Characterization and crystal structure of a thermostable glycoside hydrolase family 45 1,4-β-endoglucanase from Thielavia terrestris

•An endoglucanase TtCel45A from Theilavia terrestris was expressed in Pichia pastoris.•TtCel45A exhibits outstanding thermostability.•High resolution apo and substrate-bound TtCel45A crystal structures were solved.•V-VI loop movement is correlated to enzyme catalytic reaction.

1,4-β-Endoglucanase is one of the most important biocatalysts in modern industries. Here, a glycoside hydrolase (GH) family 45 endoglucanase from thermophilic fungus Theilavia terrestris (TtCel45A) was expressed in Pichia pastoris. The recombinant protein shows optimal activity at 60 °C, pH 4-5. The enzyme exhibits extraordinary thermostability that more than 80% activity was detected after heating at 80 °C for 2.5 h. The high resolution crystal structures of apo-form enzyme and that in complex with cellobiose and cellotetraose were solved to 1.36-1.58 Å. The protein folds into two overall regions: one is a six-stranded β-barrel, and the other one consists of several extended loops. Between the two regions lies the substrate-binding channel, which is an open cleft spanning across the protein surface. A continuous substrate-binding cleft from subsite −4 to +3 were clearly identified in the complex structures. Notably, the flexible V-VI loop (113Gly-114Gly-115Asp-116Leu-117Gly-118Ser) is found to open in the presence of −1 sugar, with D115 and L116 swung away to yield a space to accommodate the catalytic acid D122 and the 2,5B boat conformation of −1 sugar during transition state. Collectively, we characterized the enzyme properties of P. pastoris-expressed TtCel45A and solved high-resolution crystal structures of the enzyme. These results are of great interests in industrial applications and provide new insights into the fundamental understanding of enzyme catalytic mechanism of GH45 endoglucanases.