Two Distinct Hydrolysis Mechanisms of the Neutral Endoglucanases EG1 and EG2 of Stachybotrys microspora

Abstract

Two neutral endoglucanases (EG1 and EG2) from Stachybotrys microspora were purified and studied for their hydrolysis capacity and product profiles. Contrary to most of cellulases, especially the fungal endoglucanases such as those from Trichoderma reesei the most studied fungus, both EG1 and EG2 are more active on β-glucan from barely (β-1,3; β-1,4) than on carboxymethyl cellulose (CMC) (β-1,4). This result is much researched. Both enzymes are non specific. They hydrolyze less efficiently (30%) laminarin and only EG1 was able to attack xylan at 38%. The effectiveness of both cellulases was compared using these carbohydrates as substrates. EG1 had greater activity than EG2 on both substrates. Additionally, maximal activity (~fourfold) was observed in the presence of β-glucan from barely. The distinguishing properties of EG1 and EG2 are presented in the kinetic parameters (km, Vmax), turnover number (kcat) and catalytic efficiency (kcat/Km). EG1 catalyzed the hydrolysis of CMC with a Km of 8.048 mg/mL and a Vmax of 16.58 U/min/mg, more efficiently than EG2 with a Km of 13.71 mg/mL and a Vmax of 2.524 U/min/mg. Both purified cellulases have two distinct catalytic mechanisms. EG1 was able to hydrolyze CMC and β-glucan into soluble cellooligosaccharides (cellobiose, cellotriose) and (cellotriose, cellotetraose, cellobiose) as dominants products respectively, with a tiny amounts of glucose. In contrast, EG2 was able to efficiently hydrolyze both substrates into glucose. The relative amount of each product was dependent on the particular endoglucanase, substrate, kinetic constants and reaction time. Thus, these tow enzymatic hydrolysis behaviours should provide two alternatives for biotechnological applications.