Characterization of Active Lentinula edodes Glucoamylase Expressed and Secreted by Saccharomyces cerevisiae

The gene encoding Lentinula edodes glucoamylase (GLA) was cloned into Saccharomyces cerevisiae, expressed constitutively and secreted in an active form. The enzyme was purified to homogeneity by (NH₄)₂SO₄ fractionation, anion exchange and affinity chromatography. The protein had a correct N-terminal sequence of WAQSSVIDAYVAS, indicating that the signal peptide was efficiently cleaved. The recombinant enzyme was glycosylated with a 2.4% carbohydrate content. It had a pH optimum of 4.6 and a pH 3.4–6.4 stability range. The temperature optimum was 50°C with stability ≤50°C. The enzyme showed considerable loss of activity when incubated with glucose (44%), glucosamine (68%), galactose (22%), and xylose (64%). The addition of Mn⁺⁺ activated the enzyme by 45%, while Li⁺, Zn⁺⁺, Mg⁺⁺, Cu⁺, Ca⁺⁺, and EDTA had no effect. The enzyme hydrolyzed amylopectin at rates 1.5 and 8.0 times that of soluble starch and amylose, respectively. Soluble starch was hydrolyzed 16 and 29 times faster than wheat and corn starch granules, respectively, with the hydrolysis of starch granules using 10× the amount of GLA. Apparent K_m and V_max for soluble starch were estimated to be 3.0 mg/ml and 0.13 mg/ml/min (40°C, pH 5.3), with an apparent k_cat of 2.9×10⁵ min⁻¹.