249
Hydrolysis and Fermentation Technologies for Alcohols
function without dissociating from its substrate. The chance for reaction
is significantly increased by an active enzyme adsorbed onto the surface
of the substrate.
3. Beta-glucosidase or cellobiase hydrolyzes the disaccharides and tetrasaccharides into individual monosaccharides.
4. Oxidative cellulase depolymerizes cellulose by the free radical reactions as
in the case of a cellobiose dehydrogenase (acceptor), an enzyme that catalyzes the dehydrogenation of cellobiose.
5. Cellulose phosphorylase depolymerizes cellulose using phosphates instead
of water.
In most cases, the enzyme complex breaks down cellulose to beta-glucose. This type
of cellulose enzyme is produced mainly by symbiotic bacteria. Enzymes that break
down hemicellulose are called hemicellulase, which are still classified under cellulases. The principal challenge in building an enzyme system is how to make these
different enzymes work together for hydrolytic degradation of biomass.
The enzymes described above can also be classified as progressive (or processive)
and nonprogressive (or nonprocessive). Progressive cellulase will continue to interact
with a single polysaccharide strand, whereas nonprogressive cellulase will interact
once, disengage, and then engage another polysaccharide strand. Based on the enzymatic capability, cellulase enzyme is characterized into two groups: C1 enzyme (or
factor) and Cx enzyme (or factor). The C1 factor is regarded as an “affinity” or prehydrolysis factor that transforms crystalline cellulose (i.e., cotton fiber or Avicel) into
linear and hydroglucose chains. The C1 factor has very little effect on the soluble
derivatives. The Cx (hydrolytic) factor breaks down the linear chains into soluble
carbohydrates, usually cellobiose and glucose. Microbes rich in the C1 factor are
more useful in the production of glucose from the cellulose. This is generally a ratecontrolling step. Trichoderma reesei microbes contain the best amount of C1 factor.
This is an industrially important fungus that is capable of screening large amounts
of cellulases and hemicellulases [33]. The site of action of cellulolytic enzymes is
important in the design of Cx factor. If the enzyme is within cell mass, the material
to be reacted must diffuse into the cell mass. Therefore, the enzymatic hydrolysis of
cellulose usually takes place extracellularly, where enzyme is diffused from the cell
mass into the external medium.
Another important factor in the enzymatic reaction is whether the enzyme
is adaptive or constitutive. A constitutive enzyme is present in a cell at all times,
whereas an adaptive enzyme is only found in the presence of a given substance and
the synthesis of enzyme is triggered by an inducing agent. Most fungal cellulases
are adaptive [31,34]. Cellobiose is an inducing agent for microbes T. reesei. For high
concentration (>0.5%–1%), it can also be an inhibitor. In most practical situations, it
acts as an inducing agent.
9.4.2.2.2 Enzyme Production and Inhibition
As mentioned earlier, the most useful enzyme system for hydrolysis of cellulose
is cellulase. Cellulase is a multicomponent enzyme system consisting of endobeta-1,4-glycanases, exo-beta-1,4-glucan glucohydrolases, and exo-beta-1,4-glucan
Hydrolysis and Fermentation Technologies for Alcohols
function without dissociating from its substrate. The chance for reaction
is significantly increased by an active enzyme adsorbed onto the surface
of the substrate.
3. Beta-glucosidase or cellobiase hydrolyzes the disaccharides and tetrasaccharides into individual monosaccharides.
4. Oxidative cellulase depolymerizes cellulose by the free radical reactions as
in the case of a cellobiose dehydrogenase (acceptor), an enzyme that catalyzes the dehydrogenation of cellobiose.
5. Cellulose phosphorylase depolymerizes cellulose using phosphates instead
of water.
In most cases, the enzyme complex breaks down cellulose to beta-glucose. This type
of cellulose enzyme is produced mainly by symbiotic bacteria. Enzymes that break
down hemicellulose are called hemicellulase, which are still classified under cellulases. The principal challenge in building an enzyme system is how to make these
different enzymes work together for hydrolytic degradation of biomass.
The enzymes described above can also be classified as progressive (or processive)
and nonprogressive (or nonprocessive). Progressive cellulase will continue to interact
with a single polysaccharide strand, whereas nonprogressive cellulase will interact
once, disengage, and then engage another polysaccharide strand. Based on the enzymatic capability, cellulase enzyme is characterized into two groups: C1 enzyme (or
factor) and Cx enzyme (or factor). The C1 factor is regarded as an “affinity” or prehydrolysis factor that transforms crystalline cellulose (i.e., cotton fiber or Avicel) into
linear and hydroglucose chains. The C1 factor has very little effect on the soluble
derivatives. The Cx (hydrolytic) factor breaks down the linear chains into soluble
carbohydrates, usually cellobiose and glucose. Microbes rich in the C1 factor are
more useful in the production of glucose from the cellulose. This is generally a ratecontrolling step. Trichoderma reesei microbes contain the best amount of C1 factor.
This is an industrially important fungus that is capable of screening large amounts
of cellulases and hemicellulases [33]. The site of action of cellulolytic enzymes is
important in the design of Cx factor. If the enzyme is within cell mass, the material
to be reacted must diffuse into the cell mass. Therefore, the enzymatic hydrolysis of
cellulose usually takes place extracellularly, where enzyme is diffused from the cell
mass into the external medium.
Another important factor in the enzymatic reaction is whether the enzyme
is adaptive or constitutive. A constitutive enzyme is present in a cell at all times,
whereas an adaptive enzyme is only found in the presence of a given substance and
the synthesis of enzyme is triggered by an inducing agent. Most fungal cellulases
are adaptive [31,34]. Cellobiose is an inducing agent for microbes T. reesei. For high
concentration (>0.5%–1%), it can also be an inhibitor. In most practical situations, it
acts as an inducing agent.
9.4.2.2.2 Enzyme Production and Inhibition
As mentioned earlier, the most useful enzyme system for hydrolysis of cellulose
is cellulase. Cellulase is a multicomponent enzyme system consisting of endobeta-1,4-glycanases, exo-beta-1,4-glucan glucohydrolases, and exo-beta-1,4-glucan