BackgroundPretreating biomass with ionic liquids (IL) increases enzyme accessibility and cellulose is typically recovered through precipitation with an anti-solvent. An industrially feasible pretreatment and hydrolysis process requires robust cellulases that are stable and active in the presence of either small amounts of ILs co-precipitated with recovered cellulose or for saccharifications in the presence of IL. β-glucosidase (BG) hydrolyzes cellobiose into two molecules of glucose (Glc) and is the last step of biomass hydrolysis. These enzymes are prone not only to product inhibition by glucose but also to inactivation by ILs. With increasing interest in IL-based pretreatment methods, there is increasing focus toward a search for Glc-tolerant and IL-tolerant BG.ResultsWe identified a BG belonging to the GH1 family, H0HC94, encoded in Agrobacterium tumefaciens 5A, and cloned and overexpressed the protein in Escherichia coli. H0HC94 exhibited high enzymatic activity with β-glycosidic substrates (248 µmol/min/mg on pNPGlc and 262 µmol/min/mg on cellobiose) and tolerant to Glc (apparent Ki = 686 mM). Further evidence of Glc-based stabilization came from the increase in melting temperature of H0HC94, with increasing Glc concentrations. The half-life of H0HC94 also increased between 2- and 20-fold in the presence of increasing concentrations of Glc. In the presence of 0.9 M of different [C2mim]-based ionic liquids, the specific activity of H0HC94 decreased by around 20–30 %. However, the addition of 100 mM glucose to the IL-enzyme mix resulted in a more stable enzyme as evidenced by the slight recovery of H0HC94 melting temperature and up to tenfold increase in half-life. This higher stability came at a cost of 2–10 % decrease in specific activity. The steady-state kinetic analyses for a subset of the ionic liquids tested indicate that the enzyme undergoes uncompetitive inhibition by glucose and ionic liquid, indicating the possibility of binding of the ionic liquid and glucose to the enzyme–substrate complex.ConclusionsH0HC94 is a Glc-stabilized BG that is also tolerant up to 0.9 M concentrations of different IL’s and indicates the possibilities of using an IL–Glc-based cellulose solvent that displays enzyme-compatibility.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-016-0484-3) contains supplementary material, which is available to authorized users.
Most β-glucosidases are subjected to inhibition by the final hydrolysis product glucose resulting in the accumulation of cellobiose and oligosaccharides. This accumulated cellobiose and oligosaccharides further inhibit the activities of endoglucanase and cellobiohydrolases, resulting in the inhibition of cellulose degradation and a more expensive biofuel. To elucidate the mechanism(s) of glucose tolerance, we designed and characterised six mutations of a moderately glucose-tolerant β-glucosidase (H0HC94) from the mesophilic bacterium Agrobacterium tumefaciens 5A. The hydrophobicity and steric were varied across non-conserved residues in specific regions of the active site tunnel. In contrast to the uncompetitive inhibition of WT enzyme by glucose, C174V and H229S are competitively inhibited pointing towards a possible glucose-binding site in the protein at these positions. Increasing hydrophobicity at the +1 subsite and increasing hydrophobicity and steric at +2 subsites seemed to be critical for glucose tolerance for this BG. Additionally, in L178E, specific activity was 1.8 times higher on the natural substrate cellobiose while both W127F and L178E mutants showed an enhancement in thermostability. The kinetic stability of W127F, V176A, L178A and L178E also increased between 2- and 3-folds compared to WT. Our results indicate that while the structure between subsites +1 and +2 is critical for the glucose tolerance, the specific residues may not be identical across such enzymes.
β-glucosidase (EC 3.2.1.21; BG) cleaves β-glucosidic linkages in disaccharide or glucose-substituted molecules. In an effort towards designing better BGs, we focused on the role of non-conserved residues across an otherwise homologous BG active site tunnel and designed mutants across the aglycone-binding site (V169C) and the gatekeeper residues (I246A) of the active site tunnel. We expressed in Escherichia coli, the Hore_15280 gene encoding a β-glucosidase (BG) in Halothermothrix orenii. The overexpressed and purified wild-type (B8CYA8) has a high specific activity of 345 μmol/min/mg on pNPGlc and a half-life of 1.13 h when assayed with pNPGlc at pH 7.1 and 70 °C. The specific activities of V169C and I246A were 1.7 and 1.2 times higher than that of wild-type (WT) enzyme with the model substrate pNPGlc, while the activity on the natural substrate cellobiose was slightly higher to the WT. The two mutants were kinetically stable with 4.4- to 11-fold longer half-life compared to the WT enzyme. When the two mutations were combined to generate the V169C/I246A mutant, the specific activity increased to nearly twofold higher than WT on both substrates and the half-life increased fivefold. The two single mutants also show enhanced saccharification of insoluble natural biomass on supplementation of Trichoderma viride cellulase cocktail. These enhanced properties suggest the need for a closer look at the active site tunnel of these enzymes, especially across residues that are not conserved towards improving catalytic efficiencies.
β-Glucosidases are often inhibited by their reaction product glucose and a barrier to the efficient lignocellulosic biomass hydrolysis to glucose. We had previously reported the mutants, C174V, and H229S, with a nearly 2-fold increased glucose tolerance over the wild type (WT), H0HC94, encoded in Agrobacterium tumefaciens 5A (apparent K i , Glc = 686 mM). We report our steady−state and time-resolved intrinsic fluorescence spectroscopy, circular dichroism, and isothermal titration calorimetry (ITC) studies to further understand increased glucose tolerance. Changes in the mutants' emission intensity and the differential change in quenching rate in the absence and presence of glucose reflect changes in protein conformation by glucose. Time-resolved lifetime and anisotropy measurements further indicated the microenvironment differences across solvent-exposed tryptophan residues and a higher hydrodynamic radius due to glucose binding, respectively. ITC measurements confirmed the increase of glucose binding sites in the mutants. The experiment results were supported by molecular dynamics simulations, which revealed significant variations in the glucose−protein hydrogen-bonding profiles. Protein structure network analysis of the simulated structures further indicates the mutants' conformation change than the WT. Computational studies also indicated additional glucose binding sites in mutants. Our results indicate the role of glucose binding in modulating the enzyme response to glucose.
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