Addition of substrate-binding domains increases substrate-binding capacity and specific activity of a chitinase from Trichoderma harzianum

Genome-Wide Identification and Analysis of Chitinase GH18 Gene Family in Mycogone perniciosa

Mycogone perniciosa causes wet bubble disease in Agaricus bisporus and various Agaricomycetes species. In a previous work, we identified 41 GH18 chitinase genes and other pathogenicity-related genes in the genome of M. perniciosa Hp10. Chitinases are enzymes that degrade chitin, and they have diverse functions in nutrition, morphogenesis, and pathogenesis. However, these important genes in M. perniciosa have not been fully characterized, and their functions remain unclear. Here, we performed a genome-wide analysis of M. perniciosa GH18 genes and analyzed the transcriptome profiles and GH18 expression patterns in M. perniciosa during the time course of infection in A. bisporus. Phylogenetic analysis of the 41 GH18 genes with those of 15 other species showed that the genes were clustered into three groups and eight subgroups based on their conserved domains. The GH18 genes clustered in the same group shared different gene structures but had the same protein motifs. All GH18 genes were localized in different organelles, were unevenly distributed on 11 contigs, and had orthologs in the other 13 species. Twelve duplication events were identified, and these had undergone both positive and purifying selection. The transcriptome analyses revealed that numerous genes, including transporters, cell wall degrading enzymes (CWDEs), cytochrome P450, pathogenicity-related genes, secondary metabolites, and transcription factors, were significantly upregulated at different stages of M. perniciosa Hp10 infection of A. bisporus. Twenty-three out of the 41 GH18 genes were differentially expressed. The expression patterns of the 23 GH18 genes were different and were significantly expressed from 3 days post-inoculation of M. perniciosa Hp10 in A. bisporus. Five differentially expressed GH18 genes were selected for RT-PCR and gene cloning to verify RNA-seq data accuracy. The results showed that those genes were successively expressed in different infection stages, consistent with the previous sequencing results. Our study provides a comprehensive analysis of pathogenicity-related and GH18 chitinase genes’ influence on M. perniciosa mycoparasitism of A. bisporus. Our findings may serve as a basis for further studies of M. perniciosa mycoparasitism, and the results have potential value for improving resistance in A. bisporus and developing efficient disease-management strategies to mitigate wet bubble disease.

Biochemical Characterization of a Bifunctional Enzyme Constructed by the Fusion of a Glucuronan Lyase and a Chitinase from Trichoderma sp

Bifunctional enzymes created by the fusion of a glucuronan lyase (TrGL) and a chitinase (ThCHIT42) from Trichoderma sp. have been constructed with the aim to validate a proof of concept regarding the potential of the chimera lyase/hydrolase by analyzing the functionality and the efficiency of the chimeric constructions compared to parental enzymes. All the chimeric enzymes, including or nor linker (GGGGS), were shown functional with activities equivalent or higher to native enzymes. The velocity of glucuronan lyase was considerably increased for chimeras, and may involved structural modifications at the active site. The fusion has induced a slightly decrease of the thermostability of glucuronan lyase, without modifying its catalytic activity regarding pH variations ranging from 5 to 8. The biochemical properties of chitinase seemed to be more disparate between the different fusion constructions suggesting an impact of the linkers or structural interactions with the linked glucuronan lyase. The chimeric enzymes displayed a decreased stability to temperature and pH variations, compared to parental one. Overall, TrGL-ThCHIT42 offered the better compromise in terms of biochemical stability and enhanced activity, and could be a promising candidate for further experiments in the field of fungi Cell Wall-Degrading Enzymes (CWDEs).

In silico Proteomic Analysis Provides Insights Into Phylogenomics and Plant Biomass Deconstruction Potentials of the Tremelalles

Basidiomycetes populate a wide range of ecological niches but unlike ascomycetes, their capabilities to decay plant polymers and their potential for biotechnological approaches receive less attention. Particularly, identification and isolation of CAZymes is of biotechnological relevance and has the potential to improve the cache of currently available commercial enzyme cocktails toward enhanced plant biomass utilization. The order Tremellales comprises phylogenetically diverse fungi living as human pathogens, mycoparasites, saprophytes or associated with insects. Here, we have employed comparative genomics approaches to highlight the phylogenomic relationships among thirty-five Tremellales and to identify putative enzymes of biotechnological interest encoded on their genomes. Evaluation of the predicted proteomes of the thirty-five Tremellales revealed 6,918 putative carbohydrate-active enzymes (CAZYmes) and 7,066 peptidases. Two soil isolates, Saitozyma podzolica DSM 27192 and Cryptococcus sp. JCM 24511, show higher numbers harboring an average of 317 compared to a range of 267-121 CAZYmes for the rest of the strains. Similarly, the proteomes of the two soil isolates along with two plant associated strains contain higher number of peptidases sharing an average of 234 peptidases compared to a range of 226-167 for the rest of the strains. Despite these huge differences and the apparent enrichment of these enzymes among the soil isolates, the data revealed a diversity of the various enzyme families that does not reflect specific habitat type. Growth experiment on various carbohydrates to validate the predictions provides support for this view. Overall, the data indicates that the Tremellales could serve as a rich source of both CAZYmes and peptidases with wide range of potential biotechnological relevance.

Enzymatic Modification of Native Chitin and Conversion to Specialty Chemical Products

Chitin is one of the most abundant biomolecules on earth, occurring in crustacean shells and cell walls of fungi. While the polysaccharide is threatening to pollute coastal ecosystems in the form of accumulating shell-waste, it has the potential to be converted into highly profitable derivatives with applications in medicine, biotechnology, and wastewater treatment, among others. Traditionally this is still mostly done by the employment of aggressive chemicals, yielding low quality while producing toxic by-products. In the last decades, the enzymatic conversion of chitin has been on the rise, albeit still not on the same level of cost-effectiveness compared to the traditional methods due to its multi-step character. Another severe drawback of the biotechnological approach is the highly ordered structure of chitin, which renders it nigh impossible for most glycosidic hydrolases to act upon. So far, only the Auxiliary Activity 10 family (AA10), including lytic polysaccharide monooxygenases (LPMOs), is known to hydrolyse native recalcitrant chitin, which spares the expensive first step of chemical or mechanical pre-treatment to enlarge the substrate surface. The main advantages of enzymatic conversion of chitin over conventional chemical methods are the biocompability and, more strikingly, the higher product specificity, product quality, and yield of the process. Products with a higher M_w due to no unspecific depolymerisation besides an exactly defined degree and pattern of acetylation can be yielded. This provides a new toolset of thousands of new chitin and chitosan derivatives, as the physio-chemical properties can be modified according to the desired application. This review aims to provide an overview of the biotechnological tools currently at hand, as well as challenges and crucial steps to achieve the long-term goal of enzymatic conversion of native chitin into specialty chemical products.

Differences in the chitinolytic activity of mammalian chitinases on soluble and insoluble substrates

Chitin is an abundant polysaccharide used by many organisms for structural rigidity and water repulsion. As such, the insoluble crystalline structure of chitin poses significant challenges for enzymatic degradation. Acidic mammalian chitinase, a processive glycosyl hydrolase, is the primary enzyme involved in the degradation of environmental chitin in mammalian lungs. Mutations to acidic mammalian chitinase have been associated with asthma, and genetic deletion in mice increases morbidity and mortality with age. We initially set out to reverse this phenotype by engineering hyperactive acidic mammalian chitinase variants. Using a screening approach with commercial fluorogenic substrates, we identified mutations with consistent increases in activity. To determine whether the activity increases observed were consistent with more biologically relevant chitin substrates, we developed new assays to quantify chitinase activity with insoluble chitin, and identified a one-pot fluorogenic assay that is sufficiently sensitive to quantify changes to activity due to the addition or removal of a carbohydrate-binding domain. We show that the activity increases from our directed evolution screen were lost when insoluble substrates were used. In contrast, naturally occurring gain-of-function mutations gave similar results with oligomeric and insoluble substrates. We also show that activity differences between acidic mammalian chitinase and chitotriosidase are reduced with insoluble substrate, suggesting that previously reported activity differences with oligomeric substrates may have been driven by differential substrate specificity. These results highlight the need for assays against physiological substrates when engineering metabolic enzymes, and provide a new one-pot assay that may prove to be broadly applicable to engineering glycosyl hydrolases.

Evaluating the mycostimulation potential of select carbon amendments for the degradation of a model PAH by an ascomycete strain enriched from a superfund site

Although ecological flexibility has been well documented in fungi, it remains unclear how this flexibility can be exploited for pollutant degradation, especially in the Ascomycota phylum. In this work, we assess three mycostimulation amendments for their ability to induce degradation in Trichoderma harzanium, a model fungus previously isolated from a Superfund site contaminated with polycyclic aromatic hydrocarbons. The amendments used in the present study were selected based on the documented ecological roles of ascomycetes. Chitin was selected to simulate the parasitic ecological role while cellulose and wood were selected to mimic bulk soil and wood saprobic conditions, respectively. Each amendment was tested in liquid basal medium in 0.1 and 1% (w/v) suspensions. Both chitin and cellulose amendments were shown to promote anthracene degradation in T. harzanium with the 0.1% chitin amendment resulting in over 90% removal of anthracene. None of the targets monitored for gene expression were found to be upregulated suggesting alternate pathways may be used in T. harzanium. Overall, our data suggest that mycostimulation amendments can be improved by understanding the ecological roles of indigenous fungi. However, further research is needed to better estimate specific amendment requirements for a broader group of target fungi and follow up studies are needed to determine whether the trends observed herein translate to more realistic soil systems.

Glycoside hydrolases family 20 (GH20) represent putative virulence factors that are shared by animal pathogenic oomycetes, but are absent in phytopathogens

BACKGROUND

Although interest in animal pathogenic oomycetes is increasing, the molecular basis mediating oomycete-animal relationships remains virtually unknown. Crinkler (CRN) genes, which have been traditionally associated with the cytotoxic activity displayed by plant pathogenic oomycetes, were recently detected in transcriptome sequences from the entomopathogenic oomycete Lagenidium giganteum, suggesting that these genes may represent virulence factors conserved in both animal and plant pathogenic oomycetes. In order to further characterize the L. giganteum pathogenome, an on-going genomic survey was mined to reveal novel putative virulence factors, including canonical oomycete effectors Crinkler 13 (CRN13) orthologs. These novel sequences provided a basis to initiate gene expression analyses and determine if the proposed L. giganteum virulence factors are differentially expressed in the presence of mosquito larvae (Aedes aegypti).

RESULTS

Sequence analyses revealed that L. giganteum express CRN13 transcripts. The predicted proteins, like other L. giganteum CRNs, contained a conserved LYLA motif at the N terminal, but did not display signal peptides. In contrast, other potential virulence factors, such as Glycoside Hydrolases family 20 (hexosaminidase) and 37 (trehalase) proteins (GH20 and GH37), contained identifiable signal peptides. Genome mining demonstrated that GH20 genes are absent from phytopathogenic oomycete genomes, and that the L. giganteum GH20 sequence is the only reported peronosporalean GH20 gene. All other oomycete GH20 homologs were retrieved from animal pathogenic, saprolegnialean genomes. Furthermore, phylogenetic analyses demonstrated that saprolegnialean and peronosporalean GH20 protein sequences clustered in unrelated clades. The saprolegnialean GH20 sequences appeared as a strongly supported, monophyletic group nested within an arthropod-specific clade, suggesting that this gene was acquired via a lateral gene transfer event from an insect or crustacean genome. In contrast, the L. giganteum GH20 protein sequence appeared as a sister taxon to a plant-specific clade that included exochitinases with demonstrated insecticidal activities. Finally, gene expression analyses demonstrated that the L. giganteum GH20 gene expression level is significantly modulated in the presence of mosquito larvae. In agreement with the protein secretion predictions, CRN transcripts did not show any differential expression.

CONCLUSIONS

These results identified GH20 enzymes, and not CRNs, as potential pathogenicity factors shared by all animal pathogenic oomycetes.

Coexpression of cellulases in Pichia pastoris as a self-processing protein fusion

The term cellulase refers to any component of the enzymatic complex produced by some fungi, bacteria and protozoans which act serially or synergistically to catalyze the cleavage of cellulosic materials. Cellulases have been widely used in many industrial applications ranging from food industry to the production of second generation ethanol. In an effort to develop new strategies to minimize the costs of enzyme production we describe the development of a Pichia pastoris strain able to coproduce two different cellulases. For that purpose the eglII (endoglucanase II) and cbhII (cellobiohydrolase II) genes from Trichoderma reesei were fused in-frame separated by the self-processing 2A peptide sequence from the foot-and-mouth disease virus. The protein fusion construct was placed under the control of the strong inducible AOX1 promoter. Analysis of culture supernatants from methanol-induced yeast transformants showed that the protein fusion was effectively processed. Enzymatic assay showed that the processed enzymes were fully functional with the same catalytic properties of the individual enzymes produced separately. Furthermore, when combined both enzymes acted synergistically on filter paper to produce cellobiose as the main end-product. Based on these results we propose that P. pastoris should be considered as an alternative platform for the production of cellulases at competitive costs.

Protein engineering of chit42 towards improvement of chitinase and antifungal activities

The antagonism of Trichoderma strains usually correlates with the secretion of fungal cell wall degrading enzymes such as chitinases. Chitinase Chit42 is believed to play an important role in the biocontrol activity of Trichoderma strains as a biocontrol agent against phytopathogenic fungi. Chit42 lacks a chitin-binding domain (ChBD) which is involved in its binding activity to insoluble chitin. In this study, a chimeric chitinase with improved enzyme activity was produced by fusing a ChBD from T. atroviride chitinase 18-10 to Chit42. The improved chitinase containing a ChBD displayed a 1.7-fold higher specific activity than chit42. This increase suggests that the ChBD provides a strong binding capacity to insoluble chitin. Moreover, Chit42-ChBD transformants showed higher antifungal activity towards seven phytopathogenic fungal species.

Physicochemical study of a novel chimeric chitinase with enhanced binding ability

Chitinases are slow-reacting but important enzymes as they are anticipated to have diverse applications. The role of a chitin-binding domain (ChBD) in enhancing the quality of binding is essential information for purposeful engineering of chitinases. The idea of making hybrid chitinases by fusing a known ChBD to a chitinase, which naturally lacks ChBD is of interest especially for bio-controlling purposes. Therefore, in the present study, the ChBD of Serratia marcescens chitinase B was selected and fused to the fungal chitinase, Trichoderma atroviride Chit42. Both Chit42 and chemric Chit42 (ChC) showed similar activity towards colloidal chitin with specificity constants of 0.83 and 1.07 min(-1), respectively, same optimum temperatures (40°C), and similar optimum pH (4 and 4.5, respectively). In the presence of insoluble chitin, ChC showed higher activity (70%) and obtained a remarkably higher binding constant (700 times). Spectroscopic studies indicated that chimerization of Chit42 caused some structural changes, which resulted in a reduction of α-helix in ChC structure. Chemical and thermal stability studies suggested that ChC had a more stable structure than Chit42. Hill analysis of the binding data revealed mixed-cooperativity with positive cooperativity governing at ChC concentrations below 0.5 and above 2 µM in the presence of insoluble chitin. It is suggested that the addition of the ChBD to Chit42 affords structural changes which enhance the binding ability of ChC to insoluble chitin, improving its catalytic efficiency and increasing its thermal and chemical stability.

Cellulases from thermophilic fungi: recent insights and biotechnological potential

Thermophilic fungal cellulases are promising enzymes in protein engineering efforts aimed at optimizing industrial processes, such as biomass degradation and biofuel production. The cloning and expression in recent years of new cellulase genes from thermophilic fungi have led to a better understanding of cellulose degradation in these species. Moreover, crystal structures of thermophilic fungal cellulases are now available, providing insights into their function and stability. The present paper is focused on recent progress in cloning, expression, regulation, and structure of thermophilic fungal cellulases and the current research efforts to improve their properties for better use in biotechnological applications.

CBM3d, a novel subfamily of family 3 carbohydrate-binding modules identified in Cel48A exoglucanase of Cellulosilyticum ruminicola

Previously, we found that exoglucanase Cel48A from Cellulosilyticum ruminicola H1 bound intensively to Avicel; however, no known carbohydrate-binding module (CBM) was observed in the protein. Bioinformatics suggested that a C-terminal fragment of 127 amino acids, named the Cellulosilyticum-specific paralogous module (CPM), could function in binding. CPM-appended proteins are all putative (hemi)cellulases from Cellulosilyticum spp. In the present work, we demonstrated that Cel48A without the CPM retained only exoglucanase activity and lost the Avicel-binding ability, while the isolated CPM exhibited a high affinity for Avicel. In addition, the CPM bound to chitin, but not to soluble polysaccharides, making it a type A CBM, which binds only insoluble polysaccharides. Phylogenetic analysis clustered the CPM and its homologs as a separate branch that was distantly related to CBM subfamilies 3a (28% identity), 3b (24% identity), and 3c (21% identity). Sequence alignment revealed distinct secondary structures of the new CBM 3 group, in particular, a conserved Pro66-Trp67 insert preceding strand β4', a deletion preceding strand β6, and incomplete strands β8 and β9. An alanine scan for six aromatic and three nonaromatic amino acid residues (D66, P66, and R111) by site-directed mutagenesis determined that Phe62, Pro66, Trp67, Tyr68, Arg111, and Trp117 were the functional residues for binding. Among them, Phe62, Pro66, and Trp67 were the newly determined key sites in the CPM for binding. Three-dimensional homolog modeling revealed two types of substrate-binding sites, planar and groove, in the CPM. Thus, a new subfamily, CBM family 3d, is proposed.

Comparative molecular evolution of trichoderma chitinases in response to mycoparasitic interactions

Certain species of the fungal genus Trichoderma are potent mycoparasites and are used for biological control of fungal diseases on agricultural crops. In Trichoderma, whole-genome sequencing reveal between 20 and 36 different genes encoding chitinases, hydrolytic enzymes that are involved in the mycoparasitic attack. Sequences of Trichoderma chitinase genes chi18-5, chi18-13, chi18-15 and chi18-17, which all exhibit specific expression during mycoparasitism-related conditions, were determined from up to 13 different taxa and studied with regard to their evolutionary patterns. Two of them, chi18-13 and chi18-17, are members of the B1/B2 chitinase subgroup that have expanded significantly in paralog number in mycoparasitic Hypocrea atroviridis and H. virens. Chi18-13 contains two codons that evolve under positive selection and seven groups of co-evolving sites. Chi18-15 displays a unique codon-usage and contains five codons that evolve under positive selection and three groups of co-evolving sites. Regions of high amino acid variability are preferentially localized to substrate- or product side of the catalytic clefts. Differences in amino acid diversity/conservation patterns between different Trichoderma clades are observed. These observations show that Trichoderma chitinases chi18-13 and chi18-15 evolve in a manner consistent with rapid co-evolutionary interactions and identifies putative target regions involved in determining substrate-specificity.

Comparative evolutionary histories of the fungal chitinase gene family reveal non-random size expansions and contractions due to adaptive natural selection

Gene duplication and loss play an important role in the evolution of novel functions and for shaping an organism's gene content. Recently, it was suggested that stress-related genes frequently are exposed to duplications and losses, while growth-related genes show selection against change in copy number. The fungal chitinase gene family constitutes an interesting case study of gene duplication and loss, as their biological roles include growth and development as well as more stress-responsive functions. We used genome sequence data to analyze the size of the chitinase gene family in different fungal taxa, which range from 1 in Batrachochytrium dendrobatidis and Schizosaccharomyces pombe to 20 in Hypocrea jecorina and Emericella nidulans, and to infer their phylogenetic relationships. Novel chitinase subgroups are identified and their phylogenetic relationships with previously known chitinases are discussed. We also employ a stochastic birth and death model to show that the fungal chitinase gene family indeed evolves non-randomly, and we identify six fungal lineages where larger-than-expected expansions (Pezizomycotina, H. jecorina, Gibberella zeae, Uncinocarpus reesii, E. nidulans and Rhizopus oryzae), and two contractions (Coccidioides immitis and S. pombe) potentially indicate the action of adaptive natural selection. The results indicate that antagonistic fungal-fungal interactions are an important process for soil borne ascomycetes, but not for fungal species that are pathogenic in humans. Unicellular growth is correlated with a reduction of chitinase gene copy numbers which emphasizes the requirement of the combined action of several chitinases for filamentous growth.

Quantification of the CBD-FITC conjugates surface coating on cellulose fibres

Background

Cellulose Binding Domains (CBD) were conjugated with fluorescein isothiocyanate (FITC). The surface concentration of the Binding Domains adsorbed on cellulose fibres was determined by fluorescence image analysis.

Results

For a CBD-FITC concentration of 60 mg/L, a coating fraction of 78% and 110% was estimated for Portucel and Whatman fibres, respectively. For a saturating CBD concentration, using Whatman CF11 fibres, a surface concentration of 25.2 × 10^-13 mol/mm² was estimated, the equivalent to 4 protein monolayers. This result does not imply the existence of several adsorbed protein layers.

Conclusion

It was verified that CBDs were able to penetrate the fibres, according to confocal microscopy and TEM-immunolabelling analysis. The surface concentration of adsorbed CBDs was greater on amorphous fibres (phosphoric acid swollen) than on more crystalline ones (Whatman CF11 and Sigmacell 20).

Increased insect virulence in Beauveria bassiana strains overexpressing an engineered chitinase

Entomopathogenic fungi are currently being used for the control of several insect pests as alternatives or supplements to chemical insecticides. Improvements in virulence and speed of kill can be achieved by understanding the mechanisms of fungal pathogenesis and genetically modifying targeted genes, thus improving the commercial efficacy of these biocontrol agents. Entomopathogenic fungi, such as Beauveria bassiana, penetrate the insect cuticle utilizing a plethora of hydrolytic enzymes, including chitinases, which are important virulence factors. Two chitinases (Bbchit1 and Bbchit2) have previously been characterized in B. bassiana, neither of which possesses chitin-binding domains. Here we report the construction and characterization of several B. bassiana hybrid chitinases where the chitinase Bbchit1 was fused to chitin-binding domains derived from plant, bacterial, or insect sources. A hybrid chitinase containing the chitin-binding domain (BmChBD) from the silkworm Bombyx mori chitinase fused to Bbchit1 showed the greatest ability to bind to chitin compared to other hybrid chitinases. This hybrid chitinase gene (Bbchit1-BmChBD) was then placed under the control of a fungal constitutive promoter (gpd-Bbchit1-BmChBD) and transformed into B. bassiana. Insect bioassays showed a 23% reduction in time to death in the transformant compared to the wild-type fungus. This transformant also showed greater virulence than another construct (gpd-Bbchit1) with the same constitutive promoter but lacking the chitin-binding domain. We utilized a strategy where genetic components of the host insect can be incorporated into the fungal pathogen in order to increase host cuticle penetration ability.

Costs and benefits of processivity in enzymatic degradation of recalcitrant polysaccharides

Many enzymes that hydrolyze insoluble crystalline polysaccharides such as cellulose and chitin guide detached single-polymer chains through long and deep active-site clefts, leading to processive (stepwise) degradation of the polysaccharide. We have studied the links between enzyme efficiency and processivity by analyzing the effects of mutating aromatic residues in the substrate-binding groove of a processive chitobiohydrolase, chitinase B from Serratia marcescens. Mutation of two tryptophan residues (Trp-97 and Trp-220) close to the catalytic center (subsites +1 and +2) led to reduced processivity and a reduced ability to degrade crystalline chitin, suggesting that these two properties are linked. Most remarkably, the loss of processivity in the W97A mutant was accompanied by a 29-fold increase in the degradation rate for single-polymer chains as present in the soluble chitin-derivative chitosan. The properties of the W220A mutant showed a similar trend, although mutational effects were less dramatic. Processivity is thought to contribute to the degradation of crystalline polysaccharides because detached single-polymer chains are kept from reassociating with the solid material. The present results show that this processivity comes at a large cost in terms of enzyme speed. Thus, in some cases, it might be better to focus strategies for enzymatic depolymerization of polysaccharide biomass on improving substrate accessibility for nonprocessive enzymes rather than on improving the properties of processive enzymes.

Review of fungal chitinases

Chitin is the second most abundant organic and renewable source in nature, after cellulose. Chitinases are chitin-degrading enzymes. Chitinases have important biophysiological functions and immense potential applications. In recent years, researches on fungal chitinases have made fast progress, especially in molecular levels. Therefore, the present review will focus on recent advances of fungal chitinases, containing their nomenclature and assays, purification and characterization, molecular cloning and expression, family and structure, regulation, and function and application.

Carbohydrate binding modules: biochemical properties and novel applications

Polysaccharide-degrading microorganisms express a repertoire of hydrolytic enzymes that act in synergy on plant cell wall and other natural polysaccharides to elicit the degradation of often-recalcitrant substrates. These enzymes, particularly those that hydrolyze cellulose and hemicellulose, have a complex molecular architecture comprising discrete modules which are normally joined by relatively unstructured linker sequences. This structure is typically comprised of a catalytic module and one or more carbohydrate binding modules (CBMs) that bind to the polysaccharide. CBMs, by bringing the biocatalyst into intimate and prolonged association with its substrate, allow and promote catalysis. Based on their properties, CBMs are grouped into 43 families that display substantial variation in substrate specificity, along with other properties that make them a gold mine for biotechnologists who seek natural molecular "Velcro" for diverse and unusual applications. In this article, we review recent progress in the field of CBMs and provide an up-to-date summary of the latest developments in CBM applications.

Carbohydrate esterase family 4 enzymes: substrate specificity

The substrate specificity of selected enzymes classified under Carbohydrate Esterase family 4 (CE4) has been examined. Chitin deacetylase from Mucor rouxii and both a native and a truncated form of acetyl xylan esterase from Streptomyces lividans were found to be active on both xylan and several soluble chitinous substrates. Furthermore, the activities of all enzymes examined were significantly increased in the presence of Co(2+) when chitinous substrates were employed. However, the presence of this metal ion did not result in enhancing the activities of the enzymes when xylan was used as substrate. An acetyl xylan esterase from Bacillus pumilus, classified under Carbohydrate Esterase family 7, was found to be inactive towards all chitinous substrates tested. Finally, all enzymes examined were inactive towards cell wall peptidoglycan.