8. POLYSACCHARIDASES
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substrate with regard to molecular weight, its degree of branching, to
a lesser degree its tendency to retrogradation (39), and, finally, the
presence of unbranched impurities. The latter have a higher tendency
to retrogradate than has amylopectin and, therefore, escape degradation; they then cause the iodine-staining property of the mixture to disappear more slowly than that of pure amylopectin during a-amylatic
breakdown. Compared to the a-amylatic degradation of amylose, amylopectin is more slowly attacked by this enzyme (39), and the reaction
slows down when 50 to 55% of the glucosidic bonds are split. The end
products of exhaustive degradation by swine α-amylase of corn amylopectin with 4% branching points are 19% glucose, 73% maltose, and
8% isomaltose, whereas malt α-amylase leaves somewhat less glucose
(13.5%) and larger amounts of a-l,6-glucosidic bonds containing dextrins (16.2%), having an average degree of polymerization of 2.8 (39).
Potato amylopectin containing 0.4% glucose-6-phosphate (40) yields
13% glucose, 68% maltose, and 19% of branched dextrins with an average
chain length of 6.2. It is obvious that the end products of α-amylatic degradation of glycogen are richer in branched dextrins and contain less
glucose and maltose than the products of amylopectin degradation.
Whelan and Roberts (38) found after exhaustive breakdown of liver
glycogen by salivary amylase the following split products: maltose
(4.57 X 10~
3 moles); maltotriose (2.25 X 10
-3 moles); and dextrins containing a-l,6-branch linkages, but no tetrasaccharide. The dextrins were
characterized as consisting of a mixture of a total of eight different
penta-, hexa-, hepta-, and octasaccharides, each containing one a-1,6glucosidic link.
e. a-Amylatic Degradation of Starch Pastes. Starch pastes and soluble starch are essentially mixtures of amylose and amylopectin. Whereas a solution of soluble starch behaves as such, the enzymatic degradation of starch pastes represents a special case owing to the extremely
high viscosity of this material and to its peculiar colloidal structure.
The phenomena occurring during swelling of starch granules, and the
existence in a starch paste of giant particles containing huge numbers
of amylopectin and amylose molecules held together by crystalline
regions, has been discussed by K. H. Meyer (41, 42). The action of
α-amylase on these giant particles produces a dramatic decrease of the
viscosity, since the rupture of only minute numbers of bonds therein
drastically reduces the particle size while the number of reducing end
groups is not yet measurably increased.
Once these large particles have been broken down to the size of
molecules, and the texture of the starch granule has disappeared, the
363
substrate with regard to molecular weight, its degree of branching, to
a lesser degree its tendency to retrogradation (39), and, finally, the
presence of unbranched impurities. The latter have a higher tendency
to retrogradate than has amylopectin and, therefore, escape degradation; they then cause the iodine-staining property of the mixture to disappear more slowly than that of pure amylopectin during a-amylatic
breakdown. Compared to the a-amylatic degradation of amylose, amylopectin is more slowly attacked by this enzyme (39), and the reaction
slows down when 50 to 55% of the glucosidic bonds are split. The end
products of exhaustive degradation by swine α-amylase of corn amylopectin with 4% branching points are 19% glucose, 73% maltose, and
8% isomaltose, whereas malt α-amylase leaves somewhat less glucose
(13.5%) and larger amounts of a-l,6-glucosidic bonds containing dextrins (16.2%), having an average degree of polymerization of 2.8 (39).
Potato amylopectin containing 0.4% glucose-6-phosphate (40) yields
13% glucose, 68% maltose, and 19% of branched dextrins with an average
chain length of 6.2. It is obvious that the end products of α-amylatic degradation of glycogen are richer in branched dextrins and contain less
glucose and maltose than the products of amylopectin degradation.
Whelan and Roberts (38) found after exhaustive breakdown of liver
glycogen by salivary amylase the following split products: maltose
(4.57 X 10~
3 moles); maltotriose (2.25 X 10
-3 moles); and dextrins containing a-l,6-branch linkages, but no tetrasaccharide. The dextrins were
characterized as consisting of a mixture of a total of eight different
penta-, hexa-, hepta-, and octasaccharides, each containing one a-1,6glucosidic link.
e. a-Amylatic Degradation of Starch Pastes. Starch pastes and soluble starch are essentially mixtures of amylose and amylopectin. Whereas a solution of soluble starch behaves as such, the enzymatic degradation of starch pastes represents a special case owing to the extremely
high viscosity of this material and to its peculiar colloidal structure.
The phenomena occurring during swelling of starch granules, and the
existence in a starch paste of giant particles containing huge numbers
of amylopectin and amylose molecules held together by crystalline
regions, has been discussed by K. H. Meyer (41, 42). The action of
α-amylase on these giant particles produces a dramatic decrease of the
viscosity, since the rupture of only minute numbers of bonds therein
drastically reduces the particle size while the number of reducing end
groups is not yet measurably increased.
Once these large particles have been broken down to the size of
molecules, and the texture of the starch granule has disappeared, the