Cellulose hydrolysis to glucose requires a series of cellulase enzymes, of which β-glucosidases play a crucial role. β-glucosidase (MbmgBG1) derived from the midgut of Macrotermes barneyi has higher glucose tolerance (maintaining more than 60% enzyme activity at 1.5 mol/L glucose). However, low enzyme activity and poor thermal stability limit the applications of β-glucosidase in food industries. Point mutants (F167L, T176C, E347I, R354K, N393G and V425M) were obtained by site-directed mutagenesis of non-conserved amino acids near conserved amino acids. Among them, the specific activities against to substrate pNPG of two mutants (F167L and R354K) were about 2-fold and 4-fold higher than that of MbmgBG1. Kcat/Km values were also higher than that of the wild-type, reflecting stronger affinity to the substrate and higher catalytic ability of mutants than MbmgBG1. When the glucose concentration was 1.5 mol/L, the enzyme activity of MbmgBG1 was about 60% of the original activity. F167L and R354K kept 60% enzymatic activity when the glucose concentrations of was 2.0 mol/L and 3.0 mol/L, respectively. These results lay a foundation for further studies on the catalytic efficiency of β-glucosidase.

Natural lignocellulosic materials contain cellulose, hemicellulose, and lignin. Cellulose hydrolysis to glucose requires a series of lignocellulases. Recently, the research on the synergistic effect of lignocellulases has become a new research focus. Here, four lignocellulase genes encoding β-glucosidase, endo-1,4-β-glucanase, xylanase and laccase from termite and their endosymbionts were cloned into pETDuet-1 and pRSFDuet-1 and expressed in Escherichia coli. After SDS-PAGE analysis, the corresponding protein bands consistent with the theoretical values were observed and all the proteins showed enzyme activities. We used phosphoric acid swollen cellulose (PASC) as substrate to measure the synergistic effect of crude extracts of co-expressing enzymes and the mixture of single enzyme. The co-expressed enzymes increased the degradation efficiency of PASC by 44% compared with the single enzyme mixture; while the degradation rate increased by 34% and 20%, respectively when using filter paper and corn cob pretreated with phosphoric acid as substrates. The degradation efficiency of the co-expressed enzymes was higher than the total efficiency of the single enzyme mixture.
Animals ; Cellulase ; Cellulose ; Hydrolysis ; Isoptera ; Lignin ; Symbiosis ; beta-Glucosidase