Recombinant cellobiohydrolase of Myceliophthora thermophila: characterization and applicability in cellulose saccharification

Cloning, expression and purification of cellobiohydrolase gene from Caldicellulosiruptor bescii for efficient saccharification of plant biomass

Anthropogenic activities have led to a drastic shift from natural fuels to alternative renewable energy reserves that demand heat-stable cellulases. Cellobiohydrolase is an indispensable member of cellulases that play a critical role in the degradation of cellulosic biomass. This article details the process of cloning the cellobiohydrolase gene from the thermophilic bacterium Caldicellulosiruptor bescii and expressing it in Escherichia coli (BL21) CondonPlus DE3-(RIPL) using the pET-21a(+) expression vector. Multi-alignments and structural modeling studies reveal that recombinant CbCBH contained a conserved cellulose binding domain III. The enzyme's catalytic site included Asp-372 and Glu-620, which are either involved in substrate or metal binding. The purified CbCBH, with a molecular weight of 91.8 kDa, displayed peak activity against pNPC (167.93 U/mg) at 65°C and pH 6.0. Moreover, it demonstrated remarkable stability across a broad temperature range (60-80°C) for 8 h. Additionally, the Plackett-Burman experimental model was employed to assess the saccharification of pretreated sugarcane bagasse with CbCBH, aiming to evaluate the cultivation conditions. The optimized parameters, including a pH of 6.0, a temperature of 55°C, a 24-hour incubation period, a substrate concentration of 1.5% (w/v), and enzyme activity of 120 U, resulted in an observed saccharification efficiency of 28.45%. This discovery indicates that the recombinant CbCBH holds promising potential for biofuel sector.

Functional characterisation of a new halotolerant seawater active glycoside hydrolase family 6 cellobiohydrolase from a salt marsh

Realising a fully circular bioeconomy requires the valorisation of lignocellulosic biomass. Cellulose is the most attractive component of lignocellulose but depolymerisation is inefficient, expensive and resource intensive requiring substantial volumes of potable water. Seawater is an attractive prospective replacement, however seawater tolerant enzymes are required for the development of seawater-based biorefineries. Here, we report a halophilic cellobiohydrolase SMECel6A, identified and isolated from a salt marsh meta-exo-proteome dataset with high sequence divergence to previously characterised cellobiohydrolases. SMECel6A contains a glycoside hydrolase family 6 (GH6) domain and a carbohydrate binding module family 2 (CBM2) domain. Characterisation of recombinant SMECel6A revealed SMECel6A to be active upon crystalline and amorphous cellulose. Mono- and oligosaccharide product profiles revealed cellobiose as the major hydrolysis product confirming SMECel6A as a cellobiohydrolase. We show SMECel6A to be halophilic with optimal activity achieved in 0.5X seawater displaying 80.6 ± 6.93% activity in 1 × seawater. Structural predictions revealed similarity to a characterised halophilic cellobiohydrolase despite sharing only 57% sequence identity. Sequential thermocycling revealed SMECel6A had the ability to partially reversibly denature exclusively in seawater retaining significant activity. Our study confirms that salt marsh ecosystems harbour enzymes with attractive traits with biotechnological potential for implementation in ionic solution based bioprocessing systems.

Development of an efficient protein expression system in the thermophilic fungus Myceliophthora thermophila

BACKGROUND

Thermophilic fungus Myceliophthora thermophila has been widely used in industrial applications due to its ability to produce various enzymes. However, the lack of an efficient protein expression system has limited its biotechnological applications.

RESULTS

In this study, using a laccase gene reporting system, we developed an efficient protein expression system in M. thermophila through the selection of strong constitutive promoters, 5'UTRs and signal peptides. The expression of the laccase was confirmed by enzyme activity assays. The results showed that the Mtpdc promoter (Ppdc) was able to drive high-level expression of the target protein in M. thermophila. Manipulation of the 5'UTR also has significant effects on protein expression and secretion. The best 5'UTR (NCA-7d) was identified. The transformant containing the laccase gene under the Mtpdc promoter, NCA-7d 5'UTR and its own signal peptide with the highest laccase activity (1708 U/L) was obtained. In addition, the expression system was stable and could be used for the production of various proteins, including homologous proteins like MtCbh-1, MtGh5-1, MtLPMO9B, and MtEpl1, as well as a glucoamylase from Trichoderma reesei.

CONCLUSIONS

An efficient protein expression system was established in M. thermophila for the production of various proteins. This study provides a valuable tool for protein production in M. thermophila and expands its potential for biotechnological applications.

Biochemical characteristics of Myceliophthora thermophila recombinant β-glucosidase (MtBgl3c) applicable in cellulose bioconversion

The GH3 β-glucosidase gene of Myceliophthora thermophila (MtBgl3c) has been cloned and heterologously expressed in E. coli for the first time. This study highlights the important characteristics of recombinant MtBgl3c (rMtBgl3c) which make it a promising candidate in industrial applications. Optimization of the production of rMtBgl3c led to 28,000 U L-1. On purification, it has a molecular mass of ∼100 kDa. It is a broad substrate specific thermostable enzyme that exhibits pH and temperature optima at 5.0 and 55 °C, respectively. The amino acid residues Asp287 and Glu514 act as nucleophile and catalytic acid/base, respectively in the enzyme catalysis. Its low Km value (1.28 mM) indicates a high substrate affinity as compared to those previously reported. The rMtBgl3c displays a synergistic action with the commercial enzyme cocktail in the saccharification of sugarcane bagasse suggesting its utility in the cellulose bioconversion. Tolerance to solvents, detergents as well as glucose make this enzyme applicable in wine, detergent, paper and textile industries too.

Structural and thermostability insights into cellobiohydrolase of a thermophilic mould Myceliophthora thermophila: in-silico studies

Cellobiohydrolase (CBH) is one of the cellulases with a wide range of industrial applications; it plays a pivotal role in cellulose hydrolysis and thus in biofuel production. The structural and thermostability analysis of a CBHII of the thermophilic mold Myceliophthora thermophila (MtCel6A) had been carried out using various in-silico approaches. The validation of 3 D model by the Ramachandran plot indicated 88.5% amino acid residues in the favoured regions. Docking analysis suggested MtCel6A to display a high affinity towards cellotetraose as compared to other substrates. The enzyme exhibited a high tolerance to the end product, cellobiose. The thermostability evaluation by molecular dynamic simulations and principal component analysis confirmed its tolerance to elevated temperatures. The identified thermolabile regions could be targeted for site-directed mutagenesis in order to ameliorate thermostability further. Our experimental data published earlier confirmed the present findings of in-silico studies. The structural and functional characteristics of MtCel6A highlighted its critical features that make it a useful biocatalyst in several industrial processes.Communicated by Ramaswamy H. Sarma.

Boosting enzymatic degradation of cellulose using a fungal expansin: Structural insight into the pretreatment mechanism

The recalcitrance of cellulosic biomass greatly hinders its enzymatic degradation. Expansins induce cell wall loosening and promote efficient cellulose utilization; however, the molecular mechanism underlying their action is not well understood. In this study, TlEXLX1, a fungal expansin from Talaromyces leycettanus JCM12802, was characterized in terms of phylogeny, synergy, structure, and mechanism of action. TlEXLX1 displayed varying degrees of synergism with commercial cellulase in the pretreatment of corn straw and filter paper. TlEXLX1 binds to cellulose via domain 2, mediated by CH-π interactions with residues Tyr291, Trp292, and Tyr327. Residues Asp237, Glu238, and Asp248 in domain 1 form hydrogen bonds with glucose units and break the inherent hydrogen bonding within the cellulose matrix. This study identified the expansin amino acid residues crucial for cellulose binding, and elucidated the structure and function of expansins in cell wall networks; this has potential applications in biomass utilization.