Enzymatic saccharification of cassava residues and glucose inhibitory kinetics on β-glucosidase from Hypocrea orientalis

Abstract

Cassava residues are byproducts of the starch industry containing abundant cellulose for bioproduction of green fuel. To obtain maximum sugar yields from cassava residues, the optimal conditions for hydrolyzing the residues were determined using cellulase prepared from a novel Hypocrea orientalis strain. The optimal pH value and optimal temperature for the cellulase hydrolysis were 5.0 and 50 °C, respectively. The concentration of NaOH was determined to be 1% for pretreatment of cassava residues to gain enough soluble sugars suitably. The yield of released sugars was 10 mg/mL in the optimal conditions after 24 h of reaction, which was similar to that of bagasse and wheat grass. Inhibition kinetics of H. orientalis β-glucosidase (BG) by glucose was first studied using the progress-of-substrate-reaction method as described by Tsou (Tsou, C. L. Adv. Enzymol. Related Areas Mol. Biol. 1988, 61, 381-436), and the microscopic inhibition rate constants of glucose were determined. The results showed that glucose could inhibit BG reversibly and competitively. The rate constants of forward (k+0) and reverse (k-0) reaction were measured to be 4.88 × 10-4 (mM·s)-1 and 2.7 × 10-4 s-1, respectively. Meanwhile, the inhibition was more significant than that of l-glucose, d-mannose, d-galactose, d-aminoglucose, acetyl-d-glucose, and d-fructose. This work reveals how to increase sugar yields and reduce product inhibition during enzymatic saccharification of cellulose