8. POLYSACCHARIDASES
415
polysaccharides as food, have to rely for this purpose on their intestinal
fauna and flora.
It can further be concluded from the above considerations that the
distribution in nature of the polysaccharides is completely unrelated
to the distribution of their specific hydrolytic enzymes. This is also
particularly evident in the case of dextran, which is synthesized exclusively by, and which occurs solely in, certain bacteria, whereas
dextranase is found in molds and even in mammalian tissues as well as
in bacteria.
There is considerable consistency as to the general mechanism of
action of polysaccharidases. In the case of substrates having more than
one type of bond to be hydrolyzed, there exists a separate enzyme
specific for each bond; i.e., the a-l,4-glucosidic bonds in amylopectin
and glycogen are broken down by the amylases; the a-l,6-glucosidic
bonds by more or less specific glucosidases; the glycosidic bonds of the
pectic substances and their methyl ester linkages are hydrolyzed by
polygalacturonase and by pectinesterase, respectively; hyaluronidase (of
testicular and bacterial origin) splits only the hexosaminidic bonds of
the substrate, whereas the glucuronidic bonds of the split products may
be hydrolyzed by β-glucuronidase; chondroitin sulfate is depolymerized
by chondroitinase; and chondrosulfatase hydrolyzes the sulfate ester
bonds.
Where the mechanism has been studied in some detail, it appears
that the type of linkage occurring in the majority is broken down first,
like the a-l,4-glucosidic bonds in amylopectin and glycogen. In some
cases, the depolymerization, i.e., the splitting of glycosidic bonds of the
polymer chains, may precede the hydrolysis of the accessory bonds,
i.e., of the bonds which do not contribute to the polymer nature of the
substrate; e.g., during the degradation of chondroitin sulfate where
the hydrolysis of the sulfate ester groups was found not to be an
essential prerequisite for the depolymerization; or in the breakdown of
chitin where the presence of N-acetyl groups was even necessary for
the action of chitinase. On the other hand, a reversed situation appears
to exist in the case of the pectic enzymes, in which the splitting of the
methyl ester bond has to precede the action of polygalacturonase; the
latter enzyme has been found, indeed, to hydrolyze only those glycosidic
linkages that are not adjacent to esterified galacturonate units.
The actual depolymerization of polysaccharides by enzymes is in
most, but not in all, cases achieved in a two-stage reaction. During the
first step, the glycosidic bonds of the polymer are randomly split by a
first enzyme (with the exception of β-amylase) until mainly disaccharides and a few other oligosaccharides of low degrees of polymeriza-
415
polysaccharides as food, have to rely for this purpose on their intestinal
fauna and flora.
It can further be concluded from the above considerations that the
distribution in nature of the polysaccharides is completely unrelated
to the distribution of their specific hydrolytic enzymes. This is also
particularly evident in the case of dextran, which is synthesized exclusively by, and which occurs solely in, certain bacteria, whereas
dextranase is found in molds and even in mammalian tissues as well as
in bacteria.
There is considerable consistency as to the general mechanism of
action of polysaccharidases. In the case of substrates having more than
one type of bond to be hydrolyzed, there exists a separate enzyme
specific for each bond; i.e., the a-l,4-glucosidic bonds in amylopectin
and glycogen are broken down by the amylases; the a-l,6-glucosidic
bonds by more or less specific glucosidases; the glycosidic bonds of the
pectic substances and their methyl ester linkages are hydrolyzed by
polygalacturonase and by pectinesterase, respectively; hyaluronidase (of
testicular and bacterial origin) splits only the hexosaminidic bonds of
the substrate, whereas the glucuronidic bonds of the split products may
be hydrolyzed by β-glucuronidase; chondroitin sulfate is depolymerized
by chondroitinase; and chondrosulfatase hydrolyzes the sulfate ester
bonds.
Where the mechanism has been studied in some detail, it appears
that the type of linkage occurring in the majority is broken down first,
like the a-l,4-glucosidic bonds in amylopectin and glycogen. In some
cases, the depolymerization, i.e., the splitting of glycosidic bonds of the
polymer chains, may precede the hydrolysis of the accessory bonds,
i.e., of the bonds which do not contribute to the polymer nature of the
substrate; e.g., during the degradation of chondroitin sulfate where
the hydrolysis of the sulfate ester groups was found not to be an
essential prerequisite for the depolymerization; or in the breakdown of
chitin where the presence of N-acetyl groups was even necessary for
the action of chitinase. On the other hand, a reversed situation appears
to exist in the case of the pectic enzymes, in which the splitting of the
methyl ester bond has to precede the action of polygalacturonase; the
latter enzyme has been found, indeed, to hydrolyze only those glycosidic
linkages that are not adjacent to esterified galacturonate units.
The actual depolymerization of polysaccharides by enzymes is in
most, but not in all, cases achieved in a two-stage reaction. During the
first step, the glycosidic bonds of the polymer are randomly split by a
first enzyme (with the exception of β-amylase) until mainly disaccharides and a few other oligosaccharides of low degrees of polymeriza-