S2: On the Trail of Lignin Depolymerase

In 1981, it was disclosed that some gram-negative bacterial strains can grow on lignin substrates as sole sources of carbon and energy.  In 1983, however, the activity of the white-rot fungal enzyme, lignin peroxidase, was first proposed to embody the biochemical origins of lignin cleavage.  Subsequently, manganese peroxidases and laccases were added to the suite of enzymes that could be involved in the first steps of lignin biodegradation.   Nevertheless, these enzymes are limited in the degree to which they can degrade lignin macromolecules.  Mechanistically, peroxidases and laccases are poised between depolymerizing and (re)polymerizing lignins, and usually the latter predominates.

Alternative means for enzymatic lignin depolymerization have been discovered in a bacterial flavin-dependent monooxygenase that depolymerizes high molecular weight native softwood lignin preparations.   The ability of this enzyme to hydroxylate substituted aromatic rings is consistent with what should be sufficient for bringing about lignin cleavage.  Indeed, such flavin-dependent monooxygenases are widespread in the secretomes of white-rot and brown-rot fungi. 

Through classical light-scattering, the reduction in radius of gyration of the lignin macromolecules can be monitored as they are cleaved by the lignin depolymerase.  A complicating feature in such assays arises from strong intermolecular forces between lignin substructures.  Consequently, as the radius of gyration falls, the apparent molecular weight of the lignin substrate tends to increase; the resulting effect is enhanced by nonproductive interactions between the partially cleaved lignin components and the enzyme.  However, a number of auxiliary proteins are capable of annulling, in a non-catalytic manner, these kinetically-favored enhancements in association.