A multi-modular family 9 glycoside hydrolase (GH9) enzyme of Clostridium alkalicellulosi

Clostridium alkalicellulosi DSM17461^T is a mesophilic bacterium that can grow on different carbohydrates. The draft genome sequence of this microorganism reveals several glucoside hydrolase encoding genes that are important for cellulose degradation. In this study, the gene encoding a multi-modular family 9 glycoside hydrolase (GH9) enzyme was successfully expressed. This enzyme contains GH9 catalytic module, family 3 carbohydrate binding module, and type I dockerin at its C-termini, and we designated this enzyme as CalGH9A. The enzyme actively hydrolyzed carboxymethyl cellulose (CMC) and was able to hydrolyze regenerated amorphous cellulose (RAC) with prolong incubation (3 h) Interestingly, incubation of CalGH9A with beechwood xylan (BWX) for 16 h showed release of reducing sugar. The optimum pH and temperature for CalGH9A to hydrolyze CMC, RAC, BWX were 6.0 and 55 °C. The ability of CalGH9A to generate a series of cello-oligomers (G2-G6), suggesting that the enzyme has an endo-acting capability. The hydrolysis products from regenerated amorphous cellulose (RAC) were cellotriose (a major product), cellobiose, and glucose. CalGH9A showed higher Vmax and lower Km against CMC than those against BWX, suggesting CalGH9A is an endo-glucanase. CalGH9A resisted to several metal ions such as Mn2+, Fe2+, Co2+, Fe3+, Ni2+, Urea, and EDTA; however, Zn2+ and Cu2+ inhibited its activity. CalGH9A showed good tolerance to various concentrations of NaCl and was stable at high NaCl concentrations (10% w/v).