Reducing specific binding affinity while maintaining the enzyme activity: Dual effects of lignosulfonates on enzyme hydrolysis of sulfite-pretreated lignocellulose

It is well known that enzymatic hydrolysis is hampered by soluble inhibitors, while lignosulfonate (LS) generated from the sulfite pretreatment could enhance saccharification under certain conditions. To explain the roles of the LS during the hydrolyzing process, two types of LS were tested on selected lignocellulosic substrates and investigated through surface activity analysis and designed hydrolyzing experiments. The results showed that the LS with higher surface activity bound to and saturated the enzyme at a lower dosage and more effectively influenced the enzymatic hydrolysis. Both lignosulfonates, irrespective of their molecular weight and sulfonation degree, inhibited or enhanced the enzymatic saccharification related to two opposing mechanisms, i.e., competitive inhibition by the LS and its beneficial role on the enzyme activity. According to the Michaelis-Menten equation, the rate of cellulase-substrate complex conversion into product did not change with the introduction of the LS, whereas the specific binding affinity of the enzyme to the substrate was noticeably altered. With the introduction of LS, the stability of the enzyme increased, which increased the final hydrolysis yield. The hypothesis that the inhibition effects of LS could be effectively overcome by increasing the substrate content and the buffer concentration of the hydrolysates was confirmed through additional experiments.