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Water for Energy and Fuel Production
cellobiohydrolases. Cellobiose is the dominant product of this system, but it is highly
inhibitory to the enzymes and is not usable by most organisms. Cellobiase hydrolyzes cellobiose to glucose, which is much less inhibitory and highly fermentable.
Many fungi produce this cellobiase and most of the work that is presently conducted
is on T. reesei (viride). The cellulase produced by T. reesei is much less inhibited than
other cellulases that have the major advantages for industrial purposes [35].
Cellulases can inhibit competitively [36–41], noncompetitively [39,42–44], or
uncompetitively [37]. Uncompetitive inhibition takes place when an enzyme inhibitor binds only to the complex formed between the enzyme and the substrate, whereas
noncompetitive inhibition takes place when an enzyme inhibitor and the substrate
may both be bound to the enzyme at any given time. For purified cellulose, wheat
straw and bagasse, T. reesei produced enzyme is competitively inhibited by glucose
and cellobiose. However, some enzyme is noncompetitively inhibited by cellobiose
using other substrates such as rice straw and Avicel (microcrystalline cellulose).
Trichoderma viride is uncompetitively inhibited by glucose in a cotton waste substrate [37].
Besides T. reesei, other mutants such as Rut C-30 [45] and Clostridium
thermocellum have also been extensively examined. Cellulases isolated from
C. thermocellum have high specific activities [46], especially against crystalline
forms of cellulose that have proven to be resistant to other cellulase preparations.
Low-cost but efficient enzymes for the lignocellulosic ethanol technology is continued to be developed to reduce the operational cost and improve the productivity
of the process.
9.4.2.3 mechanism of Cellulose hydrolysis
The overall cellulose hydrolysis is based on the synergistic action of three distinct
cellulase enzymes and depends on the concentration ratio and the adsorption ratio
of the component enzymes: endo-beta-gluconases, exo-beta-gluconases, and betaglucosidases. The endo-beta-gluconases attack the interior of the cellulose polymer
in a random fashion [47], exposing new chain ends. This enzyme is strongly but
reversibly adsorbed to the microcrystalline cellulose commonly known as Avicel
and catalyzes the solid-phase reaction. The strength of the adsorption is greater at
the lower temperatures. This enzyme is necessary for the hydrolysis of crystalline
substrates of cellulose, resulting in a considerable accumulation of reducing sugars,
mainly cellobiose, because the extracellular cellulase complex does not possess the
cellobiose activity. Sugars that contain aldehyde groups that are oxidized to carboxylic acids are classified as reducing sugars.
The exo-beta-gluconases remove disaccharide cellobiose units from the nonreducing ends of cellulose chains. The exo-beta-gluconases adsorb strongly on both
crystalline and amorphous substrates, and carry out the solid-phase reaction. The
mechanism of the reaction is complex because there are two distinct forms of both
endo- and exoenzymes, each with a different type of synergism with other members
of the complex. The concentration of cellobiose in the solution increases as these
enzymes continue to split off the cellobiose units. The action of exo-beta-gluconases
may be severely hampered (or stopped) by the accumulation of cellobiose in the
solution.