Environmental Engineering Reference
In-Depth Information
3 Microscopic Transport Through Plant Cell Walls
Enzyme penetration into plant cell wall is widely acknowledged to be a key bar-
rier to economical and effective biochemical conversion of lignocellulosic biomass
[5, 49]. In fact, the primary function of pretreatment of lignocellulosic biomass
is to assist subsequent enzymatic digestibility by making cell walls more accessi-
ble to saccharifying enzymes [1, 4, 44]. However, an accurate description of the
methods by which enzymes penetrate cell walls and accomplish cellulose degra-
dation has been lacking. A recent study by Donohoe and coworkers provided, for
the first time, direct visual evidence of loosening of plant cell wall structure due to
dilute acid pretreatment and the subsequently improved access by cellulases [49].
Figure 3c-f further demonstrate the penetration of cellulases into pretreated cell
walls as detected by nano-gold labeled antibodies to Cel7A and other cellulases.
This study shows that penetration of enzymes into mildly pretreated cell walls is
minimal and that cells stay largely intact even after prolonged exposure to cellu-
lases (Fig. 3a, b). In moderately pretreated cell walls, cellulases are able to partially
penetrate and disintegrate the inner secondary layers (S3) only (Fig. 3c, d); whereas
the outer layers (S1 and S2) remain impervious to enzymes. In severely pretreated
cell walls, enzymes penetrate throughout (Fig. 3e, f). These data suggest that enzy-
matic digestibility of biomass is restricted by transport of enzymes into cell walls.
While not directly evidenced by this study, these results also suggest that thermal
pretreatments (and possibly others) “loosen” cell walls in layers providing enzymes
access only to these structurally compromised zones of the cell walls. Kinetic data
on thermal pretreatments by several research groups also suggests likely mass trans-
fer limited xylan removal that can be modeled as parallel fast and slow reactions
[44, 50, 51] and the fundamental observations made by Donohoe and coworkers
[49] support this hypothesis.
4 Lignin Mobility and Impact on Biochemical Conversion
Lignin is a polymeric material composed of phenylpropanoid units derived primar-
ily from three cinnamyl alcohols (monolignols):
ρ
-coumaryl, coniferyl, and sinapyl
alcohols. Polymer formation is thought to occur via oxidative (radical-mediated)
coupling between monolignols and the growing oligomer/polymer [52, 53] and is
commonly believed to occur in a near-random fashion [54], although some recent
studies suggest an ordered and protein-regulated lignin synthesis [55]. In any case,
the resulting polymer is complex, heterogeneous, and recalcitrant to biological
degradation. Although lignin loss is minimal during thermal-acidic/neutral pre-
treatments, it can undergo structural and chemical changes [56] that significantly
influence downstream enzymatic conversion.
Although enzymes thoroughly penetrate cell walls after high severity pre-
treatments [49], incomplete cellulose conversion by cellulases suggests additional
barriers exist at the ultrastructural level. One potential barrier is occlusion of the cel-
lulose microfibrils by residual lignin or hemicellulose that would sterically prevent
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