Improvement of Selenomonas ruminantium β-xylosidase thermal stability by replacing buried free cysteines via site directed mutagenesis.

β-xylosidase is an essential enzyme for breakdown of xylan to d-xylose. It has a significant potential application value for medicine, food, paper and pulp, and biofuel industries. Due to the negative consequences caused by buried free cysteine residues, mutational substitution of such residues is often accompanied by a notable increase in thermal stability. To characterize the role of cysteine residues in the structure, function and stability of Selenomonas ruminantium β-d-Xylosidase (SXA), we prepared and evaluated wild-type and four cysteines- deficient SXA proteins. Buried cysteine residues were replaced with. In comparison with the wild-type, the Km values of the mutants remained relatively constant while their kcat values decreased. The C101V and C286V displayed higher thermal stability than the wild-type at 55 and 60 °C. Conformational changes of the secondary and tertiary structure as derived from circular dichroism and fluorescence spectroscopy revealed that changing a buried cysteine to a hydrophobic residue could lead to an increase in thermal stability with minimal perturbation of the wild-type protein structure. In addition to experimental methods, the stability of WT SXA and C101V and C286V mutants at 333 K was also studied by MD simulation. Our theoretical data had a good agreement with the experimental results.