (1970). It has been indicated that tryptophan residues are involved at the active center (Uehara et al. 1971). The four binding sites are independent of each other.

Optimum pH: 4-5.

Extinction coefficient: = 17.7 (Takeda and Hizukuri 1969).

Specificity: β-Amylase is specific for amylose chains of six glucose units (Kainuma and French 1970). See also Lee (1971). Thoma et al. (1971) has reviewed substrate structural requirements of both α- and β- amylases. See Kainuma and French (1970) for comparison of specificity with hog pancreas α-amylase, and also the review by French (1960).

Inhibitors: The enzyme is sulfydryl sensitive, and inhibited by heavy metal ions, p-mercuribenzoate, iodoacetamide and urea. Ascorbate inhibition has been attributed to cupric ion reduction and subsequent formation of an inactive cuprous enzyme complex (Rowe and Weill 1962). Cyclohexaamylose is a competitive inhibitor.