The use of small chemical oxidizers acting as redox
mediators represents a second alternative to overcome
the difficulties associated to the limited access of the
bulky lignin polymer to the activated cofactor of peroxidases
and other oxidoreductases. Enzyme-mediator
systems have been extensively investigated in the case of
laccases after the work of Bourbonnais and Paice (1990)
reporting that synthetic mediators expanded the application
potential of these low-redox-potential oxidoreductases
enabling oxidation of non-phenolic lignin model
compounds. These mediators are low-molecular-mass
compounds that: (i) form stable free radicals oxidizing
compounds that the enzyme alone is not able to oxidize,
and (ii) diffuse away from the enzyme and can easily
penetrate the lignocellulose matrix. It has been recently
shown that some phenolic lignin precursors or degradation
products can be used as ‘natural’ laccase mediators
in industrial processes, and suggested that some of them
could play a similar role in nature (Camarero et al., 2005).
However, the real significance of the laccase-mediator
systems in natural biodegradation of lignin is still to be
demonstrated.