Impact of disulfide bonds on the folding and refolding capability of a novel thermostable GH45 cellulase

A new cellulase (TaCel45) of glycoside hydrolase family 45 was identified in the thermophilic fungus Thielavia arenaria XZ7 and was successfully expressed in Pichia pastoris. The specific activities of TaCel45 towards lichenin, sodium carboxymethylcellulose (CMC-Na), and barley β-glucan were 769, 498, and 486 U/mg protein, respectively, which are higher than the values for all other reported GH45 cellulases. TaCel45 had maximum activity at pH 5.0-6.0 and 60-65 °C with barley β-glucan and CMC-Na as substrates and had a melting temperature (T_m) of 68.4 °C. However, TaCel45 exhibited extraordinary thermostability at 90 and 100 °C, retaining more than 70 and 45% of its activity after a 1-h incubation, respectively. Seven mutants (C11S, C12S, C16S, C31S, C171S, C193S, and C203S) were then constructed to investigate the effects of each disulfide bond on the structure, activity, and stability of TaCel45. As a result, six disulfide bonds (C11-C136, C16-C87, C31-C57, C88-C203, C90-C193, and C160-Cy171) were found to be indispensable for the folding, secretion, and activity of TaCel45, while C12-C48 was critical for thermal adaptation and refolding. The mutant C12S showed decreased optimal temperature and T_m values of 50 and 60.2 °C, respectively, and retained less than 50% of the thermal refolding ability of the wild type. Overall, this study demonstrated that disulfide bonds play a vital role in the folding and refolding capability and thermostability of this GH45 cellulase.