A novel thermoacidophilic and thermostable endo-β-1,4-glucanase from Phialophora sp. G5: its thermostability influenced by a distinct β-sheet and the carbohydrate-binding module

摘要

An endo-β-1,4-glucanase gene, egG5, was cloned from the fungus Phialophora sp. G5. The 1,290-bp open reading frame encodes a bimodular cellulase composed of an N-terminal family 1 carbohydrate-binding module (CBM) and a C-terminal family 5 glycoside hydrolase catalytic module. Recombinant EgG5 produced in Pichia pastoris exhibited maximal activity at pH 4.0–5.0 and 70 °C, retained 40% of the maximal activity at pH 2.0, and was stable at pH 2.0–10.0. When compared with its closest homolog in Trichoderma sp. C-4 (70.6% identity), EgG5 had better thermostability (51.6% activity at 65 °C for 12 h vs 10% activity at 60 °C for 20 min). Sequence–structure analysis indicated that the distinct β-sheet in EgG5 in place of a linking loop in Trichoderma sp. C-4 endoglucanase might be the reason. To verify its function, two mutants, EgG5-Mut (disrupting the β-sheet with four amino acid substitutions) and EgG5-CBM (removing the CBM), were constructed, expressed in P. pastoris, and characterized. Both mutants had similar pH optima (pH 4.0) and temperature optima (70 °C) but varied in pH stabilities (pH 2.0–10.0 and pH 2.0–7.0, respectively) and thermostabilities. The thermostability of EgG5-Mut (13.4% activity vs 52.5% of EgG5 at 65 °C for 12 h) confirmed the effect of β-sheet on enzyme thermostability. EgG5-CBM was more thermostable (94.9% activity at 65 °C for 12 h and 15.5% activity at 80 °C for 30 min) and had higher specific activity (711.6 vs 60.3 U mg−1 of EgG5). This study presents an excellent endoglucanase with potential use in the bioconversion of lignocellulosic materials and provides good ideas for the improvement of enzyme thermostability.