THESIS

4.1.3 Enzymes specific for both of the α-(1,4) and α-(1,6) linkages
The amyloglucosidases (E.C. 3.2.1.3) are those exoenzymes that
hydrolyze both types of linkages in α-glucan chains, releasing glucose in β- anomery.
These enzymes, also called glucoamylase, glucamylase, or γ-amylase, are mainly
produced by microorganisms, especially by molds such as Aspergillus, Penicillium,
and Rhizopus (Hayashida et al. 1990).
Structure and properties: Amyloglucoxidases from molds are
proteins whose molecular weight varies widely from 27,000 to 112,000, depending on
their origin. The amino acid sequences of several glucoamylases (Aspergillus niger,
Aspergillus phoenicis, and Endomycopsis sp.) are know, and although the results vary
greatly with enzyme source, some characteristics stand out. The amyloglucosidases
generally contain methionine and tryptophan residues, and half a cystein residue.
Amyloglucosidases are chiefly made up to isoenzymes, I and II, which differ in their
capacity to hydrolyze starch in solid form, and in their stability.
Thus amyloglucosidase I adsorb to and hydrolyze solid starch. In
contrast, amyloglucosidase II exhibits neither of these two properties (Hayashida et
al. 1990). Aspergillus enzyme is highly recommended for structural analyses because
it lacks α-amylase. Two or three isoforms have been found in several enzymes of
fungal origin, and the isoform with the highest molecular weight exhibits high rawstarch-digesting
activity because it has a starch-binding domain. The enzyme is used
for the determination of starch because it hydrolyzes starch specifically and
commercial preparations from Rhizopus contain weak α-amylase and completely
hydrolyze starch (Manelius et al. 1997).
The pH and temperature optimum: The properties of the
amyloglucosidase are a function of enzyme origin. Their pH optima lie between 4.5
and 5.5, and their temperature optima between 40 and 60 o C. The presence of
oligosaccharudes in the medium stabilizes the enzymes. In contrast, the presence of
Ca 2+ ions inhibits them, and favors their denaturation (Table 10) (Abe et al. 1988).
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