The last piece in the cellulase puzzle: the characterisation of beta-glucosidase from the herbivorous gecarcinid land crab Gecarcoidea natalis

Fenton-ultrasound treatment of corn stalks enhances humification during composting by stimulating the inheritance and synthesis of polyphenolic compounds-preliminary evidence from a laboratory trial

The impact of Fenton-ultrasound treatment on the production of polyphenols and humic acid (HA) during corn stalk composting was investigated by analyzing the potential for microbial assimilation of polysaccharides in corn stalks to generate polyphenols using a13C-glucose tracer. The results showed that Fenton-ultrasound treatment promoted the decomposition of lignocellulose and increased the HA content, degree of polymerization (DP), and humification index (HI). The primary factor could be attributed to Fenton-ultrasound treatment-induced enhanced the abundance of lignocellulose-degrading microorganisms, as Firmicutes, Actinobacteria phylum and Aspergillis genus, which serve as the primary driving forces behind polyphenol and HA formation. Additionally, the utilization of a13C isotope tracer revealed that corn stalk polysaccharide decomposition products can be assimilated by microbes and subsequently secrete polyphenolic compounds. This study highlights the potential of microbial activity to generate phenolic compounds, offering a theoretical basis for increasing polyphenol production and promoting HA formation during composting.

Enhanced β-glucosidase in Western flower thrips affects its interaction with the redox-based strategies of kidney beans under elevated CO2

β-Glucosidase is validated as an elicitor for early immune responses in plants and it was detected in the salivary glands of Frankliniella occidentalis in previous research. Seven differentially expressed genes encoding β-glucosidase were obtained by comparing the transcriptomes of F. occidentalis adults grown under two different CO₂ concentrations (800 vs. 400 ppm), which might be associated with the differences in the interaction between F. occidentalis adults and its host plant, Phaseolus vulgaris under different CO₂ levels. To verify this speculation, changes in defense responses based on the production and elimination of reactive oxygen species (ROS) in P. vulgaris leaves treated with three levels of β-glucosidase activity under ambient CO₂ (aCO₂ ) and elevated CO₂ (eCO₂ ) were measured in this study. According to the results, significantly higher levels of ROS were noticed under eCO₂ compared to aCO₂ , which was caused by the increased β-glucosidase activity in thrips due to increased cellulose content in P. vulgaris leaves under eCO₂ . Together with the lower activities of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) in injured leaves under eCO₂ , P. vulgaris leaves would be negatively affected on redox-based defense by eCO₂ , thus facilitating thrips damage under climate change.

De novo transcriptome assembly of the midgut glands of herbivorous land crabs, Chiromantes haematocheir, and identification of laccase genes involved in lignin degradation

Herbivorous land crabs such as Chiromantes haematocheir and C. dehaani show biomass-degrading activities. In this study, we performed RNA-seq analysis to detect biomass-degrading enzymes. A de novo transcriptome assembly in the midgut glands of molting and non-molting C. haematocheir crabs was constructed using RNA sequencing. Illumina sequencing generated 44,937,002 and 44,394,310 reads from the two midgut glands. In total, 178,710 contigs with an average length of 750 bp and an N50 value of 1,235 bp were assembled, of which 37,890 contigs were annotated using BLASTx search against the NCBI database. We identified 22 contigs (11 genes) belonging to the laccase family and 44 contigs (22 genes) belonging to the peroxidase family. Sixteen contigs (three genes) belonging to the GH9 cellulase family were also detected. We selected the gene accounting for the majority of expressed laccase and analyzed its properties. The 24131-laccase transcript (2465 bp) had one complete open reading frame, nt 149-1987, encoding a protein of 613 amino acids with a deduced molecular mass of 67.708 kDa. The enzyme was shown to belong to the multicopper oxidase family. The 24131-laccase protein was confirmed to have oxidation activity against 2,6-dimethoxyphenol by ectopic expression in Escherichia coli. Laccase activity was significantly enhanced by feeding land crabs with plant diets. These data suggest that the enzyme plays an important role in the digestion of lignin in the guts of land crabs.

Current perspective on production and applications of microbial cellulases: a review

The potential of cellulolytic enzymes has been widely studied and explored for bioconversion processes and plays a key role in various industrial applications. Cellulase, a key enzyme for cellulose-rich waste feedstock-based biorefinery, has increasing demand in various industries, e.g., paper and pulp, juice clarification, etc. Also, there has been constant progress in developing new strategies to enhance its production, such as the application of waste feedstock as the substrate for the production of individual or enzyme cocktails, process parameters control, and genetic manipulations for enzyme production with enhanced yield, efficiency, and specificity. Further, an insight into immobilization techniques has also been presented for improved reusability of cellulase, a critical factor that controls the cost of the enzyme at an industrial scale. In addition, the review also gives an insight into the status of the significant application of cellulase in the industrial sector, with its techno-economic analysis for future applications. The present review gives a complete overview of current perspectives on the production of microbial cellulases as a promising tool to develop a sustainable and greener concept for industrial applications.

Presence and activity of endo-β-1,4-mannase, an important digestive carbohydrase within the digestive fluid of terrestrial crustaceans

Within the midgut gland of the Christmas Island red crab, Gecarcoidea natalis, a single transcript for a GH5_10 endo-β-1,4-mannase had the highest expression out of all of the carbohydrase enzymes (Gan et al. in Mar Biotechnol 20:654-665, 2018). The activity, and potential digestive importance of this hemicellulase, compared with other carbohydrases, has yet to be established. The digestive fluid of G. natalis contained substantial endo-β-1,4-mannase activities (630 ± 55 (6) nmol reducing sugars. min^-1. mg^-1 protein). It was present as a single isozyme of 66.3 ± 0.7 kDa (n = 6). Endo-β-1,4-mannase activities were higher than that for lichenase and endo-β-1,4-glucanase but lower than that for β-1,3-glucanase and amylase. The digestive fluid was able to hydrolyse, galactomannan, into its component monosaccharides. Hence, this confirms expression data that this enzyme is one of the most important digestive cellulases/ hemicellulases. Expression of GH5_10 endo-β-1,4-mannase mRNA was consistent with that of a digestive enzyme, as it was expressed in the digestive midgut gland but not in muscle and gill. Endo-β-1,4-mannase activities were also present within the digestive fluid of the terrestrial hermit crabs, Coenobita perlatus and Coenobita brevimanus. Endo-β-1,4-mannase activities (1351 ± 136 (n=3) nmol reducing sugars. min^-1 mg^-1 protein for C. perlatus. 665 ± 32 n=(5) nmol reducing sugars. min^-1 mg^-1 protein for C. brevimanus) were higher than that for endo-β-1,4-glucanase and amylase but were lower than β-1,3-glucanase activities. Animals within the terrestrial hermit crab family, Coenobitidae consume legume and palm seeds which contain substantial amounts of mannan. Hence, high endo-β-1,4-mannase activities suggest that digestion of mannan within these species may represent an important source of carbohydrate.

Response surface optimization of cellulase production from Aneurinibacillus aneurinilyticus BKT-9: An isolate of urban Himalayan freshwater

Due to their vast industrial potential, cellulases have been regarded as the potential biocatalysts by both the academicians and the industrial research groups. In the present study, culturable bacterial strains of Himalayan Urban freshwater lake were investigated for cellulose degrading activities. Initially, a total of 140 bacterial strains were isolated and only 45 isolates were found to possess cellulose degrading property. On the basis of preliminary screening involving cellulase activity assay on CMC agar (with clear zone of hydrolysis) and biosafety assessment testing, only single isolate named as BKT-9 was selected for the cellulase production studies. Strain BKT-9 was characterized at the molecular level using rRNA gene sequencing and its sequence homology analysis revealed its identity as Aneurinibacillus aneurinilyticus. Further, various physico-chemical parameters and culture conditions were optimized using one factor approach to enhance cellulase production levels in the strain BKT-9. Subsequently, RSM based statistical optimization led to formulation of cellulase production medium, wherein the bacterial strain exhibited ~60 folds increase in enzyme activity as compared to un-optimized culture medium. Further studies are being suggested to scale up cellulase production in A. aneurinilyticus strain BKT-9 so that it can be utilized for biomass saccharification at an industrial level.

Exosomes in the phloem and xylem of woody plants

Exosomes in the secondary phloem and secondary xylem of angiosperms and gymnosperms have physiological roles in the storage and transport of endoglucanases. Knowledge of plant extracellular vesicles (EVs) is limited by their presence in the apoplastic fluid of seeds and leaves. The contents of plant EVs and their biological functions are unclear. The aim of the present study was to expand our knowledge of EVs in woody plants. Sample splits were prepared from branch and stem samples from angiosperms and gymnosperms after cryomechanical destruction with liquid nitrogen. The study methods included scanning electron (SEM), atomic force microscopy (AFM), endoglucanase activity measurement. EVs visualized on the internal layers of the cell walls proved to be exosomes according to their diameter (65-145 nm). SEM revealed cup-shaped structures characteristic of exosomes in a dry state. Plant exosomes in the form of globules in the native state were visualized for the first time by AFM. Exosomes were present both in the active and dormant cambium. Erosion zones were observed at the sites of exosome localization. The activity of endo-1,4-β-glucanase was detected in Picea xylem, while the RNA level was very low, suggesting that endo-1,4-β-glucanases were preserved in the exosomes. There are grounds to assert that endo-1,4-β-glucanases delivered by exosomes participated in pit cavity formation in the S₁ layer of xylary fibres. A possible mechanism of endo-1,4-β-glucanase action in the biosynthesis of the secondary wall is proposed. These results demonstrate that the physiological role of the exosomes in the phloem and xylem is the storage and transport of endo-1,4-β-glucanases participating in cell wall remodeling in woody plants. Present study expands our knowledge about plant exosomes.

Biofilm Improves Isopod Growth Independent of the Dietary Cellulose Content

Cellulose is an abundant source of carbon, accounting for more than 50% of foliage and 90% of woody tissues of plants. Despite the diversity of species that include living or dead plant tissue in their diets, the ability to digest cellulose through self-produced enzymatic machinery is considered rare in the animal kingdom. The majority of animals studied to date rely on the cellulolytic activity of symbiotic microorganisms in their digestive tract, with some evidence for a complementary action of endogenous cellulases. Terrestrial isopods have evolved a lifestyle including feeding on a lignocellulose diet. Whether isopods utilize both external and internal cellulases for digestion of a diet is still not understood. We experimentally manipulated the content of cellulose (30%, 60%, or 90%) and the amount of biofilm (small or large) in the offered food source and quantified growth and cellulolytic activity in the gut of the isopod Porcellio scaber. The presence of a visible biofilm significantly promoted isopod growth, regardless of the cellulose content in the diet. The activity of gut cellulases was not significantly affected by the amount of biofilm or the cellulose content. Our results do not support a significant contribution of either ingested or host enzymes to cellulose utilization in P. scaber. Cellulose might not represent a key nutrient for isopods and does not seem to affect the nutritional value of the diet-associated biofilm. We propose that it is the biofilm community that determines the quality of plant diet in terrestrial isopods and potentially also in other detrital plant feeders.

Guaiacol oxidation activity of herbivorous land crabs, Chiromantes haematocheir and Chiromantes dehaani

The land crabs, Chiromantes haematocheir (Akate-gani) and Chiromantes dehaani (Kurobenkei-gani) inhabit seaside forests in Japan. The crabs mainly consume plant material and its detritus. Therefore, they are expected to possess the ability to degrade the major components of biomass, cellulose and lignin in order to digest plant materials. In this study, we analyzed biomass-degrading activities of the land crabs, especially guaiacol oxidation activity, which seems to be related to lignin degradation. Cellulase activity was detected from almost all gut samples including the stomach, midgut gland and intestine of all dissected crabs. Conversely, high guaiacol oxidation activity was detected in the midgut gland of all C. dehaani and several female C. haematocheir crabs. This is consistent with a previous study showing that female crabs were more herbivorous than male crabs were and observation that C. dehaani crabs are more herbivorous than C. haematocheir. Guaiacol oxidation activity might play an important role in the herbivorous behavior of land crabs.

Transcriptome-Guided Identification of Carbohydrate Active Enzymes (CAZy) from the Christmas Island Red Crab, Gecarcoidea natalis and a Vote for the Inclusion of Transcriptome-Derived Crustacean CAZys in Comparative Studies

The Christmas Island red crab, Gecarcoidea natalis, is an herbivorous land crab that consumes mostly fallen leaf litter. In order to subsist, G. natalis would need to have developed specialised digestive enzymes capable of supplying significant amounts of metabolisable sugars from this diet. To gain insights into the carbohydrate metabolism of G. natalis, a transcriptome assembly was performed, with a specific focus on identifying transcripts coding for carbohydrate active enzyme (CAZy) using in silico approaches. Transcriptome sequencing of the midgut gland identified 70 CAZy-coding transcripts with varying expression values. At least three newly discovered putative GH9 endo-β-1,4-glucanase ("classic cellulase") transcripts were highly expressed in the midgut gland in addition to the previously characterised GH9 and GH16 (β-1,3-glucanase) transcripts, and underscoring the utility of whole transcriptome in uncovering new CAZy-coding transcripts. A highly expressed transcript coding for GH5_10 previously missed by conventional screening of cellulase activity was inferred to be a novel endo-β-1,4-mannase in G. natalis with in silico support from homology modelling and amino acid alignment with other functionally validated GH5_10 proteins. Maximum likelihood tree reconstruction of the GH5_10 proteins demonstrates the phylogenetic affiliation of the G. natalis GH5_10 transcript to that of other decapods, supporting endogenous expression. Surprisingly, crustacean-derived GH5_10 transcripts were near absent in the current CAZy database and yet mining of the transcriptome shotgun assembly (TSA) recovered more than 100 crustacean GH5_10s in addition to several other biotechnological relevant CAZys, underscoring the unappreciated potential of the TSA database as a valuable resource for crustacean CAZys.

Lignocellulose degradation at the holobiont level: teamwork in a keystone soil invertebrate

BACKGROUND

Woodlice are recognized as keystone species in terrestrial ecosystems due to their role in the decomposition of organic matter. Thus, they contribute to lignocellulose degradation and nutrient cycling in the environment together with other macroarthropods. Lignocellulose is the main component of plants and is composed of cellulose, lignin and hemicellulose. Its digestion requires the action of multiple Carbohydrate-Active enZymes (called CAZymes), typically acting together as a cocktail with complementary, synergistic activities and modes of action. Some invertebrates express a few endogenous lignocellulose-degrading enzymes but in most species, an efficient degradation and digestion of lignocellulose can only be achieved through mutualistic associations with endosymbionts. Similar to termites, it has been suspected that several bacterial symbionts may be involved in lignocellulose degradation in terrestrial isopods, by completing the CAZyme repertoire of their hosts.

RESULTS

To test this hypothesis, host transcriptomic and microbiome shotgun metagenomic datasets were obtained and investigated from the pill bug Armadillidium vulgare. Many genes of bacterial and archaeal origin coding for CAZymes were identified in the metagenomes of several host tissues and the gut content of specimens from both laboratory lineages and a natural population of A. vulgare. Some of them may be involved in the degradation of cellulose, hemicellulose, and lignin. Reconstructing a lignocellulose-degrading microbial community based on the prokaryotic taxa contributing relevant CAZymes revealed two taxonomically distinct but functionally redundant microbial communities depending on host origin. In parallel, endogenous CAZymes were identified from the transcriptome of the host and their expression in digestive tissues was demonstrated by RT-qPCR, demonstrating a complementary enzyme repertoire for lignocellulose degradation from both the host and the microbiome in A. vulgare.

CONCLUSIONS

Our results provide new insights into the role of the microbiome in the evolution of terrestrial isopods and their adaptive radiation in terrestrial habitats.

Origin, evolution, and divergence of plant class C GH9 endoglucanases

BACKGROUND

Glycoside hydrolases of the GH9 family encode cellulases that predominantly function as endoglucanases and have wide applications in the food, paper, pharmaceutical, and biofuel industries. The partitioning of plant GH9 endoglucanases, into classes A, B, and C, is based on the differential presence of transmembrane, signal peptide, and the carbohydrate binding module (CBM49). There is considerable debate on the distribution and the functions of these enzymes which may vary in different organisms. In light of these findings we examined the origin, emergence, and subsequent divergence of plant GH9 endoglucanases, with an emphasis on elucidating the role of CBM49 in the digestion of crystalline cellulose by class C members.

RESULTS

Since, the digestion of crystalline cellulose mandates the presence of a well-defined set of aromatic and polar amino acids and/or an attributable domain that can mediate this conversion, we hypothesize a vertical mode of transfer of genes that could favour the emergence of class C like GH9 endoglucanase activity in land plants from potentially ancestral non plant taxa. We demonstrated the concomitant occurrence of a GH9 domain with CBM49 and other homologous carbohydrate binding modules, in putative endoglucanase sequences from several non-plant taxa. In the absence of comparable full length CBMs, we have characterized several low strength patterns that could approximate the CBM49, thereby, extending support for digestion of crystalline cellulose to other segments of the protein. We also provide data suggestive of the ancestral role of putative class C GH9 endoglucanases in land plants, which includes detailed phylogenetics and the presence and subsequent loss of CBM49, transmembrane, and signal peptide regions in certain populations of early land plants. These findings suggest that classes A and B of modern vascular land plants may have emerged by diverging directly from CBM49 encompassing putative class C enzymes.

CONCLUSION

Our detailed phylogenetic and bioinformatics analysis of putative GH9 endoglucanase sequences across major taxa suggests that plant class C enzymes, despite their recent discovery, could function as the last common ancestor of classes A and B. Additionally, research into their ability to digest or inter-convert crystalline and amorphous forms of cellulose could make them lucrative candidates for engineering biofuel feedstock.

Digestive enzymes of two brachyuran and two anomuran land crabs from Christmas Island, Indian Ocean

The digestive ability of four sympatric land crabs species (the gecarcinids, Gecarcoidea natalis and Discoplax celeste and the anomurans, Birgus latro and Coenobita perlatus) was examined by determining the activity of their digestive enzymes. The gecarcinids are detritivores that consume mainly leaf litter; the robber crab, B. latro, is an omnivore that preferentially consumes items high in lipid, carbohydrate and/or protein; C. perlatus is also an omnivore/detritivore. All species possess protease, lipase and amylase activity for hydrolysing ubiquitous protein, lipid and storage polysaccharides (glycogen and starch). Similarly all species possess enzymes such as N-acetyl-β-D-glucosaminidase, the cellulases, endo-β-1,4-glucanase and β-glucohydrolase and hemicellulases, lichenase and laminarinase for the respective hydrolysis of structural substrates chitin, cellulose and hemicelluloses, lichenan and laminarin. Except for the enzyme activities of C. perlatus, enzyme activity could not be correlated to dietary preference. Perhaps others factors such as olfactory and locomotor ability and metabolic status may determine the observed dietary preferences. The digestive fluid of C. perlatus possessed higher endo-β-1,4-glucanase, lichenase and laminarinase activities compared to that of the other species. Thus, C. perlatus may be efficient at digestion of cellulose and hemicellulose within plant material. Zymography indicated that the majority of protease, lipase, phosphatase, amylase, endo-β-1,4-glucanase, β-glucohydrolase and N-acetyl-β-D-glucosaminidase isozymes were common to all species, and hence were inherited from a common aquatic ancestor. Differences were observed for the phosphatase, lipase and endo-β-1,4-glucanase isozymes. These differences are discussed in relation to phylogeny and possible evolution to cope with the adoption of a terrestrial diet.

Comparative digestive physiology

In vertebrates and invertebrates, morphological and functional features of gastrointestinal (GI) tracts generally reflect food chemistry, such as content of carbohydrates, proteins, fats, and material(s) refractory to rapid digestion (e.g., cellulose). The expression of digestive enzymes and nutrient transporters approximately matches the dietary load of their respective substrates, with relatively modest excess capacity. Mechanisms explaining differences in hydrolase activity between populations and species include gene copy number variations and single-nucleotide polymorphisms. Transcriptional and posttranscriptional adjustments mediate phenotypic changes in the expression of hydrolases and transporters in response to dietary signals. Many species respond to higher food intake by flexibly increasing digestive compartment size. Fermentative processes by symbiotic microorganisms are important for cellulose degradation but are relatively slow, so animals that rely on those processes typically possess special enlarged compartment(s) to maintain a microbiota and other GI structures that slow digesta flow. The taxon richness of the gut microbiota, usually identified by 16S rRNA gene sequencing, is typically an order of magnitude greater in vertebrates than invertebrates, and the interspecific variation in microbial composition is strongly influenced by diet. Many of the nutrient transporters are orthologous across different animal phyla, though functional details may vary (e.g., glucose and amino acid transport with K+ rather than Na+ as a counter ion). Paracellular absorption is important in many birds. Natural toxins are ubiquitous in foods and may influence key features such as digesta transit, enzymatic breakdown, microbial fermentation, and absorption.

Comprehensive enzymatic analysis of the cellulolytic system in digestive fluid of the Sea Hare Aplysia kurodai. Efficient glucose release from sea lettuce by synergistic action of 45 kDa endoglucanase and 210 kDa ß-glucosidase

Although many endo-ß-1,4-glucanases have been isolated in invertebrates, their cellulolytic systems are not fully understood. In particular, gastropod feeding on seaweed is considered an excellent model system for production of bioethanol and renewable bioenergy from third-generation feedstocks (microalgae and seaweeds). In this study, enzymes involved in the conversion of cellulose and other polysaccharides to glucose in digestive fluids of the sea hare (Aplysia kurodai) were screened and characterized to determine how the sea hare obtains glucose from sea lettuce (Ulva pertusa). Four endo-ß-1,4-glucanases (21K, 45K, 65K, and 95K cellulase) and 2 ß-glucosidases (110K and 210K) were purified to a homogeneous state, and the synergistic action of these enzymes during cellulose digestion was analyzed. All cellulases exhibited cellulase and lichenase activities and showed distinct cleavage specificities against cellooligosaccharides and filter paper. Filter paper was digested to cellobiose, cellotriose, and cellotetraose by 21K cellulase, whereas 45K and 65K enzymes hydrolyzed the filter paper to cellobiose and glucose. 210K ß-glucosidase showed unique substrate specificity against synthetic and natural substrates, and 4-methylumbelliferyl (4MU)-ß-glucoside, 4MU–ß-galactoside, cello-oligosaccharides, laminarin, and lichenan were suitable substrates. Furthermore, 210K ß-glucosidase possesses lactase activity. Although ß-glucosidase and cellulase are necessary for efficient hydrolysis of carboxymethylcellulose to glucose, laminarin is hydrolyzed to glucose only by 210K ß-glucosidase. Kinetic analysis of the inhibition of 210K ß-glucosidase by D-glucono-1,5-lactone suggested the presence of 2 active sites similar to those of mammalian lactase-phlorizin hydrolase. Saccharification of sea lettuce was considerably stimulated by the synergistic action of 45K cellulase and 210K ß-glucosidase. Our results indicate that 45K cellulase and 210K ß-glucosidase are the core components of the sea hare digestive system for efficient production of glucose from sea lettuce. These findings contribute important new insights into the development of biofuel processing biotechnologies from seaweed.

Effect of Fenton's pretreatment on cotton cellulosic substrates to enhance its enzymatic hydrolysis response

Fenton's reagent that generates reactive hydroxyl radical species was evaluated for its effectiveness as a pretreatment agent on cotton cellulosic substrates to increase its susceptibility to cellulase enzyme. Response surface methodology was used to optimize four different process variables viz., time of reaction; substrate size and concentrations of Fe2+ and H2O2. Overall, the cellulose substrates treated at 0.5 mM concentration of Fe2+, 2% concentration of H2O2 for a reaction period of 48 h gave the highest enzyme activity as determined using the response surface methodology. Cellulose substrates with high aspect ratio recorded better enzyme response than that with low aspect ratio which is supported by copper number estimation. The cellulosic substrate prepared using a combination of optimized Fenton's pretreatment conditions and/or enzyme hydrolysis were studied and characterized by atomic force microscopy and scanning electron microscopy. Additionally, degree of polymerization analysis gives further insight into the degradation during Fenton's reaction.

Synergistic interaction of an endo-β-1,4-glucanase and a β-glucohydrolase leads to more efficient hydrolysis of cellulose-like polymers in the gecarcinid land crab, Gecarcoidea natalis

This study investigated synergism between endo-β-1,4-glucanase and β-glucohydrolase enzymes from Gecarcoidea natalis. Together, these enzymes efficiently hydrolyse the cellulose-like polymer, carboxymethyl cellulose, to glucose. Endo-β-1,4-glucanase and β-glucohydrolase, isolated previously from G. natalis, were incubated in vitro using a ratio of the measured activities that matches that found in their digestive juice (5.4 : 1). Their combined activity, measured as the release of glucose from carboxymethyl cellulose, was greater than the sum of their separate activities. Hence they synergistically released glucose from carboxymethyl cellulose (degree of synergy: 1.27). This may be due to the complementary nature of the products of endo-β-1,4-glucanase activity and the preferred substrates of the β-glucohydrolase. β-glucohydrolase may also enhance cellulose hydrolysis by removing cellobiose, a potential competitive inhibitor of endo-β-1,4-glucanase. The synergistic interaction of these two enzymes further supports the previous suggestion that this species possesses a novel two-enzyme cellulase system that differs from the traditional three-enzyme fungal model.

Characterisation of cellulose and hemicellulose digestion in land crabs with special reference to Gecarcoidea natalis

This article reviews the current knowledge of cellulose and hemicellulose digestion by herbivorous land crabs using the gecarcinid Gecarcoidea natalis as a model species for this group. Cellulose digestion in the gecarcinids is hypothesised to require mechanical fragmentation and enzymatic hydrolysis. Mechanical fragmentation is achieved by the chelae, mandibles and gastric mill, which reduce the material to particles less than 53 µm. The gastric mill shows adaptations towards a plant diet; in particular, there are transverse ridges on the medial and lateral teeth and ventral cusps on the lateral teeth that complement and interlock to provide efficient cutting surfaces. Enzymatic hydrolysis of cellulose and hemicellulose is achieved through cellulase and hemicellulase enzymes. In the gecarcinids, 2–3 endo-β-1,4-glucanases, one β-glucohydrolase and a laminarinase have been identified. The endo-β-1,4-glucanases are multifunctional, with both endo-β-1,4-glucanase and lichenase activity. Complete cellulose hydrolysis is achieved through the synergistic action of the endo-β-1,4-glucanase and β-glucohydrolase. The evidence for the endogenous production of the cellulase and hemicellulase enzymes, their evolutionary origin and possible evolution in invertebrates as they colonised land is also discussed.