500
13 The Structure and Metabolism of Carbohydrates
The digestion of native cellulose is initiated by
the hydrolysis of bonds in less strictly organized
regions of the cellulose chain; this is achieved by
endoglucanase, which creates sites for cellobiohydrolase activity. This cleaves off cellobiose at
these sites, thereby destroying the strict organization of the crystalline regions and revealing new
sites for attack by the endoglucanase. By the
combined activities of the Cx and C1 enzymes,
the cellulose fibres are finally completely degraded to cellobiose and the corresponding oligosaccharides; these are then hydrolysed by the
joint action of Cx and cellobiase to give free glucose residues [176].
The classical method for the determination of
cellulase activity using preparations of soluble
cellulose measures only the activity of Cx; the
C1 enzyme can be assayed only indirectly by the
release of reducing sugars from crystalline cellulose [176]. Endoglucanase (Cx) and cellobiase
activities have been detected in numerous species, especially amongst the molluscs and the
arthropods, including species whose capacity for
cellulose utilization is not particularly great or is
completely absent. In the latter cases, it is
important to know whether C1 activity is present
and, thus, whether the hydrolysis of native cellulose is possible. Appropriate investigations have
shown, quite surprisingly, that the eight carnivorous species of molluscs (five cephalopods and
three prosobranchia) have a complete cellulolytic
system which is probably never active in the living
animal [1, 176]. Where the complete cellulase
complex is found, it needs to be ascertained
whether the individual enzyme components are
all produced by the animal itself or whether they
originate exogenously. Many animals digest cellulose either with the help of C1-producing protozoans or fungi, or through the involvement of
bacteria which can hydrolyse cellulose without
the secretion of cellulolytic enzymes.
It is now quite certain that the thysanurans
Thermobia domestica and Ctenolepisma lineata
and the floor mite Steganacarus magnus (Oribatidae) do not require bacteria or other symbionts
for cellulose utilization [176, 299]. However, the
situation is more complicated for the termites and
some other insects. In the posterior gut of the
lower termites (Mastotermitidae, Kalotermitidae, Hodotermitidae and Rhinotermitidae) and
the wood cockroach Cryptocerus punctulatus
there are anaerobic protozoans which ferment
cellulose to acetate and are indispensable for cellulose digestion. These protozoans belong to certain genera of the flagellate group of the Polymastigina and are found only in this habitat. The total
cellulase activity and more than 90 % of the cellobiase activity in the termite posterior gut are generated by the anaerobic protozoans present.
Feeding these insects with starch results in the
disappearance of the protozoans and loss of the
capacity for cellulose utilization by the host; reinoculation with the protozoans restores the cellulose utilization capacity. The bacteria present in
the termite gut are not involved in cellulose digestion. Termites also produce their own cellulolytic
enzymes; the activities of endoglucanase and cellobiase in the salivary glands and in the anterior
gut and midgut are not affected by removal of the
protozoans [176, 278]. For the fungus-farming
termites, such as Macrotermes natalensis, and the
wood wasps (Siricidae), the greater part of the
cellulolytic enzymes found in the gut originate
from the fungal diet. The amphipod crustaceans
of the genus Gammarus also contain cellulolytic
enzymes derived from fungi [230].
The degree to which symbiontic gut bacteria
are important for the utilization of cellulose by
invertebrates is in most cases not known. A critical role may be assumed when the extent of cellulose degradation correlates with the extent of
the bacterial flora, e.g. in the beetle Oryctes, the
diplopod Glomeris and the isopod Tracheoniscus
[176]. The isolation of animal cellulases has seldom been attempted. The endo-~(1-4)-glucanase
of the edible snail Helix pomatia has all the typical properties of this enzyme type: the molecular
mass is 51 kDa; it cleaves soluble ~(1-4)-glucans
down to cellotetraose with a pH optimum of 6;
and it is inactivated by EDTA and can be reactivated by calcium ions. Three cellulolytic enzymes
have been isolated from the long-horned beetle
Ergates faber. Enzyme A has a mass of 25 kDa
and is a typical cellobiohydrolase (C1 enzyme);
enzymes Band C have masses of 57 and 70 kDa,
respectively, and are exoglucanases of broad
specificity which cleave off terminal glucose residues from longer or shorter ~(1-4)-glucans.
Many ~-glucosidases of the invertebrates are active against cellobiose; one such enzyme from a
beetle larva has been purified and characterized
[47].
Most endo-~( 1-4 )-glucanases are active
against the lichenin found in Irish moss, which is
essentially composed of ~(1-3)-linked cellotriose
sequences. The hydrolysis of ~(1-3)-glucans
found, for example, in the cell wall of fungi, in
the brown algae (laminarin) and in the phloem of
higher plants, requires specific 1l(1-3)-glucanases
(Iaminarinases). Such enzymes have been found
13 The Structure and Metabolism of Carbohydrates
The digestion of native cellulose is initiated by
the hydrolysis of bonds in less strictly organized
regions of the cellulose chain; this is achieved by
endoglucanase, which creates sites for cellobiohydrolase activity. This cleaves off cellobiose at
these sites, thereby destroying the strict organization of the crystalline regions and revealing new
sites for attack by the endoglucanase. By the
combined activities of the Cx and C1 enzymes,
the cellulose fibres are finally completely degraded to cellobiose and the corresponding oligosaccharides; these are then hydrolysed by the
joint action of Cx and cellobiase to give free glucose residues [176].
The classical method for the determination of
cellulase activity using preparations of soluble
cellulose measures only the activity of Cx; the
C1 enzyme can be assayed only indirectly by the
release of reducing sugars from crystalline cellulose [176]. Endoglucanase (Cx) and cellobiase
activities have been detected in numerous species, especially amongst the molluscs and the
arthropods, including species whose capacity for
cellulose utilization is not particularly great or is
completely absent. In the latter cases, it is
important to know whether C1 activity is present
and, thus, whether the hydrolysis of native cellulose is possible. Appropriate investigations have
shown, quite surprisingly, that the eight carnivorous species of molluscs (five cephalopods and
three prosobranchia) have a complete cellulolytic
system which is probably never active in the living
animal [1, 176]. Where the complete cellulase
complex is found, it needs to be ascertained
whether the individual enzyme components are
all produced by the animal itself or whether they
originate exogenously. Many animals digest cellulose either with the help of C1-producing protozoans or fungi, or through the involvement of
bacteria which can hydrolyse cellulose without
the secretion of cellulolytic enzymes.
It is now quite certain that the thysanurans
Thermobia domestica and Ctenolepisma lineata
and the floor mite Steganacarus magnus (Oribatidae) do not require bacteria or other symbionts
for cellulose utilization [176, 299]. However, the
situation is more complicated for the termites and
some other insects. In the posterior gut of the
lower termites (Mastotermitidae, Kalotermitidae, Hodotermitidae and Rhinotermitidae) and
the wood cockroach Cryptocerus punctulatus
there are anaerobic protozoans which ferment
cellulose to acetate and are indispensable for cellulose digestion. These protozoans belong to certain genera of the flagellate group of the Polymastigina and are found only in this habitat. The total
cellulase activity and more than 90 % of the cellobiase activity in the termite posterior gut are generated by the anaerobic protozoans present.
Feeding these insects with starch results in the
disappearance of the protozoans and loss of the
capacity for cellulose utilization by the host; reinoculation with the protozoans restores the cellulose utilization capacity. The bacteria present in
the termite gut are not involved in cellulose digestion. Termites also produce their own cellulolytic
enzymes; the activities of endoglucanase and cellobiase in the salivary glands and in the anterior
gut and midgut are not affected by removal of the
protozoans [176, 278]. For the fungus-farming
termites, such as Macrotermes natalensis, and the
wood wasps (Siricidae), the greater part of the
cellulolytic enzymes found in the gut originate
from the fungal diet. The amphipod crustaceans
of the genus Gammarus also contain cellulolytic
enzymes derived from fungi [230].
The degree to which symbiontic gut bacteria
are important for the utilization of cellulose by
invertebrates is in most cases not known. A critical role may be assumed when the extent of cellulose degradation correlates with the extent of
the bacterial flora, e.g. in the beetle Oryctes, the
diplopod Glomeris and the isopod Tracheoniscus
[176]. The isolation of animal cellulases has seldom been attempted. The endo-~(1-4)-glucanase
of the edible snail Helix pomatia has all the typical properties of this enzyme type: the molecular
mass is 51 kDa; it cleaves soluble ~(1-4)-glucans
down to cellotetraose with a pH optimum of 6;
and it is inactivated by EDTA and can be reactivated by calcium ions. Three cellulolytic enzymes
have been isolated from the long-horned beetle
Ergates faber. Enzyme A has a mass of 25 kDa
and is a typical cellobiohydrolase (C1 enzyme);
enzymes Band C have masses of 57 and 70 kDa,
respectively, and are exoglucanases of broad
specificity which cleave off terminal glucose residues from longer or shorter ~(1-4)-glucans.
Many ~-glucosidases of the invertebrates are active against cellobiose; one such enzyme from a
beetle larva has been purified and characterized
[47].
Most endo-~( 1-4 )-glucanases are active
against the lichenin found in Irish moss, which is
essentially composed of ~(1-3)-linked cellotriose
sequences. The hydrolysis of ~(1-3)-glucans
found, for example, in the cell wall of fungi, in
the brown algae (laminarin) and in the phloem of
higher plants, requires specific 1l(1-3)-glucanases
(Iaminarinases). Such enzymes have been found