Changing the role of lignin in enzymatic hydrolysis for a sustainable and efficient sugar platform

Hydrolyzing lignocellulose, the most abundant biomass raw material in nature, into monosaccharides by enzymes and then transforming the sugars into chemicals and fuels by biological or chemical methods is one of the most important parts of biorefining. In lignocellulosic biomass, cellulose is often tightly wrapped by lignin along with hemicelluloses. Although some pretreatments can partially remove lignin, residual lignin in pretreated substrates still physically blocks the accessibility of cellulose, non-productively adsorbs cellulases, and sterically hinders the enzymatic hydrolysis of cellulose. So, lignin is generally considered to be an inhibitor to the enzymatic hydrolysis of lignocellulosic substrates. However, new research shows that under the right conditions, lignin does not inhibit the enzymatic hydrolysis of lignocellulose, and can even increase the final sugar yield. For example, recently, some novel lignin-target pretreatments have been developed, they can reduce or overcome the inhibitory effect of lignin. And researchers also find that lignin can be functionalized and used as an activator/promoter of enzymatic hydrolysis of lignocellulose, their effectiveness is comparable to common high-efficiency surfactants. What's more, different lignin-based carriers were also synthesized to carry and recycle cellulases during the hydrolysis, they can obviously reduce the load and cost of cellulases. These works indicate that lignin can play a positive role in the enzymatic hydrolysis of lignocellulose. In order for readers to dialectically view lignin and understand its multiplicity, this review is to report the progress in the studies of how researchers tame the “lignin dog” to facilitate enzymatic hydrolysis of lignocellulose.