Introduction to Fungi, Third Edition

IMPORTANCE OF HOMOBASIDIOMYCETES
531
Fig19.13 Reaction cycle of lignin peroxidase.The reduction of H 2
O 2
to H 2
O withdraws two electrons from the ground-state
enzyme (LiP), one from the ferric ion to give the ferryl ion (Fe4þ), and the other from the protoporphyrin group itself which is
converted to the porphyrin cation radical.This results in the highly oxidized LiP I state which catalyses two separate one-electron
oxidations of a reductant, e.g. veratryl alcohol (VA), becoming reduced via the LiP II state to the ground state.TheVA cation
radical is regenerated when it withdraws one electron from lignin itself or from other molecules serving as redox charge carriers
(not shown). H 2
O 2
can be generated by several means, including the oxidation of 3-chloro-anisylalcohol (CA alc) to give its
corresponding aldehyde (CA ald).
has not yet been fully characterized, although it
is likely to be pivotal as most, if not all, white-rot
fungi produce them, and in some white-rot
species such as Pycnoporus cinnabarinus they may
be the only kind of ligninolytic enzyme present
(Eggert et al., 1997). All three lignin-attacking
enzymes are usually secreted as multiple
isoforms which differ in their pH optima or
catalytic properties (Kirk & Cullen, 1998). It
should be noted that lignin degradation is
always co-metabolic, i.e. white-rot fungi cannot
utilize lignin as the sole source of carbon or
energy, and they probably remove it for the
purpose of gaining access to more easily
degraded substrates such as cellulose. Although
white-rot fungi are generally considered to be
more ‘efficient’ than brown-rots, the latter, in
circumventing the effort of lignin degradation,
probably obtain more glucose per unit metabolic
effort.
The attack on lignin by means of charge
transfer molecules being a non-specific reaction
mechanism, ligninolytic enzymes will also
degrade other substances, including man-made
recalcitrant environmental pollutants such
as polycyclic aromatic hydrocarbons, organohalogens,
or the explosive TNT (see Gadd, 2001).
White-rot fungi are therefore being investigated
extensively for their potential in the bioremediation
of contaminated environments. Another