Lignin–Enzyme Interactions in the Hydrolysis of Lignocellulosic Biomass

Lignin is central to overcoming recalcitrance in the enzyme hydrolysis of lignocellulose. While the term implies a physical barrier in the cell wall structure, there are also important biochemical components that direct interactions between lignin and the hydrolytic enzymes that attack cellulose in plant cell walls. Progress toward a deeper understanding of the lignin synthesis pathway – and the consistency between a range of observations over the past 40 years in the very extensive literature on cellulose hydrolysis – is resulting in advances in reducing a major impediment to cellulose conversion: the cost of enzymes. This review addresses lignin and its role in the hydrolysis of hardwood and other lignocellulosic residues. Lignin and lignin-derived phenolic compounds inhibit lignocellulolytic enzymes. While lignin nonspecifically adsorbs enzymes, phenolic compounds inhibit and/or deactivate them. The effect will vary depending on type of phenolic compounds, their concentration, and possible synergistic effects. The effect will also depend on the type of enzyme and microorganism from which they were produced. Enzymes from Trichoderma reesei are more susceptible to the inhibitory and/or deactivating effects than those from Aspergillus niger. Noncatalytic proteins, such as bovine serum albumin or soy-derived proteins, minimize the nonspecific adsorption of the hydrolytic enzymes on lignin. Understanding these mechanisms of enzyme inhibition or deactivation and approaches to mitigate them potentially favor continued reduction of cellulose conversion-associated costs.