Lignin and Lignin-Modifying Enzymes

There is increasing worldwide interest in the use of ligninolytic fungi for bioremediation purposes and for biopulping applications. Three families of fungal enzymes, designated lignin-modifying enzymes (LMEs), consist of lignin peroxidases (LiPs), manganese peroxidases (MnPs), and laccases (LACs), and these play a key role in lignin biotransformation. Demethoxylation is the most obvious consequence of attack on lignin by these fungi. Other methods such as nuclear magnetic resonance spectroscopy have also been used to study the degradation of polymeric lignin, but these methods are not easily amenable for detailed physiological and biochemical studies on white rot fungi and their enzymes. The disadvantage in the use of dimeric lignin model compounds is the fact that, unlike the lignin polymer, they can be taken up and metabolized intracellularly by microorganisms, which can make it difficult to determine whether the degradation products observed really reflect actual ligninolytic activity. Therefore, ideally, lignin model compounds should be sufficiently macromolecular but at the same time facilitate efficient product analysis. A heme peroxidase different from other microbial, plant, and animal peroxidases, termed versatile peroxidase (VP), was discovered in Pleurotus and Bjerkandera species. Hydroxylation of both phenolic and nonphenolic lignin resulting in new phenolic substructures on the lignin polymer may make it susceptible to attack by LAC or MnP.