Engineered Lignin in Poplar Biomass Facilitates Cu-Catalyzed Alkaline-Oxidative Pretreatment

Both untransformed poplar and genetically
modified “zip-lignin”
poplar, in which additional ester bonds were introduced into the lignin
backbone, were subjected to mild alkaline and copper-catalyzed alkaline
hydrogen peroxide (Cu-AHP) pretreatment. Our hypothesis was that the
lignin in zip-lignin poplar would be removed more easily than lignin
in untransformed poplar during this alkaline pretreatment, resulting
in higher sugar yields following enzymatic hydrolysis. We observed
improved glucose and xylose hydrolysis yields for zip-lignin poplar
compared to untransformed poplar following both alkaline-only pretreatment
(56% glucose yield for untransformed poplar compared to 67% for zip-lignin poplar)
and Cu-AHP pretreatment (77% glucose yield for untransformed poplar compared to
85% for zip-lignin poplar). Compositional analysis, glycome profiling,
and microscopy all supported the notion that the ester linkages increase
delignification and improve sugar yields. Essentially no differences
were noted in the molecular weight distributions of solubilized lignins
between the zip-lignin poplar and the control line. Significantly,
when zip-lignin poplar was utilized as the feedstock, hydrogen peroxide,
catalyst, and enzyme loadings could all be substantially reduced while
maintaining high sugar yields.