10-01: Novel lignin modifying redox-catalytic mechanism in termite to overcome plant cell wall recalcitrance

Wood-feeding termites have been well recognized for their ability to efficiently utilize lignocellulosic sugars. However, the exact mechanisms of plant cell wall (PCW) deconstruction in termite remains unknown. Our previous work revealed selective lignin modifications occurring in the termite Coptotermes formosanus (Shiraki), which were believed to enhance cellulose utilization in the hindgut. We present here a speculation on how termites accomplish the selective reactions to reduce biomass recalcitrance. Concentrated H₂O₂, Fe (II), and pyridine-like compounds were detected in the midgut, where lignin-unlocking primarily occurs. As well, similar lignin-modification mechanism was demonstrated in-vitro by the oxidative-catalytic system consisting of H₂O₂ and Fe(II)-bipyridyl (HPFP). The saccharification results of pretreated ligninsulfonate and softwood strongly suggest that this system with μmol chemicals at mild temperature/pressure significantly reduced lignin inhibition effect on the cellulases and enhanced the cellulose hydrolysis. We herein formulated our hypothesis that this non-enzymatic lignin-modifying system works in the termite midgut as an advanced and unique unlocking system to promote the selective modification on lignin network and component-association for efficient biomass sugar release. The HPFP system with small molecular size is able to penetrate the PCW. Extra H₂O₂ can also escape from the decomposed lignocellulose to act as a cofactor for peroxidases, until the erosion of PCW allows for the entry of lignases. The main factor responsible for lignin-unlocking is believed to be O₂·^- generated from H₂O₂ along with the catalysis of Fe(II)-pyridine analogs. This research results can provide new insight for the design of a new generation of lignocelluloses processing systems.