Ligninolysis

Ligninolysis is the biological and chemical process that deconstructs lignin, the highly cross-linked, aromatic polymer that reinforces plant secondary cell walls. Lignin’s recalcitrance makes ligninolysis a slow process, but it is a central component of the global carbon cycle and a target for biotechnological lignin valorization.

In nature, ligninolysis is mainly carried out by white-rot fungi, some brown-rot fungi, and certain bacteria. The primary enzymatic systems are lignin peroxidase (LiP), manganese peroxidase (MnP), and laccase (with redox mediators), collectively called lignin-modifying enzymes. LiP oxidizes non-phenolic lignin units at high redox potential; MnP oxidizes Mn2+ to Mn3+, which oxidizes phenolic structures; laccase catalyzes phenolic oxidation and, with small-molecule mediators, can oxidize non-phenolic lignin as well. Diverse peroxidases and DyP-type enzymes in bacteria also contribute. Some brown-rot fungi rely more on non-enzymatic Fenton chemistry to cleave polysaccharides and selectively modify lignin, rather than fully mineralize it.

Lignin’s structure is heterogeneous, consisting of monolignols—p-coumaryl, coniferyl, and sinapyl alcohols—linked by bonds such as beta-O-4. The recalcitrant network requires concerted oxidative attacks to disrupt C-C and C-O bonds, producing a range of phenolic and aliphatic fragments that can be further metabolized.

Applications include biopulping and biobleaching, pretreatment for lignocellulosic biofuels, and lignin valorization for value-added products. Understanding ligninolysis informs ecological studies and the development of industrial enzymes, though efficiency and cost remain challenges.