Structural and functional insights of the catalytic GH5 and Calx-ß domains from the metagenome-derived endoglucanase CelE2.

Cellulose is the most abundant natural polymer on Earth, representing an attractive feedstock for bioproducts and biofuel production. Cellulases promote the depolymerization of cellulose, generating short oligosaccharides and glucose, which are useful in biotechnological applications. Among the classical cellulases, those from glycoside hydrolase family 5 (GH5) are one of the most abundant in Nature, displaying several modular architectures with other accessory domains attached to its catalytic core, such as carbohydrate-binding modules (CBMs), Ig-like, FN3-like, and Calx-ß domains, which can influence the enzyme activity. The metagenome-derived endoglucanase CelE2 has in its modular architecture an N-terminal domain belonging to the GH5 family and a C-terminal domain with a high identity to the Calx-ß domain. In this study, the GH5 and the Calx-ß domains were subcloned and heterologously expressed in E. coli, to evaluate the structural and functional properties of the individualized domains of CelE2. Thermostability analysis by circular dichroism (CD) revealed a decrease in the denaturation temperature values around 4.6 °C for the catalytic domain (CelE21-381) compared to CelE2 full-length. The CD analyses revealed that the Calx-ß domain (CelE2382-477) was unfolded, suggesting that this domain requires to be attached to the catalytic core to become structurally stable. The three-dimensional structure of the catalytic domain CelE21-381 was determined at 2.1 Å resolution, showing a typical (α/ß)8-barrel fold and a narrow active site compared to other cellulases from the same family. The biochemical characterization showed that the deletion of the Calx-ß domain increased more than 3-fold the activity of the catalytic domain CelE21-381 towards the insoluble substrate Avicel. The main functional properties of CelE2, such as substrate specificity, optimal pH and temperature, thermal stability, and activation by CaCl2, were not altered after the deletion of the accessory domain. Furthermore, the Small Angle X-ray Scattering (SAXS) analyses showed that the addition of CaCl2 was beneficial CelE21-381 protein solvency. This work contributed to fundamental concepts about the structure and function of cellulases, which are useful in applications involving lignocellulosic materials degradation into food and feedstuffs and biofuel production.

Multi-omic Directed Discovery of Cellulosomes, Polysaccharide Utilization Loci, and Lignocellulases from an Enriched Rumen Anaerobic Consortium.

Lignocellulose is one of the most abundant renewable carbon sources, representing an alternative to petroleum for the production of fuel and chemicals. Nonetheless, the lignocellulose saccharification process, to release sugars for downstream applications, is one of the most crucial factors economically challenging to its use. The synergism required among the various carbohydrate-active enzymes (CAZymes) for efficient lignocellulose breakdown is often not satisfactorily achieved with an enzyme mixture from a single strain. To overcome this challenge, enrichment strategies can be applied to develop microbial communities with an efficient CAZyme arsenal, incorporating complementary and synergistic properties, to improve lignocellulose deconstruction. We report a comprehensive and deep analysis of an enriched rumen anaerobic consortium (ERAC) established on sugarcane bagasse (SB). The lignocellulolytic abilities of the ERAC were confirmed by analyzing the depolymerization of bagasse by scanning electron microscopy, enzymatic assays, and mass spectrometry. Taxonomic analysis based on 16S rRNA sequencing elucidated the community enrichment process, which was marked by a higher abundance of Firmicutes and Synergistetes species. Shotgun metagenomic sequencing of the ERAC disclosed 41 metagenome-assembled genomes (MAGs) harboring cellulosomes and polysaccharide utilization loci (PULs), along with a high diversity of CAZymes. The amino acid sequences of the majority of the predicted CAZymes (60% of the total) shared less than 90% identity with the sequences found in public databases. Additionally, a clostridial MAG identified in this study produced proteins during consortium development with scaffoldin domains and CAZymes appended to dockerin modules, thus representing a novel cellulosome-producing microorganism.IMPORTANCE The lignocellulolytic ERAC displays a unique set of plant polysaccharide-degrading enzymes (with multimodular characteristics), cellulosomal complexes, and PULs. The MAGs described here represent an expansion of the genetic content of rumen bacterial genomes dedicated to plant polysaccharide degradation, therefore providing a valuable resource for the development of biocatalytic toolbox strategies to be applied to lignocellulose-based biorefineries.

Biochemical and biophysical properties of a metagenome-derived GH5 endoglucanase displaying an unconventional domain architecture.

Endoglucanases are key enzymes in the degradation of cellulose, the most abundant polymer on Earth. The aim of this work was to perform the biochemical and biophysical characterization of CelE2, a soil metagenome derived endoglucanase. CelE2 harbors a conserved domain from glycoside hydrolase family 5 (GH5) and a C-terminal domain with identity to Calx-beta domains. The recombinant CelE2 displayed preference for hydrolysis of oat beta-glucan, followed by lichenan and carboxymethyl cellulose. Optimum values of enzymatic activity were observed at 45°C and pH 5.3, and CelE2 exhibited considerable thermal stability at 40°C for up to 360min. Regarding the cleavage pattern on polysaccharides, the release of oligosaccharides with a wide degree of polymerization indicated a characteristic of endoglucanase activity. Furthermore, the analysis of products generated from the cleavage of cellooligosaccharides suggested that CelE2 exhibited transglycosylation activity. Interestingly, the presence of CaCl2 positively affect CelE2, including in the presence of surfactants. SAXS experiments provided key information on the effect of CaCl2 on the stability of CelE2 and dummy atom and rigid-body models were generated. To the best of our knowledge this is the first biochemical and biophysical characterization of an endoglucanase from family GH5 displaying this unconventional modular organization.

Chemical stability of a cold-active cellulase with high tolerance toward surfactants and chaotropic agent.

CelE1 is a cold-active endo-acting glucanase with high activity at a broad temperature range and under alkaline conditions. Here, we examined the effects of pH on the secondary and tertiary structures, net charge, and activity of CelE1. Although variation in pH showed a small effect in the enzyme structure, the activity was highly influenced at acidic conditions, while reached the optimum activity at pH 8. Furthermore, to estimate whether CelE1 could be used as detergent additives, CelE1 activity was evaluated in the presence of surfactants. Ionic and nonionic surfactants were not able to reduce CelE1 activity significantly. Therefore, CelE1 was found to be promising candidate for use as detergent additives. Finally, we reported a thermodynamic analysis based on the structural stability and the chemical unfolding/refolding process of CelE1. The results indicated that the chemical unfolding proceeds as a reversible two-state process. These data can be useful for biotechnological applications.