Cloning, Expression, Characterization, and Mutagenesis of a Thermostable Exoinulinase From Kluyveromyces cicerisporus.

Inulinase is an enzyme that belongs to glycoside hydrolase family 32. It converts inulin into high-fructose syrups and fructoligosaccharides, both of which are widely used in pharmaceutical and food industries. In this study, the kcINU1 gene (GenBank accession number AF178979) encoding an exoinulinase was cloned from Kluyveromyces cicerisporus CBS4857 and expressed in Pichia pastoris X-33, yielding a maximum of 45.2 ± 0.6 U mL(-1) of inulinase activity of culture supernatant. The expressed inulinase was purified and characterized. The enzyme had an optimum temperature of 55 °C and an optimum pH of 4.5. It had a K m of 0.322 mM and a V max of 4317 μM min(-1) mg(-1) protein when inulin was used as a substrate. It retained nearly 90 % of the maximal activity after pre-incubation at 50 °C for 1 h or at pH ranging from 3.0 to 6.0 at 4 °C for 24 h, demonstrating that KcINU1 was stable at high temperature and low pH. Moreover, we constructed two KcINU1 mutants, Asp30Ala and Glu215Ala, by site-directed mutagenesis and confirmed via zymogram analysis that Asp-30 and Glu-215 of the enzyme were the catalytic active center. The present study has provided important information for understanding the catalytic mechanism of exoinulinase.