8. POLYSACCHARIDASES
385
ase, which liberates the reducing groups in the «-configuration (20).
Since the most characteristic property of ß-amylase, besides producing
the Waiden inversion, is the liberation of reducing sugar, Ohlsson called
it saccharogenic amylase (21, 22).
3. Mechanism of ß-Amylase Action
ß-Amylase is capable of splitting only the penultimate «-1,4-glucosidic linkage from nonreducing chain ends (21, 22, 26, 121, 195b),
thus liberating maltose as the sole reducing sugar. Maltose appears in
the downward rotating ß-form because of the Waiden inversion occurring simultaneously with the cleavage of the bond. After the removal
of one molecule of ß-maltose from the end of the chain, the latter is
again available to further attack by /?-amylase. In this way, the substrate is broken down step by step from the nonreducing chain end
until completion (amylose) or until the enzyme encounters an obstacle,
such as a branching point or, in certain species of amylopectin, a phosphate ester group. The ß-amylolytic breakdown of amylopectin or glycogen leaves behind, therefore, a limit dextrin of high molecular weight
which is not further attacked by ß-amylase alone. In conjunction with
«-amylase or «-glucosidases, however, ß-amylase may act further upon
limit dextrin because new nonreducing «-1,4-glucosidic chain ends have
been created by those enzymes. Thus, the action of «-amylase supplements or potentiates that of ß-amylase.
The degradation of amylose, maltohexaose, or maltotetraose (195c)
by ß-amylase yields 100% maltose. Failure to arrive at complete degradation has frequently been reported in the literature. It may be due
either to contamination by branched polysaccharides or, more likely, to
the retrogradation of amylose. The latter phenomenon can be eliminated
by repeated rejuvenations (37). Peat et al. (196) claim that amylose
contains a certain number of bonds different from «-1,4-glucosidic linkages. According to these authors, ß-amylase action is arrested by those
undefined linkages, while the so-called Z-enzyme is believed to split the
unknown bonds.
4. Distribution of ß-Amylase
The most important sources of ^-amylase are higher plants. The
occurrence of this enzyme in microorganisms is much rarer than that of
«-amylase, and ß-amylases from animal sources are unknown.
a. ß-Amylase in Higher Plants
(1). Cereals. Although cereal malts, in particular barley malt, are
known to contain both «- and /?-amylases, ungerminated grains of bar-
385
ase, which liberates the reducing groups in the «-configuration (20).
Since the most characteristic property of ß-amylase, besides producing
the Waiden inversion, is the liberation of reducing sugar, Ohlsson called
it saccharogenic amylase (21, 22).
3. Mechanism of ß-Amylase Action
ß-Amylase is capable of splitting only the penultimate «-1,4-glucosidic linkage from nonreducing chain ends (21, 22, 26, 121, 195b),
thus liberating maltose as the sole reducing sugar. Maltose appears in
the downward rotating ß-form because of the Waiden inversion occurring simultaneously with the cleavage of the bond. After the removal
of one molecule of ß-maltose from the end of the chain, the latter is
again available to further attack by /?-amylase. In this way, the substrate is broken down step by step from the nonreducing chain end
until completion (amylose) or until the enzyme encounters an obstacle,
such as a branching point or, in certain species of amylopectin, a phosphate ester group. The ß-amylolytic breakdown of amylopectin or glycogen leaves behind, therefore, a limit dextrin of high molecular weight
which is not further attacked by ß-amylase alone. In conjunction with
«-amylase or «-glucosidases, however, ß-amylase may act further upon
limit dextrin because new nonreducing «-1,4-glucosidic chain ends have
been created by those enzymes. Thus, the action of «-amylase supplements or potentiates that of ß-amylase.
The degradation of amylose, maltohexaose, or maltotetraose (195c)
by ß-amylase yields 100% maltose. Failure to arrive at complete degradation has frequently been reported in the literature. It may be due
either to contamination by branched polysaccharides or, more likely, to
the retrogradation of amylose. The latter phenomenon can be eliminated
by repeated rejuvenations (37). Peat et al. (196) claim that amylose
contains a certain number of bonds different from «-1,4-glucosidic linkages. According to these authors, ß-amylase action is arrested by those
undefined linkages, while the so-called Z-enzyme is believed to split the
unknown bonds.
4. Distribution of ß-Amylase
The most important sources of ^-amylase are higher plants. The
occurrence of this enzyme in microorganisms is much rarer than that of
«-amylase, and ß-amylases from animal sources are unknown.
a. ß-Amylase in Higher Plants
(1). Cereals. Although cereal malts, in particular barley malt, are
known to contain both «- and /?-amylases, ungerminated grains of bar-