Cloning and characterization of a thermostable and halo-tolerant endoglucanase from Thermoanaerobacter tengcongensis MB4

A β-1,4-endoglucanase (Cel5A) was cloned from the genomic DNA of saccharolytic thermophilic eubacterium Thermoanaerobacter tengcongensis MB4 and functionally expressed in Escherichia coli. Substrate specificity analysis revealed that Cel5A cleaves specifically the β-1,4-glycosidic linkage in cellulose with high activity (294 U mg⁻¹; carboxymethyl cellulose sodium (CMC)). On CMC, kinetics of Cel5A was determined (K _m 1.39 ± 0.12 g l⁻¹; k _cat/K _m 1.41 ± 0.13 g⁻¹ s⁻¹). Cel5A displays an activity optimum between 75 and 80 °C. Residues Glu187 and Glu289 were identified as key catalytic amino acids by sequence alignment. Interestingly, derived from a non-halophilic bacterium, Cel5A exhibits high residual activities in molar concentration of NaCl (3 M, 49.3%) and KCl (4 M, 48.6%). In 1 M NaCl, 82% of Cel5A activity is retained after 24 h incubation. Molecular Dynamics studies performed at 0 and 3 M NaCl, correlate the Cel5A stability to the formation of R-COO⁻···Na⁺ ···⁻OOC-R salt bridges within the Cel5A tertiary structure, while activity possibly relates to the number of Na⁺ ions trapped into the negatively charged active site, involving a competition mechanism between substrate and Na⁺. Additionally, Cel5A is remarkably resistant in ionic liquids 1-butyl-3-methyllimidazolium chloride (1 M, 54.4%) and 1-allyl-3-methylimidazolium chloride (1 M, 65.1%) which are promising solvents for cellulose degradation and making Cel5A an attractive candidate for industrial applications.