Functional and Mechanistic Study of a Lytic Polysaccharide Monooxygenase That Contributes to Xylan Degradation by Xylanase

Lytic polysaccharide monooxygenases (LPMOs) from the AA9 family, which can oxidatively cleave recalcitrant lignocellulosic polysaccharides, are of great importance for saccharification in the lignocellulosic biorefinery. Considering their wide substrate spectrum, it has been speculated that AA9 LPMOs may also have xylan degradation activity. In this work, the LPMO mutant R17L/N25G from Myceliophthora thermophila C1 was found to effectively cooperate with xylanase BpXyn11 to degrade xylan, resulting in a 57% increase of reducing sugar yield compared with xylanase alone. Electrochemical changes during the xylan degradation process were examined, and the results showed that the addition of R17L/N25G reduced the electric resistance while increasing the peak current of the reaction system. Interestingly, the quantum chemical calculation results revealed that different from the synergism with cellulase, R17L/N25G promoted xylan degradation by xylanase by reducing the energy barrier of the rate-limiting step of the reaction, rather than directly acting on xylan to provide accessible sites for BpXyn11. This study advances our knowledge of biomass degradation and provides a new solution for improving enzyme cocktails for lignocellulosic biorefinery applications.