- Open Access
Access to cellulose limits the efficiency of enzymatic hydrolysis: the role of amorphogenesis
© Arantes and Saddler; licensee BioMed Central Ltd. 2010
- Received: 13 November 2009
- Accepted: 23 February 2010
- Published: 23 February 2010
The efficient enzymatic saccharification of cellulose at low cellulase (protein) loadings continues to be a challenge for commercialization of a process for bioconversion of lignocellulose to ethanol. Currently, effective pretreatment followed by high enzyme loading is needed to overcome several substrate and enzyme factors that limit rapid and complete hydrolysis of the cellulosic fraction of biomass substrates. One of the major barriers faced by cellulase enzymes is their limited access to much of the cellulose that is buried within the highly ordered and tightly packed fibrillar architecture of the cellulose microfibrils. Rather than a sequential 'shaving' or 'planing' of the cellulose fibrils from the outside, it has been suggested that these inaccessible regions are disrupted or loosened by non-hydrolytic proteins, thereby increasing the cellulose surface area and making it more accessible to the cellulase enzyme complex. This initial stage in enzymatic saccharification of cellulose has been termed amorphogenesis. In this review, we describe the various amorphogenesis-inducing agents that have been suggested, and their possible role in enhancing the enzymatic hydrolysis of cellulose.
- Corn Stover
- Cotton Fiber
- Cellulose Microfibril
Continuing interest in the utilization of renewable biomass resources for the production of alternative fuels has brought increasing attention on the technical bottlenecks that still need to be resolved and how the variability of different lignocellulosic materials might influence the efficiency of enzymatic hydrolysis.
Over the past 40 to 50 years, many excellent research groups have been assessing the ability of carbohydrate-degrading enzymes to depolymerize the cellulosic component of lignocellulosic substrates into soluble, fermentable sugars. However the efficient, rapid and complete enzymatic hydrolysis of lignocellulosic materials using low protein loadings has proven to be one of the major technical and economical bottlenecks in the overall bioconversion process of lignocellulose to biofuels [1–4].
Several factors related to the substrates (such as lignin/hemicellulose association, degree of cellulose crystallinity and polymerization, extent of surface area) and enzymes (such as end-product inhibition, need for synergism, irreversible enzyme adsorption) have been suggested to account for the recalcitrance of cellulose to enzymatic hydrolysis . However, there is still considerable disagreement in the literature regarding the relative importance of each of these factors, and our understanding of how enzymes completely hydrolyze cellulose is still far from complete.
Enzymatic saccharification of cellulose is generally described as a heterogeneous reaction system in which cellulases in an aqueous environment react with the insoluble, macroscopic and structured cellulose, containing highly ordered and less ordered regions. Unsatisfactorily, the majority of the research directed at understanding the mechanisms of cellulose biodegradation has given little attention to the existence and the influence that the fibrillar architecture of the cellulose fibril network will have on the enzyme reactivity and consequential course of heterogeneous cellulase reactions.
In order for cellulases to efficiently hydrolyze cellulosic substrates, they must first be able to access the cellulose chains that are tightly packed in the form of insoluble microfibrils encased in hemicellulose and lignin . Previous work has shown that the ability of cellulase enzymes such as Trichoderma reesei cellobiohydrolase (CBH)I to access the cellulose chains within the microfibrils embedded in fiber walls is significantly limited, probably due to the enzyme's ability to access only the surface layers of the microfibrils . Although cellulose could be slowly eroded by surface shaving or planing, it has been proposed that, to achieve efficient enzymatic saccharification, cellulose chains in the highly ordered and tightly packed regions of microfibrils should rather be delaminated, disrupted or loosened, thereby increasing the surface area and making the individual cellulose molecules more accessible and available for interactions with cellulose-degrading enzymes. Fiber swelling and fragmentation of cellulose aggregations into short fibers have been observed during enzymatic hydrolysis of cellulose before any detectable amount of reducing sugars is released [6–9]. This initial stage in enzymatic saccharification of cellulose has been termed amorphogenesis .
The original mechanistic model for enzymatic degradation of cellulose postulated by the pioneering work of Mandel and Reese introduced the C1-Cx model [10, 11]. They hypothesized that an unknown component of the cellulase system (C1, the so called 'swelling factor') opens up the cellulose matrix, allowing this now more accessible substrate to be depolymerized by the truly hydrolytic enzymes (Cx) [10, 11]. Although many hydrolytic enzymes that could account for the suggested Cx action have been identified and characterized, so far the identification and characterization of the C1 factor remains elusive.
CBH 1, along with a number of other proteins (expansins, expansin-like proteins, swollenin), contains a polysaccharide binding surface. These proteins have been suggested to be able to non-hydrolytically loosen or disrupt the packaging of the cellulose fibril network. The cellulose-disrupting activity of such proteins has recently been shown to interact synergistically with cellulase enzymes when they are used to hydrolyze insoluble cellulose, apparently by increasing the accessibility of the cellulose to the enzymes [12, 13]. In this review paper, we provide an overview of these amorphogenesis-inducing agents and their interactions with cellulose. In addition, their potential for possible application in the enzymatic saccharification of cellulose-containing materials for biofuel production is also discussed. The structural arrangement of the cellulose chains in the fibrillar architecture, and their accessibility and reactivity are also briefly outlined. The use of enzymes and their components is expected to radically influence the way we currently think cellulose is organized within the plant cell wall.
Cellulose, an insoluble polymer consisting of β-(1-4)-linked glucose residues [14–16], has been the subject of intense research for more than a century, and new insights into a better understanding of its molecular architecture continue to emerge [15, 16]. It is well known that native cellulose molecules (cellulose I) are found in fibril form, and that its molecular architecture has a high degree of individuality, depending on its source (cell wall layer or plant type) .
Briefly, the visually dominant structural features of cellulose in higher plants are cellulose microfibrils with diameter of 2-10 nm, cross-linked by other cell wall components such as xyloglucans [15, 16]. Microfibrils are unbranched fibrils composed of approximately 30-36 glucan chains aggregated laterally by means of hydrogen bonding and van der Waals forces to produce crystalline structures . Microcrystalline cellulose has been shown to be made up of two different crystal phases: Iα and Iβ [15, 16]. Although considerable progress has been made in elucidating the crystal structures of cellulose in microfibrils, they are still not well understood [15, 16], and a deeper understanding of cellulose structure is required if we are to overcome the natural recalcitrance of lignocellulosic substrates. It is likely that these crystal structures affect the rate of diffusion of reactants and thus play an important role in the accessibility and reactivity of cellulose.
Previous work by Krässing  has shown that a higher degree of fibrillar aggregation produces a more compact fiber structure, with fewer, smaller interstices resulting in a smaller internal accessible surface area. An important feature of the highly ordered regions is that the cellulose chains are packed so tightly that even small molecules such as water cannot penetrate these highly organized structural entities . The limited accessibility to these regions leads to alteration of their reactivity to swelling and reactive agents such as cellulases. With this type of structure, it is apparent that only the cellulose molecules situated on the surface of these aggregations would be susceptible to the degrading actions of enzymes.
If cellulose hydrolysis only occurs on the surface of the cellulose aggregations, the available surface area is a potential determinant of the maximum rate of hydrolysis that can be achieved. It has been proposed that the tightly packed cellulose regions are a major factor in contributing to the resistance of cellulose to degradation, by limiting the accessibility to cellulases [17, 18]. In 1985, Coughlan  coined the term 'amorphogenesis' to suggest a possible mechanism by which the dispersion, swelling or delamination of cellulosic substrate occurred, resulting in a reduction in the degree of fibrillar aggregation and/or crystallinity, and the creation of a larger accessible surface by increasing the reactive internal surface. Consequently, amorphogenesis enhances the reactivity of the fibrous cellulosic substrates by increasing the amount of cellulose directly accessible to the enzymes.
Many carbohydrate-hydrolyzing enzymes, such as cellulases and xylanases, are modular proteins with at least two distinct modules: the catalytic module and the carbohydrate-binding module (CBM) . CBMs are thought to have one or more of the following functions: enzyme concentration on the surface of the substrate/proximity effect (the phase transfer); substrate targeting/selectivity; and disruption of non-hydrolytic crystalline substrate. CBMs that are specific for insoluble cellulose can be grouped into two general categories: those that interact with crystalline cellulose (type A CBMs) and those that interact with non-crystalline cellulose (cello-oligosaccharides in addition to insoluble cellulose) (type B CBMs,  the so-called targeting function). These non-catalytic modules readily adsorb to accessible sites on a cellulose-containing substrate to form a complex held together by specific, non-covalent, thermodynamically favorable bonds . Consequently, the catalytic module is aligned with the substrate to establish a high, local concentration of the enzyme on the cellulose surface (the so-called proximity function).
Various researchers have shown that removal of the CBM component of individual cellulases reduces the hydrolytic activity of the catalytic module on insoluble, crystalline substrates such as microcrystalline cellulose (Avicel), cotton, and filter paper, whereas their activity on soluble or amorphous cellulose remains largely unaffected [5, 22, 23]. In addition, CBMs isolated from both bacteria and fungi have been suggested to facilitate cellulose hydrolysis by physically disrupting the structure of the fibrous cellulosic network and releasing small particles, without showing any detectable hydrolytic activity, which is normally quantified by the release of reducing sugars (the so-called disruptive function) [7, 8]. In recent studies investigating the morphological and structural changes of cotton fibers after treatment with purified CBM from fungal CBH1, it was found that CBM could promote non-hydrolytic disruption of crystalline cellulose by weakening and splitting the hydrogen bonds (as observed by infra-red spectroscopy and X-ray diffraction), thereby freeing cellulose chains [24, 25]. Molecular dynamic simulations also provided a nanoscopic view of the mechanism, showing that strong and medium hydrogen bonds decreased dramatically when CBM was bound to the cellulose surface of cotton fibers . Furthermore, CBM treatment of cellulosic fibers (Whatman CF11) has also been shown to reduce the interfiber interaction (disaggregation of agglomerates between the fibers, as observed by scanning electron microscopy) through steric and hydrophobic effects, which would increase the cellulose surface area .
Earlier workers  proposed that CBMs bind to the cellulose fibers and penetrate the fibrillar network at surface discontinuities, subsequently releasing cellulose fragments that are non-covalently associated with the fiber but bonded to the underlying microfibrils. They also suggested that further penetration by the CBM then exfoliates the fiber structure, releasing the ends of cellulose chains, which remain bound to the fiber, resulting in a roughening of the surface. In related work, Lee et al. , using atomic force microscopy, observed slightly elongated holes that were left throughout the surface of cotton fibers after they were treated with hexachloropalladate-inactivated T. reesei CBH I. It was suggested that these holes were a result of the penetration of the CBM into the cellulose fibers . Incubation of cotton fibers with cellulase from Thermotoga maritima, which lacks CBM, had no effect on the surface of cotton fibers.
In a similar fashion to the proposed disruptive activity of CBM from CBH1, other CBMs have also been shown to display disruptive activity upon binding onto other non-soluble polysaccharides such as chitin (a cellulose derivative where the 2-hydroxy group is substituted with an acetamido group) and starch. One such component, CBP21 (~20 kDa), produced by Serratia marcescens, belongs to CBM family 33 and is known to bind to crystalline β-chitin (which has chitin chains arranged in a similar fashion to that of cellulose I) and strongly enhance chitin hydrolysis by chitinases [28, 29]. This is thought to be due to increased substrate availability after disruption of the crystalline chitin structure [28, 29]. It was suggested that the binding of CBP21 to chitin led to the disruption of the crystalline substrate structure through specific polar interactions that were not only important for binding, but also for alteration of the substrate structure . In related work, Zeltins and Schrempf  showed that the chitin-binding protein CHB1 secreted by Streptomyces olivaceoviridis interacts specifically with crystalline α-chitin by binding and penetrating into the structure of the substrate. Another example of a CBM with disruptive activity comes from the starch-binding domain (SBD) of Aspergillus niger glucoamylase I, an exo-acting enzyme that releases glucose from the non-reducing ends of the polymer chains [30, 31]. This glucoamylase contains an SBD with two binding sites for starch [30, 31]. These sites have been shown to help with crystalline starch hydrolysis and also help promote disruption of α-glucan interchain binding at the surface of granular starch, thereby enhancing enzymatic degradation of crystalline starches by glucoamylase I [30, 31].
Recent computational simulations have indicated that the water solutions in contact with microcrystalline cellulose surfaces are highly structured and that these structured water layers might inhibit molecular diffusion close to the cellulose surface . During enzymatic hydrolysis, this would limit the approach of cellulases towards the cellulose surfaces . More recent computer simulation studies with T. reesei CBH I action on microcrystalline cellulose Iβ showed that the CBM derived from this enzyme showed no tendency to dissociate from the cellulose surface , although it was observed to move about slightly on the surface . This finding indicated that the suggested structured water layers may not be as problematic as originally suggested. Once the enzyme is adsorbed onto the cellulose surface, a processive hydrolysis mechanism would be faster than a mechanism that requires diffusion away from and subsequent repenetration of the hydration layers .
Although the functions of CBMs during enzymatic hydrolysis of cellulose have not been fully elucidated and continue to be the subject of research, it seems reasonable to believe that the primary role of a CBM is to anchor the catalytic module to cellulose. This anchoring by the CBM is generally accepted to increase the effective concentration of cellulases onto the solid substrate, thereby assisting the enzyme through the phase transfer from the soluble fraction (enzyme) to the insoluble fraction (substrate) . Complementing this, CBM may also have a more active role in the depolymerization of cellulose by influencing the cellulose structure through the non-hydrolytic release of single cellulose chains from the highly ordered and tightly packed regions of microfibrils, which might occur by disrupting the intermicrofibrillar associative forces [6, 32–36] and by feeding the newly exposed chains through the tunnel-shaped catalytic module for hydrolysis.
Expansins are plant-derived proteins, which were first identified in the early 1990s and are primarily known for their unique 'loosening' effect on the cellulosic network within plant cell walls during growth [41–43].
Two families of expansins have currently been characterized: α- and β-expansins [44, 45]. Although they share only about 20% of their amino acid identity, they are of similar size (~27 kDa), contain a number of conserved residues and characteristic motifs distributed throughout the length of the protein, and their predicted secondary structures share up to 75% identity. However, they appear to act on different cell wall components [44, 45].
Expansins usually consist of two domains (D1 and D2) connected by a peptide linker [44, 45]. D1 shows structural and sequence similarity to the catalytic site found in family-45 glycosyl hydrolyses (GH45) whose members have been characterized as endoglucanases [44–48]. Although D1 has conserved much of the GH45 catalytic site, it lacks hydrolytic activity on cell wall polysaccharides [42, 43]. Recently, Yennwar et al.  suggested that expansins do not display hydrolytic activity due to lack of a second aspartate (a key part of the catalytic machinery required for glucan hydrolysis by GH45 enzymes) in D1 . D2 was initially speculated to be a CBM on the basis of conserved aromatic and polar residues on the surface of the protein [44, 46]. However, recent studies on the structure of expansins have identified two potential polysaccharide binding surfaces, one of which corresponds to the buried D2 face contacting D1 . In addition, the findings that the linker coupling D1 and D2 is very short and that the multiple contacts between D1 and D2 could enable close coupling of the two domains suggest that the two domains, when closely packed and aligned together, could form a potential polysaccharide-binding surface spanning D1 and D2 .
Most evidence suggests a non-hydrolytic action of expansins that enlarges cell wall cavities by binding polysaccharides and disrupting non-covalent bonds within cellulose microfibrils and between other cell wall polysaccharides attached to the microfibrils [44, 45]. In addition, Yennawar et al.  suggested that expansins act as a molecular device that uses the strain energy stored in a taut cellulose-binding glycan to help dissociate the glycan from the surface of cellulose.
Despite the lack of hydrolytic activity in expansins themselves, some studies have shown that expansins enhance the enzymatic hydrolysis of crystalline cellulose by cellulases . It has been proposed that this synergistic action is a result of the expansins making the glucan chains within the microfibrils more accessible to the cellulases . In this model, expansins are believed to act like a zipper opening the crosslinking of cellulose microfibrils by ungluing the chains that stick them together, which in turn enhances cellulose accessibility, thereby speeding cellulase action [50, 51]. For instance, Baker et al. , using yellow poplar sawdust pretreated with dilute acid, showed that extremely small additions of expansin along with T. reesei cellulases (ratio ~0.012) was sufficient to induce up to a 13% increase in cellulose conversion compared with the sugar yield obtained when cellulase was used alone.
Some proteins produced by bacteria and fungi have been shown to have sequence similarity to plant expansins [10, 11, 52–56]. Kerff et al.  determined the structure and activities of one of the proteins secreted by Bacillus subtilis, a Gram-positive soil bacterium capable of colonizing the surface of plant roots. These authors considered this protein (EXLX1) to be a member of the expansin superfamily , based on its structural similarity to plant expansins (including its two-domain structure, with the precise spatial alignment of the two domains resulting in an open binding surface spanning both domains), its ability to bind to cell walls, its plant cell wall extension activity and its lack of hydrolytic activity against major polysaccharides of the plant cell wall.
When EXLX1 was used along with low levels of T. reesei cellulase enzymes (ratio 10:1) to hydrolyze microcrystalline cellulose (Avicel), it enhanced cellulose hydrolysis but not beyond the enhancement observed with bovine serum albumin (BSA), which was used as a nonspecific control . This lower cellulolytic enhancing activity was attributed to the weak plant cell wall extension activity of EXLX1 (10-fold weaker than that of plant β-expansins) . Under such hydrolysis conditions (high EXLX1:cellulase enzymes ratio), it cannot be ruled out that the higher concentration of EXLX1 in comparison to cellulase enzymes might have resulted in competition for binding sites between cellulases and EXLX1. Because BSA only loosely binds to Avicel , competition for binding sites between cellulases and BSA is not expected. This would explain why the cellulolytic enhancing activity of EXLX1 was lower than that of BSA. In contrast to the cellulolytic enhancing activity observed with EXLX1, the enhancement obtained by the addition of BSA was not a result of disrupting activity. It has previously been shown that cellulase enzymes adsorb to the inner wall of the reaction vessel during hydrolysis [58, 59]. Thus, when high concentrations of BSA relative to cellulase enzymes (10:1) were used, it is likely that this high protein addition prevented or at least reduced adsorption of cellulases to the wall of the reaction vessel, resulting in more enzymes being available to react with the Avicel. This would lead to higher cellulose conversion compared with hydrolysis being carried out in the absence of BSA.
In related work, the EXLX1 gene was expressed in Escherichia coli; the purified recombinant protein displayed cellulose-binding and cellulose-weakening activities towards filter paper, indicating its functional homology with plant expansins . Moreover, at much lower EXLX1:cellulase enzymes ratios than those used in previous work , the recombinant EXLX1 protein was found to promote significant cellulolytic enhancing activity when mixed with a commercial T. reesei cellulase mixture during hydrolysis of filter paper. This was shown when it was compared with the control containing only filter paper and cellulase and with the negative control containing filter paper, BSA and cellulase enzymes . The ratio of the recombinant EXLX1 protein and cellulase enzymes was found to be a crucial determinant of the cellulolytic enhancing activity, with the highest synergistic activity (5.9-fold) observed at the lowest cellulase loading (0.012 filter paper units (FPU)/g filter paper) and the highest recombinant protein loading (300 μg/g filter paper). However, under this low cellulase loading, the cellulose conversion was < 10% of the theoretical maximum, and at higher cellulase loading (0.6 FPU/g filter paper, giving ~20% cellulose conversion), the synergistic activity was insignificant .
Another example of expansin-like proteins is a protein isolated from T. reesei, a well-known cellulolytic fungus . This expansin-like protein (named swollenin due to its ability to swell cotton fibers) contains an amino-terminal fungal-type cellulose-binding module linked to the plant expansin homologous module . Saloheimo et al.  reported the sequence similarity of swollenin to the fibronectin (Fn)III-type repeats of mammalian titin proteins. These latter proteins have been shown to be able to unfold and refold easily, allowing the protein to stretch. This ability might be important for swollenin if its function is to allow slippage of cellulose microfibrils in plant cell walls, as suggested for expansins.
Swollenin has also been shown to disrupt the structure of the cotton fibers, weaken filter paper and promote an apparent dispersion of Valonia cell wall structure . This ability to disrupt solid substrates is unlikely to be the result of hydrolytic activity, as no reducing sugars were detected . This would seem to indicate that swollenin is inactive against the β-1,4-glycosidic bonds in cellulose, suggesting that swollenin may share a similar role with expansins in swelling the cellulosic network within cell walls. Saloheimo et al.  have reported that swollenin is an important component in the enzyme mixture required for degradation of lignocellulosic biomass and hence, a potential candidate for the C1-induced dispersion proposed by Reese et al. . In addition, there is evidence  that the swollenin gene is regulated in a manner similar to that of the T. reesei cellulase genes, so that low expression levels occur in the presence of glucose and high expression levels occur in the presence of cellulose .
The observation that microbial proteins containing an expansin-like domain, such as swollenin in T. reesei  and EXLX1 protein in B. subtilis , can enhance root colonization, suggest that expansin-type modules have been adapted by diverse microbes to facilitate their interactions with plants . It seems that several swollenin-like activities are displayed by T. reesei, which may vary in their modes of action but would contribute synergistically to the efficient hydrolysis of the plant polysaccharides . Similarly to the potential role of expansins in enhancing the efficiency of enzymatic hydrolysis of cellulose, it has been suggested that swollenin would increase the access of cellulases to cellulose chains by promoting dispersion of cellulose aggregations, exposing individual cellulose chains to interactions with cellulases. Although the cellulolytic enhancing activity of swollenin has not been assessed, recently a chimeric enzyme associating T. reesei swollenin with an Aspergillus niger feruloyl esterase was constructed and found to significantly increase the efficiency of ferulic acid release from lignocellulosic substrate .
It has been shown that some cellulolytic bacteria, especially strains of the thermophilic anaerobic Clostridium thermocellum, produce an unidentified, yellow, water-insoluble substance when growing on cellulose [61, 62]. Similarly to CBMs in fungal cellulases, this yellow substance has been shown to have a strong affinity for crystalline cellulose and to be part of the bacterial cellulolytic system required for efficient enzymatic degradation of cellulose [61–63]. Production of this 'yellow affinity substance' has been observed to precede the production of cellulases and also to be involved in the hydrolysis of cellulose by facilitating the binding of the cellulolytic enzyme complexes to cellulose [61, 64]. Kopecny et al.  showed that endoglucanase and cellobiohydrolase activities were increased in the presence of the yellow affinity substance.
Despite some similarities in functions to that of CBMs, no substantial research has subsequently been conducted to investigate the exact means by which the yellow substance enhances cellulose saccharification.
Recently, an unknown non-hydrolytic protein (Zea h), of approximately 56 kDa, purified from fresh postharvest corn stover (the unused plant parts left after harvest), was shown to decrease the hydrogen-bond intensity and crystallinity index of filter paper . It also increased the adsorption of cellulase onto cellulosic substrates, which in turn increased the conversion of cellulose to glucose by a factor of 3.2, and accelerated hydrolysis by increasing hydrolysis rate of cellulases by a factor of 2 . Although the Zea h protein appears to have potential to enhance the cellulolytic activity of cellulase enzymes, the mechanism involved in this enhancement and the three-dimensional structure of the protein remain to be resolved.
Several fungal proteins with homology to family 61 glycosyl hydrolase (GH61) have also been reported to show cellulolytic enhancing activity on a variety of pretreated lignocellulosic substrates when combined with T. reesei cellulases [66, 67]. For instance, the expression of Thielavia terrestris GH61 in T. reesei allowed for a reduction in protein loading of 1.4-fold to reach 90% conversion of the cellulose in corn stover pretreated with steam . Based on the lack of hydrolytic activity of GH61 on pretreated lignocellulosic substrates and on a variety of cellulosic and hemicellulosic model substrates, it was suggested that the cellulase-enhancing effect of such proteins is limited to substrates containing other cell wall-derived materials such as hemicellulose or lignin . However, no clear correlation was observed between the proportion of these non-cellulolytic components and the degree of enhancement observed . Although it has not been experimentally established, rather than acting on cellulose microfibrils themselves, GH61 proteins could be acting via disruption of non-covalent bonds between cellulose and the non-cellulolytic materials (as observed with some expansins ), resulting in increased access of cellulases to the cellulose microfibrils and enhancing the overall cellulolytic activity of the cellulase complex.
T. reesei Cel61B, which was previously thought to be an endoglucanase , is the only GH61 protein so far to have its three-dimensional structure resolved . The structure appears to lack any suitable catalytic centre. However, a possible catalytic role has been speculated for the bound cation (nickel or other transition metals), given the highly conserved binding site in the GH61 proteins . CBP21, a non-catalytic carbohydrate binding protein reported to disrupt the insoluble crystalline β-chitin structure and enhance chitin hydrolysis by chitinases as described earlier, is the protein whose structure is most similar to that of Cel61B. It is possible therefore that Cel61B may also have some direct or indirect role in the enzymatic degradation of cellulose . However, the exact mechanism and function of Cel61B and other related GH61 proteins has yet to be fully resolved.
Low molecular weight peptides or phenolate-type compounds produced by 'brown rot' wood-decaying fungi (mainly Basidiomycota) are thought to mediate the non-hydrolytic/nonenzymatic attack of the lignocellulose matrix [71–73]. This attack is thought to increase pore size, consequently enhancing the diffusion of cellulases within the substrate [71–78]. These nonhydrolytic/nonenzymatic reactions mediated by low molecular weight compounds have been shown to enhance the activity of commercial cellulases and brown rot endoglucanases during hydrolysis of pure cellulose and various lignocellulosic substrates [79, 80]. In addition, it has been suggested that this initial attack swells the ordered packing of the cellulose chains, exposing new end-groups of the fibrous cellulosic substrate (enhancing accessibility) to the attack of cellulases, as evidenced by a significant decrease in the crystallinity of the cotton fibers . When the overall modification of milled spruce wood was examined using pyrolysis-molecular beam mass spectrometry coupled with multivariate analyses, it was apparent that the non-hydrolytic/nonenzymatic-mediated reactions could more readily open the structure of the lignocellulosic matrix, freeing cellulose fibrils , which indicated that this non-hydrolytic/nonenzymatic mechanism could be, in brown rot fungi, a potential candidate for the C1-induced disruption proposed by Reese et al. .
Considerable progress has been made in elucidating the nature, type and mechanism of cellulases when soluble, short chain oligosaccharides are assumed to be the substrate. However, when the recalcitrant, largely inaccessible nature of the cellulosic substrate is considered, the exact biochemical mechanisms involved in the delamination, dispersion and swelling of cellulose has been much discussed but still remains largely unknown. It has been suggested that disruption of the highly ordered and tightly packed regions of the cellulose structure facilitates the exposure of inaccessible cellulose chains buried within these regions, thereby enhancing enzyme access to cellulose, which is expected to speed the hydrolytic attack of cellulases. In this context, some proteins have been proposed as having an active role in the solubilization of cellulose by affecting (weakening, swelling) the cellulose structure via the non-hydrolytic release of the previously enzyme-inaccessible individual cellulose chains. Although the mechanism by which each of these proteins attack cellulose has yet to be resolved, the observation that most of these swelling or delaminating agents contain a (potential) carbohydrate-binding surface may indicate that this binding module may play an important role in this non-hydrolytic amorphogenesis activity. It is apparent that further research is needed to better understand the possible mechanisms of these proposed amorphogenesis-inducing agents. Moreover, it is also possible that the enzymatic hydrolysis of cellulose occurs as just an external surface phenomenon. However this is unlikely as, although relatively slow, the rate of cellulose hydrolysis indicates that there must be some creation of new surfaces within the cellulose matrix. However, how this delamination, swelling and dispersion action of the cellulase complex occurs has yet to be fully determined.
Natural Sciences and Engineering Research Council of Canada (NSERC), Natural Resources Canada (NRCan), International Energy Agency (IEA), Genome BC and Novozymes are all gratefully acknowledged for the support of our group's work. We thank our colleagues within the UBC Forest Products Biotechnology/Bioenergy Group for stimulating discussions on the topic of this review.
- Walker LP, Wilson DB: Enzymatic hydrolysis of cellulose: An Overview. Biores Technol. 1991, 36: 3-14. 10.1016/0960-8524(91)90095-2.View ArticleGoogle Scholar
- Xia L, Len P: Cellulose production by solid-state fermentation on lignocellulosic waste from the xylose industry. Process Biochem. 1999, 34: 909-912. 10.1016/S0032-9592(99)00015-1.View ArticleGoogle Scholar
- Berlin A, Maximenko V, Gilkes N, Saddler J: Optimization of enzyme complexes for lignocellulose hydrolysis. Biotechnol Bioeng. 2007, 97: 287-296. 10.1002/bit.21238.View ArticleGoogle Scholar
- Zhang X, Qin W, Paice M, Saddler JN: High consistency enzymatic hydrolysis of hardwood substrates. Biores Technol. 2009, 100: 5890-5897. 10.1016/j.biortech.2009.06.082.View ArticleGoogle Scholar
- Mansfield SD, Mooney C, Saddler JN: Substrate and enzyme characteristics that limit cellulose hydrolysis. Biotechnol Prog. 1999, 15: 804-816. 10.1021/bp9900864.View ArticleGoogle Scholar
- Coughlan MP: The properties of fungal and bacterial cellulases with comment on their production and application. Biotechnology and genetic engineering reviews. Edited by: Russell GE. 1985, Newcastle-upon-Tyne: Interscience, 3: 37-109.Google Scholar
- Din N, Gilkes NR, Tekant B, Miller RC, Warren RA, Kilburn DG: Non-hydrolytic disruption of cellulose fibres by the binding domain of a bacterial cellulase. Bio/Technology. 1991, 9: 1096-1099. 10.1038/nbt1191-1096.View ArticleGoogle Scholar
- Teeri TT, Reinikainen T, Ruohonen L, Alwyn Jones T, Knowles JKC: Domain function in Trichoderma reesei cellobiohydrolases. J Biotechnol. 1992, 21: 169-176. 10.1016/0168-1656(92)90120-X.View ArticleGoogle Scholar
- Walker LP, Wilson DB, Irwin DC: Measuring fragmentation of cellulose by Thermomonospora fusca cellulase. Enzyme Microb Technol. 1990, 12: 378-386. 10.1016/0141-0229(90)90168-P.View ArticleGoogle Scholar
- Reese ET, Sui RGH, Levinson HS: The biological degradation of soluble cellulose derivatives and its relationship to the mechanisms of cellulose hydrolysis. J Bacteriol. 1950, 59: 485-497.Google Scholar
- Mandels M, Reese ET: Fungal cellulases and the microbial decomposition of cellulosic fabric. Development in Industrial Mycology. 1964, Washington: American Institute of Biological Sciences, 5: 5-20.Google Scholar
- Baker JO, King MR, Adney WS, Decker SR, Vinzant TB, Lantz SE, Nieves RE, Thomas SR, Li LC, Cosgrove DJ, Himmel ME: Investigation of the cell-wall loosening protein expansin as a possible additive in the enzymatic saccharification of lignocellulosic biomass. Appl Biochem Biotechnol. 2000, 84: 217-223. 10.1385/ABAB:84-86:1-9:217.View ArticleGoogle Scholar
- Kim ES, Lee HJ, Bang WG, Choi IG, Kim KH: Functional characterization of a bacterial expansin from Bacillus subtilis for enhanced enzymatic hydrolysis of cellulose. Biotechnol Bioeng. 2009, 102: 1342-1353. 10.1002/bit.22193.View ArticleGoogle Scholar
- Krässing HA: Effect of structure and morphology on accessibility and reactivity. Cellulose: Structure, Accessibility, and Reactivity. Edited by: Krässing HA. 1993, Gordon and Breach Science Publishers, 11: 167-324.Google Scholar
- Somerville C, Bauer S, Brininstool G, Facette M, Hamann T, Milne J, Osborne E, Paredez A, Person S, Raab T, Vormerk S, Youngs H: Toward a systems approach to understanding plant cell walls. Science. 2004, 306: 2206-2211. 10.1126/science.1102765.View ArticleGoogle Scholar
- Ding S-Y, Himmel ME: The maize primary cell wall microfibril: a new model derived from direct visualization. J Agric Food Chem. 2006, 54: 597-606. 10.1021/jf051851z.View ArticleGoogle Scholar
- Wood TM, McCrae SI, Bhat KM: The mechanism of fungal cellulase action. Synergism between enzyme components of Penicillium pinophilum cellulase in solubilizing hydrogen-bond ordered cellulose. Biochem J. 1989, 260: 37-43.View ArticleGoogle Scholar
- Laureano-Perez L, Teymouri F, Alizadeh H, Dale BE: Understanding factors that limit enzymatic hydrolysis of biomass. Appl Biochem Biotechnol. 2005, 121: 1081-1099. 10.1385/ABAB:124:1-3:1081.View ArticleGoogle Scholar
- Gilkes NR, Henrissat B, Kilburn DG, Miller RC, Warren RAJ: Domains in microbial b-1,4-glycanases: sequence conservation, function, and enzyme families. Microbiol Rev. 1991, 55: 303-315.Google Scholar
- Boraston AB, Bolam DN, Gilbert HJ, Davies GJ: Carbohydrate-binding modules: fine tuning polysaccharide recognition. Biochem J. 2004, 382: 769-782. 10.1042/BJ20040892.View ArticleGoogle Scholar
- Lynd LR, Weimer PJ, van Zyl WH, Pretorius IS: Microbial cellulose utilization: fundamentals and biotechnology. Microbiol Mol Biol R. 2002, 66: 506-577. 10.1128/MMBR.66.3.506-577.2002.View ArticleGoogle Scholar
- Raghothama S, Simpson PJ, Szabo L, Nagy T, Gilbert H, Williamson MP: Solution structure of the CBM10 cellulose binding module from Pseudomonas xylanase A. Biochem. 2000, 39: 978-984. 10.1021/bi992163+.View ArticleGoogle Scholar
- Irwin DC, Spezio M, Walker LP, Wilson DB: Activity studies of eight purified cellulases: specificity, synergism, and binding domain effects. Biotechnol Bioeng. 1993, 42: 1002-1013. 10.1002/bit.260420811.View ArticleGoogle Scholar
- Wang LS, Zhang YZ, Gao PJ: A novel function for the cellulose binding module of cellobiohydro late I. Sci China Ser C-Life Sci. 2008, 51: 620-629. 10.1007/s11427-008-0088-3.View ArticleGoogle Scholar
- Gao PJ, Chen GJ, Wang TH, Zhang YS, Liu J: Non-hydrolytic disruption of crystalline structure of cellulose by cellulose binding domain and liker sequence of cellobiohy drolase I from Penicillium janthinellum. Sheng Wu Hua Xue Yu Sheng Wu Wu Li Xue Bao (Shanghai). 2001, 33: 13-18.Google Scholar
- Pinto R, Moreira S, Mota M, Gama M: Studies on the cellulose-binding domains adsorption to cellulose. Langmuir. 2004, 20: 1409-1413. 10.1021/la035611u.View ArticleGoogle Scholar
- Lee L, Evans BR, Woodward J: The mechanism of cellulose action on cotton fibers: evidence from atomic force microscopy. Ultramicroscopy. 2000, 82: 213-221. 10.1016/S0304-3991(99)00158-8.View ArticleGoogle Scholar
- Zeltins A, Schrempf H: Visualization of α-chitin-binding protein (CHB1) from Streptomyces olivaceoviridis. Anal Biochem. 1995, 231: 287-294. 10.1006/abio.1995.0053.View ArticleGoogle Scholar
- Vaaje-Kolstad G, Horn SJ, van Aalten DMF, Synstad B, Eijsink VGH: The non-catalytic chiting-binding proteinCBP21 from Serratia marcescens is essential for chiting degradation. J Biol Chem. 2005, 280: 28492-28497. 10.1074/jbc.M504468200.View ArticleGoogle Scholar
- Southall SM, Simpson PJ, Gilbert HJ, Williamson G, Williamson MP: The starch-binding domain from glucoamylase disrupts the structure of starch. FEBS Lett. 1999, 447: 58-60. 10.1016/S0014-5793(99)00263-X.View ArticleGoogle Scholar
- Sorimachi K, Le Gal-Coeffet MF, Williamson G, Archer DB, Williamson MP: Solution structure of the granular starch binding domain of Aspergillus niger glucoamylase bound to β-cyclodextrin. Structure. 1997, 5: 647-661. 10.1016/S0969-2126(97)00220-7.View ArticleGoogle Scholar
- Klyosov AA: Trends in biochemistry and enzymology of cellulose degradation. Biochem. 1990, 29: 10577-10585. 10.1021/bi00499a001.View ArticleGoogle Scholar
- Rabinovich ML, Nguen VV, Klyosov AA: Adsorption of cellulolytic enzymes on cellulose and kinetics of the adsorbed enzymes. Two types of interactions of the enzymes with the insoluble substrate. Biokhimiya. 1982, 48: 369-377. (in Russian)Google Scholar
- Rabinovich ML: Mechanisms of enzymatic degradation of cellulose. Microbiology and Biochemistry of Degradation of Plant Raw Materials. Edited by: Skryabin GK, Golovlev EL, Klyosov A Nauka. 1988, 70-108. (in Russian)Google Scholar
- Klyosov AA, Rabinovich ML: Enzymatic conversion of cellulose to glucose: present state of the art and potential. Enzyme Engineering: Future Directions. Edited by: Wingard LB, Berezin IV, Klyosov AA. 1980, New York: Plenum Press, 83-165.View ArticleGoogle Scholar
- Esteghalian AR, Srivastava V, Gilkes N, Gregg DJ, Saddler JN: An overview of factors influencing the enzymatic hydrolysis of lignocellulosic feedstocks. Glycosyl Hydrolases for Biomass Conversion. Edited by: Himmel ME, Baker JO, Saddler JN. 2000, Washington DC: Americal Chemical Society, 100-111.View ArticleGoogle Scholar
- Mattews J, Skopec CE, Mason PE, Zuccato P, Torget RW, Sugiyama J, Himmel ME, Brady JW: Computer simulation studies of microcrystalline cellulose Iβ. Carbohyd Res. 2006, 324: 138-152. 10.1016/j.carres.2005.09.028.View ArticleGoogle Scholar
- Zhong L, Matthews JF, Crowley MF, Rignall T, Talón C, Cleary JM, Walker RC, Chukkapall G, McCabe C, Nimlos MR, Brooks CL, Himmel ME, Brady JW: Interactions of the complete cellobiohydrolase I from Trichodera reesei with microcrystalline cellulose. Cellulose. 2008, 15: 261-273. 10.1007/s10570-007-9186-0.View ArticleGoogle Scholar
- Zhong L, Matthews JF, Hansen PI, Crowley MF, Cleary JM, Walker RC, Nimlos MR, Books CL, Adney WS, Himmel ME, Brady JW: Computational simulations of the Trichoderma reesei cellobiohydrolase I acting on microcrystalline cellulose Iβ: the enzyme-substrate complex. Carbohyd Res. 2009, 344: 1984-1992. 10.1016/j.carres.2009.07.005.View ArticleGoogle Scholar
- Levy I, Shoseyov O: Cellulose-binding domains - Biotechnological applications. Biotechnol Adv. 2002, 20: 191-213. 10.1016/S0734-9750(02)00006-X.View ArticleGoogle Scholar
- Li ZC, Durachko DM, Cosgrove DJ: An oat coleoptile wall protein that induces wall extension in vitro and that is antigenically related to a similar protein from cucumber hypocotyls. Planta. 1993, 191: 349-356. 10.1007/BF00195692.View ArticleGoogle Scholar
- McQueen-Mason S, Durachko DM, Cosgrove DJ: Two endogenous proteins that induce cell wall expansion in plants. Plant Cell. 1992, 4: 1425-1433. 10.1105/tpc.4.11.1425.View ArticleGoogle Scholar
- Cosgrove DJ: Growth of the plant cell wall. Nat Rev Mol Cell Biol. 2005, 6: 850-861. 10.1038/nrm1746.View ArticleGoogle Scholar
- Yennawar NH, Li LC, Dudzinski DM, Tabuchi A, Cosgrove DJ: Crystal structure and activities of EXP1 (Zea m 1), a β-expansin and group-1 pollen allergen from maize. Proc Natl Acad Sci. 2006, 103: 14664-114671. 10.1073/pnas.0605979103.View ArticleGoogle Scholar
- Cosgrove DJ: New genes and new biological roles for expansins. Curr Opin Plant Biol. 2000, 3: 73-78. 10.1016/S1369-5266(99)00039-4.View ArticleGoogle Scholar
- Cosgrove DJ: Relaxation in a high-stress environment: the molecular bases of extensible cell walls and cell enlargement. Plant Cell. 1997, 9: 1031-1041. 10.1105/tpc.9.7.1031.View ArticleGoogle Scholar
- Saloheimo A, Henrissat B, Hoffren AM, Teleman O, Penttilä M: A novel, small endoglucanase gene, egl5, from Trichoderma reesei isolated by expression in yeast. Mol Microbiol. 1994, 13: 219-228. 10.1111/j.1365-2958.1994.tb00417.x.View ArticleGoogle Scholar
- Tatusova TA, Madden TL: Blast 2 sequences - a new tool for comparing protein and nucleotide sequences. FEMS Microbiol Lett. 1999, 174: 247-250. 10.1111/j.1574-6968.1999.tb13575.x.View ArticleGoogle Scholar
- Cosgrove DJ, Durachko DM, Li LC: Expansins may have cryptic endoglucanase activity and can synergize the breakdown of cellulose by fungal cellulases. Annu Meeting Am Soc Plant Physiol Abstr. 1998, 171-Google Scholar
- Sampedro J, Cosgrove DJ: The expansin superfamily. Genome Biol. 2005, 6: 242.1-242.11. 10.1186/gb-2005-6-12-242.View ArticleGoogle Scholar
- Cosgrove DJ: Enhancement of accessibility of cellulose by expansins. United States Patent. Patent No. US 6,326,470 B1. 2001Google Scholar
- Kerff F, Amoroso A, Herman R, Sauvage E, Petrella S, Filée P, Charlier P, Joris B, Tabuchi A, Nikolaidis N, Cosgrove DJ: Crystal structure and activity of Bacillus subtilis YoaJ (EXLX1), a bacterial expansin that promotes root colonization. Proc Natl Acad Sci USA. 2008, 105: 16876-81. 10.1073/pnas.0809382105.View ArticleGoogle Scholar
- Saloheimo M, Paloheimo M, Hakola S, Pere J, Swanson B, Nyyssönen E, Bhatia A, Ward M, Penttilä M: Swollenin, a Trichoderma reesei protein with sequence similarity to the plant expansins, exhibits disruption activity on cellulosic materials. Eur J Biochem. 2002, 269: 4202-11. 10.1046/j.1432-1033.2002.03095.x.View ArticleGoogle Scholar
- Laine MJ, Haapalainen M, Wahlroos T, Kankare K, Nissinen R, Kassuwi S, Metzler MC: The cellulase encoded by the native plasmid of Clavibacter michiganensis ssp. sepedonicus plays a role in virulence and contains an expansin-like domain. Physiol Mol Plant P. 2000, 57: 221-233. 10.1006/pmpp.2000.0301.View ArticleGoogle Scholar
- Yao Q, Sun TT, Liu WF, Chen GJ: Gene Cloning and heterologous Expression of a novel endoglucanase, swollenin, from Trichoderma pseudokoningii S38. Biosci Biotech Bioch. 2008, 72: 2799-2805. 10.1271/bbb.80124.View ArticleGoogle Scholar
- Brotman Y, Briff E, Viterbo A, Chet I: Role of swollenin, an expansin-Like protein from Trichoderma, in plant root colonization. Plant Physiol. 2008, 147: 779-789. 10.1104/pp.108.116293.View ArticleGoogle Scholar
- Yang B, Wyman CE: BSA treatment to enhance enzymatic hydrolysis of cellulose in lignin containing substrates. Biotechnol Bioeng. 2006, 94: 611-617. 10.1002/bit.20750.View ArticleGoogle Scholar
- Bommarius AS, Katona A, Cheben SE, Patel AS, Ragauskas AJ, Knudson K, Pu Y: Cellulase kinetics as a function of cellulose pretreatment. Metab Eng. 2008, 10: 371-381. 10.1016/j.ymben.2008.06.008.View ArticleGoogle Scholar
- Xu F, Ding H: A new kinetic model for heterogeneous (or spatially confined) enzymatic catalysis: contributions from the fractal and jamming (overcrowding) effects. Appl Catal A-Gen. 2007, 317: 70-81. 10.1016/j.apcata.2006.10.014.View ArticleGoogle Scholar
- Levasseur A, Saloheimo M, Navarro D, Andberg M, Monot F, Nakari-Setala T, Asther M, Record E: Production of a chimeric enzyme tool associating the Trichoderma reesei swollenin with the Aspergillus niger feruloyl esterase A for release of ferulic acid. Appl Microbiol Biotechnol. 2006, 73: 872-880. 10.1007/s00253-006-0546-8.View ArticleGoogle Scholar
- Ljungdahl LG, Petersson B, Eriksson KE, Wiegel J: A yellow affinity substance involved in the cellulolytic system of Clostridium thermocellum. Curr Microbiol. 1983, 9: 195-200. 10.1007/BF01567580.View ArticleGoogle Scholar
- Kopecny J, Simunek J, Hodrova B: Production and properties of yellow affinity substances by Ruminococcus flavefaciens. Ann Zootech. 1996, 45 (Suppl): 299-299. 10.1051/animres:19960652.View ArticleGoogle Scholar
- Kopecny J, Hodrova B: The effect of yellow affinity substance on cellulases of Ruminococcus flavefaciens. Lett Appl Microbiol. 1997, 25: 191-196. 10.1046/j.1472-765X.1997.00202.x.View ArticleGoogle Scholar
- Lamed R, Kenig R, Setter E, Bayer EA: Major characteristics of the cellulolytic system of Clostridium thermocellum coincide with those of the purified cellulosome. Enzyme Microbial Technol. 1985, 7: 37-41. 10.1016/0141-0229(85)90008-0.View ArticleGoogle Scholar
- Han Y, Chen HZ: Synergism between corn stover protein and cellulase. Enzyme Microbial Technol. 2007, 41: 638-645. 10.1016/j.enzmictec.2007.05.012.View ArticleGoogle Scholar
- De Leon AL, Ding H, Brown K: Polypeptides having cellulolytic enhancing activity and nucleic acids encoding same. US 2009/0019608 A1. 2009Google Scholar
- Merino ST, Cherry J: Progress and challenges in enzyme development for biomass utilization. Adv Biochem Engin/Biotechnol. 2007, 108: 95-120. 10.1007/10_2007_066.View ArticleGoogle Scholar
- Cosgrove DJ: Loosening of plant cell walls by expansins. Nature. 2000, 407: 321-326. 10.1038/35030000.View ArticleGoogle Scholar
- Foreman PK, Brown D, Dankmeyer L, Dean R, Diener S, Dunn-Coleman NS, Goedegebuur F, Houfek TD, England GJ, Kelley AS, Meerman HJ, Mitchell T, Mitchinson C, Olivares HA, Teunissen PJM, Yao J, Ward M: Transcriptional regulation of biomass-degrading enzymes in the filamentous fungus Trichoderma reesei. J Biol Chem. 2003, 278: 31988-31997. 10.1074/jbc.M304750200.View ArticleGoogle Scholar
- Karkehadabi S, Hansson H, Kim S, Piens K, Mitchinson C, Sandgren M: The first structure of a glycoside hydrolase family 61 member, Cel61B from Hypocrea jecorina, at 1.6 Å resolution. J Mol Biol. 2008, 383: 144-154. 10.1016/j.jmb.2008.08.016.View ArticleGoogle Scholar
- Goodell B, Jellison J, Liu J, Daniel G, Paszczynski A, Fekete F, Krishnamurthy S, Jun L, Xu G: Low molecular weight chelators and phenolic compounds isolated from wood decay fungi and their role in the fungal biodegradation of wood. J Biotechnol. 1997, 53: 133-162. 10.1016/S0168-1656(97)01681-7.View ArticleGoogle Scholar
- Enoki A, Itajura S, Tanaka H: The involvement of extracelullar substances for reducing molecular oxygen to hydroxyl radical and ferric ion to ferrous iron in wood degradation by wood decay fungi. J Biotechnol. 1997, 53: 265-272. 10.1016/S0168-1656(97)01682-9.View ArticleGoogle Scholar
- Arantes V, Milagres AMF: The relevance of low molecular weight compounds in wood biodegradation by fungi. Quim Nova. 2009, 32: 1586-1595. 10.1590/S0100-40422009000600043.View ArticleGoogle Scholar
- Fluornoy DS, Kirk TK, Highley TL: Wood decay by brown-rot fungi: changes in pore structure and cell wall volume. Holzforschung. 1991, 45: 383-388. 10.1515/hfsg.19220.127.116.113.View ArticleGoogle Scholar
- Blanchette RA, Krueger EW, Haight JE, Akhtar M, Akin DE: Cell wall alterations in loblolly pine wood decayed by the white-rot fungus, Ceriporiopsis subvermispora. J Biotechnol. 1997, 53: 203-213. 10.1016/S0168-1656(97)01674-X.View ArticleGoogle Scholar
- Hammel KE, Kapich AN, Jensen KA, Ryan ZC: Reactive oxygen species as agents of wood decay by fungi. Enzyme Microbial Technol. 2002, 30: 445-453. 10.1016/S0141-0229(02)00011-X.View ArticleGoogle Scholar
- Goodell B: Brown rot degradation of wood: Our evolving view. Wood Deterioration and Preservation: Advances in Our Changing World. Edited by: Goodell B, Nicholas D, Schultz T. 2003, American Chemical Society Series. Oxford University Press, 97-118. full_text.View ArticleGoogle Scholar
- Arantes V, Qian Y, Kelley SS, Milagres AMF, Filley TR, Jellison J, Goodell B: Biomimetic oxidative treatment of spruce wood studied by pyrolysis-molecular beam mass spectrometry coupled with multivariate analysis and 13C-labeled tetramethylammonium hydroxide thermochemolysis: implications for fungal degradation of wood. J Biol Inorg Chem. 2009, 8: 1253-1263. 10.1007/s00775-009-0569-6.View ArticleGoogle Scholar
- Rätto M, Ritschkoff AC, Viikari L: The effect of oxidative pretreatment on cellulose degradadion by Poria placenta and Trichoderma reesei cellulases. Appl Microbiol Biotechnol. 1997, 48: 53-57. 10.1007/s002530051014.View ArticleGoogle Scholar
- Schilling JS, Tewalt JP, Duncan SM: Synergy between pretreatment lignocellulose modifications and saccharification efficiency in two brown rot fungal systems. Appl Microbiol Biotechnol. 2009, 84: 465-475. 10.1007/s00253-009-1979-7.View ArticleGoogle Scholar
- Wang W, Gao PJ: Function and mechanism of a low-molecular weight peptide produced by Gloephyllum trabeum in biodegrdation of cellulose. J Biotechnol. 2003, 101: 119-130. 10.1016/S0168-1656(02)00321-8.View ArticleGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.