Role of functional groups in lignin inhibition of enzymatic hydrolysis of cellulose to glucose

The inhibitory effect of lignin and lignin model compounds on enzymatic hydrolysis of cellulose (Avicel) was investigated. Results indicated that the inhibition is not only dependent on lignin content but also on lignin sources (structures). Addition of 20% lignin (w/w) resulted in 10-23% drop, depending on source of the lignins, in 72-hour cellulose-to-glucose hydrolysis at cellulase loading of 15 FPU/g cellulose. The lignin model compounds studied all had negative impact on enzymes. Phenolic hydroxyl groups are critical to their inhibitory effects. At 10 mM concentration, phenolic compounds showed 1-5% more inhibition than nonphenolic ones. Chemical block of free phenolic hydroxyl groups by hydroxypropylation could significantly remove the inhibitory effect of lignin. No clear correlation was found between methoxyl groups and the inhibitory effect. The results suggested that the interference caused by phenolic hydroxyl groups may play more important role than physical barrier and non-specific adsorption in lignin inhibition of enzymatic hydrolysis of cellulose.