The S-S bridge mutation between the A2 and A4 loops (T416C-I432C) of Cel7A of Aspergillus fumigatus enhances catalytic activity and thermostability.

Disulfide bonds are important for maintaining the structural conformation and stability of the protein. The introduction of the disulfide bond is a promising strategy to increase the thermostability of the protein. In this report, cysteine residues are introduced to form disulfide bonds in the Glycoside Hydrolase family GH 7 cellobiohydrolase (GH7 CBHs) or Cel7A of Aspergillus fumigatus. Disulfide by Design 2.0 (DbD2), an online tool is used for the detection of the mutation sites. Mutations are created (D276C-G279C; DSB1, D322C-G327C; DSB2, T416C-I432C; DSB3, G460C-S465C; DSB4) inside and outside of the peripheral loops but, not in the catalytic region. The introduction of cysteine in the A2 and A4 loop of DSB3 mutant showed higher thermostability (70% activity at 70°C), higher substrate affinity (Km = 0.081 mM) and higher catalytic activity (Kcat = 9.75 min-1; Kcat/Km = 120.37 mM min-1) compared to wild-type AfCel7A (50% activity at 70°C; Km = 0.128 mM; Kcat = 4.833 min-1; Kcat/Km = 37.75 mM min-1). The other three mutants with high B factor showed loss of thermostability and catalytic activity. Molecular dynamic simulations revealed that the mutation T416C-I432C makes the tunnel wider (DSB3: 13.6 Å; Wt: 5.3 Å) at the product exit site, giving flexibility in the entrance region or mobility of the substrate in the exit region. It may facilitate substrate entry into the catalytic tunnel and release the product faster than the wild type, whereas in other mutants, the tunnel is not prominent (DSB4), the exit is lost (DSB1), and the ligand binding site is absent (DSB2). This is the first report of the gain of function of both thermostability and enzyme activity of cellobiohydrolase Cel7A by disulfide bond engineering in the loop.IMPORTANCEBioethanol is one of the cleanest renewable energy and alternatives to fossil fuels. Cost efficient bioethanol production can be achieved through simultaneous saccharification and co-fermentation that needs active polysaccharide degrading enzymes. Cellulase enzyme complex is a crucial enzyme for second-generation bioethanol production from lignocellulosic biomass. Cellobiohydrolase (Cel7A) is an important member of this complex. In this work, we engineered (disulfide bond engineering) the Cel7A to increase its thermostability and catalytic activity which is required for its industrial application.

Biosorptive uptake of ibuprofen by steam activated biochar derived from mung bean husk: Equilibrium, kinetics, thermodynamics, modeling and eco-toxicological studies.

The present study explores the use of steam activated mung bean husk biochar (SA-MBHB) as a potential sorbent for the removal of non-steroidal and anti-inflammatory drug ibuprofen from aqueous solution. SA-MBHB was characterized by SEM, FTIR, BET, TGA, point of zero charge (pHPZC) and UV-Vis spectrophotometer. The relation between removal percentages of ibuprofen and parameters such as adsorbent dose (0.05 g-250 g), contact time (5 min-210 min), pH (2-10), speed of agitation (40-280 rpm), temperature (293-308 K) and initial ibuprofen concentration (5-100 ppm) was investigated and optimized by a series of batch sorption experiments. The optimized conditions achieved were: adsorbent dose 0.1 g/L, agitation speed 200 rpm, pH 2, initial ibuprofen concentration 20 mg L(-1), equilibrium time 120 min and temperature 20 °C for more than 99% adsorptive removal of ibuprofen. The equilibrium adsorption data were well fitted into the Langmuir isotherm model while kinetic data suggested the removal process to follow pseudo second order reaction. The adsorption phenomena were optimized and simulated by using response surface methodology (RSM) and artificial neural network (ANN). Effect of process variables viz. dose, agitation speed and pH on the sorbed amount of IBP was studied through a 2(3) full factorial central composite design (CCD). The comparative analysis was done for ibuprofen removal by constructing ANN model training using same experimental matrix of CCD. The growth of Scenedesmus abundans was also observed to be affected by the IBP solution whereas the biochar treated with IBP solution did not significantly affect the growth of the Scenedesmus abundans. The results revealed that SA-MBHB could be a cost-effective, efficient and non-hazardous adsorbent for the removal of ibuprofen from aqueous solution.

Study of bio-degradation and bio-decolourization of azo dye by Enterobacter sp. SXCR.

The objective of this study was to evaluate the decolourization potential of textile dyes by a relatively newly identified bacteria species, Enterobacter sp. SXCR which was isolated from the petroleum polluted soil samples. The bacterial strain was identified by 16S rRNA gene sequence analysis. The effects of operational conditions like initial dye concentration, pH, and temperature were optimized to develop an economically feasible decolourization process. The isolate was able to decolourize sulphonated azo dye (Congo red) over a wide range (0.1-1 gl(-1)), pH 5-9, and temperature 22-40 degrees C in static condition. Anaerobic condition with minimal salt medium supplemented with 2 gl(-1) glucose, pH 7 and 34 degrees C were considered to be the optimum decolourizing condition. The bacterial isolate SXCR showed a strong ability to decolourize dye (0.2 gl(-1)) within 93 h. The biodegradation was monitored by UV-vis, fourier transform infra-red spectroscopy (FTIR) spectroscopy and high performance liquid chromatography (HPLC). Furthermore, the involvement of azoreductase in the decolourization process was identified in this strain. Cells of Enterobacter cloacae were immobilized by entrapment in calcium-alginate beads. Immobilized bacterial cells were able to reduced azo bonds enzymatically and used as a biocatalyst for decolourization of azo dye Congo red. Michaelis-Menten kinetics was used to describe the correlation between the decolourization rate and the dye concentration.

Computational approach for identification, characterization, three-dimensional structure modelling and machine learning-based thermostability prediction of xylanases from the genome of Aspergillus fumigatus.

Identification of thermostable and alkaline xylanases from different fungal and bacterial species have gained an interest for the researchers because of its biotechnological relevance in many industries, such as pulp, paper, and bioethanol. In this study, we have identified and characterized xylanases from the genome of the thermophilic fungus of Aspergillus fumigatus by in silico analysis. Genome data mining revealed that the A fumigatus genome has six xylanase genes that belong to GH10, GH11, GH43 glycoside hydrolase families. In general, most of the bacterial and fungal GH11 xylanases are alkaline, and GH10 xylanases are acidic; however, we found that one identified xylanase from A fumigatus that belongs to the GH10 family is alkaline while the rest are acidic. Moreover, physicochemical properties also stated that most of the xylanases identified have lower molecular weight except one that belongs to the GH43 family. Structure prediction by homology modelling gave optimized structures of the xylanases. It suggests that GH10 family structure models adapt (ß∕α) 8 barrel type, GH11 homology models adapt ß-jelly type, and the GH43 family has a fivefold ß-propeller type structure. Molecular docking of identified xylanases with xylan revealed that GH11 xylanases have strong interaction (-9.6 kcal/mol) with xylan than the GH10 (-8.5 and -9.3 kcal/mol) and GH43 (-8.8 kcal/mol). We used the machine learning approach based TAXyl server to predict the thermostability of the xylanases. It revealed that two GH10 xylanases and one GH11 xylanase are thermo-active up to 75ᵒC. We have explored the physiochemical properties responsible for maintaining thermostability for bacterial and fungal GH10 and GH11 xylanases by comparing crystal structures. All the analyzed parameters specified that GH10 xylanases from both the fungi and bacteria are more thermostable due to higher hydrogen bonds, salt bridges, and helical content.

Improved catalytic activity and stability of cellobiohydrolase (Cel6A) from the Aspergillus fumigatus by rational design.

Cheap production of glucose is the current challenge for the production of cheap bioethanol. Ideal protein engineering approaches are required for improving the efficiency of the members of the cellulase, the enzyme complex involved in the saccharification process of cellulose. An attempt was made to improve the efficiency of the cellobiohydrolase (Cel6A), the important member of the cellulase isolated from Aspergillus fumigatus (AfCel6A). Structure-based variants of AfCel6A were designed. Amino acids surrounding the catalytic site and conserved residues in the cellulose-binding domain were targeted (N449V, N168G, Y50W and W24YW32Y). I mutant 3 server was used to identify the potential variants based on the free energy values (∆∆G). In silico structural analyses and molecular dynamics simulations evaluated the potentiality of the variants for increasing thermostability and catalytic activity of Cel6A. Further enzyme studies with purified protein identified the N449V is highly thermo stable (60°C) and pH tolerant (pH 5-7). Kinetic studies with Avicel determined that substrate affinity of N449V (Km =0.90 ± 0.02) is higher than the wild type (1.17 ± 0.04) and the catalytic efficiency (Kcat/Km) of N449V is ~2-fold higher than wild type. All these results suggested that our strategy for the development of recombinant enzyme is a right approach for protein engineering.

Insights from the Molecular Dynamics Simulation of Cellobiohydrolase Cel6A Molecular Structural Model from Aspergillus fumigatus NITDGPKA3.

Global demand for bioethanol is increasing tremendously as it could help to replace the conventional fossil fuel and at the same time supporting the bioremediation of huge volume of cellulosic wastes generated from different sources. Ideal genetic engineering approaches are essential to improve the efficacy of the bioethanol production processes for real time applications. A locally isolated fungal strain Aspergillus fumigatus NITDGPKA3 was used in our laboratory for the hydrolysis of lignocellulose with good cellulolytic activity when compared with other contemporary fungal strains. An attempt is made to sequence the cellobiohydrolases (CBHs) of A. fumigatus NITDGPKA3, model its structure to predict its catalytic activity towards improving the protein by genetic engineering approaches. Herein, the structure of the sequenced Cellobiohydrolases (CBHs) of A. fumigatus NITDGPKA3, modelled by homology modelling and its validation is reported. Further the catalytic activity of the modelled CBH enzyme was assessed by molecular docking analysis. Phylogenetic analysis showed that CBH from A. fumigatus NITDGPKA3 belongs to the Glycohydro 6 (Cel6A) super family. Molecular modeling and molecular dynamics simulation suggest the structural and functional mechanism of the enzyme. The structures of both the cellulose binding (CBD) and catalytic domain (CD) have been compared with most widely studied CBH of Trichoderma reesei. The molecular docking with cellulose suggests that Gln 248, Pro 287, Val236, Asn284, and Ala288 are the main amino acids involved in the hydrolysis of the ß, 1-4, glycosidic bonds of cellulose.

Kinetic study of Acid hydrolysis of rice straw.

Rice straw is a renewable, cheap, and abundant waste in tropical countries. The pentose content of rice straw can be used as a substrate for many types of value-added products such as xylitol and biofuel. Dilute acid hydrolysis mainly releases pentose from rice straw. The objective of the study was to determine the effect of H2SO4 concentration and reaction time on the xylose production. The variation of the main product xylose with the reaction time was described by a kinetic model and kinetic parameters were calculated to describe the variation of the xylose production with time. The optimum yield (19.35 g/L) was obtained at 0.24 mol/L H2SO4 and 30 minutes.