Introduction to Fungi, Third Edition

530 HOMOBASIDIOMYCETES
are common on the surfaces of many fungal
hyphae. Oxalic acid may also chelate other ions
such as Cu 2þ , which is used as a preservative for
the treatment of wood. The oxalic acid concentration
may bring about strongly acidic conditions,
sometimes as low as pH 1.7, which would
be sufficient for acid hydrolysis of pectin and
even cellulose (Green et al., 1991). Both endoand
exo-enzymes of the cellulase complex are
generally produced by brown-rot fungi (Hegarty
et al., 1987), and these may act in the usual
synergistic way to convert cellulose to oligosaccharides
and thence to glucose (Radford et al.,
1996). Hemicellulases are thought to act in a
similar way. However, these enzymes may
not gain access to their substrate where it is
masked by the lignin-containing secondary wall.
Although the detailed mechanism of brown-rot
decay is still unknown, there is evidence that
at least the initial attack on cellulose
(and hemicellulose) is mediated by small molecules
capable of penetrating the lignin layer.
These may be the hydronium ion (H 3 O þ ) generated
by oxalic acid in water, or another molecule
such as the hydroxyl radical (HO) released from
hydrogen peroxide (H 2 O 2 ) by the Fenton reaction
(Fe 2þ þ H 2 O 2 ! Fe 3þ þ HO þ HO ). The
mechanism of brown-rot decay is all the more
mysterious because many brown-rot fungi do,
in fact, produce enzymes capable of degrading
lignin (Mtui & Nakamura, 2004), but these may
have other functions, such as the detoxification
of antimicrobial phenolics which are often
present in wood at high concentrations
(Rabinovich et al., 2004).
White-rot
The suite of enzymes required to break down
lignin has been most thoroughly examined in
the white-rot fungus Phanerochaete chrysosporium
(see de Jong et al., 1994a; Heinzkill & Messner,
1997). There are numerous conflicting ideas
about the enzymology of lignin degradation,
and we can only generalize here. Initial attack on
lignin is mediated by lignin peroxidases (LiP)
and/or manganese peroxidases (MnP) which do
not themselves enter the lignin layer. Instead,
small diffusible molecules probably act as redox
charge carriers between the enzymes and their
substrate. A possible reaction scheme for LiP is
shown in Fig. 19.13 (Heinzkill & Messner, 1997;
ten Have & Teunissen, 2001). The enzyme consists
of a single polypeptide chain and a protoporphyrin
IX (haem) group which is buried deep
inside the enzyme, accessible to small diffusible
molecules through a narrow pore. LiP loses two
electrons when it reduces H 2 O 2 to water. This
highly oxidized LiP I state is returned to the
ground state in two steps, each associated with
the one-electron oxidation of a reductant into
its cation radical. Veratryl alcohol, produced
in abundance by most white-rot fungi, is such
a reductant. The two cation radicals leave the
enzyme and either themselves attack the lignin
structure, or pass on their charge to other small
molecules. Either way, the extraction of one
electron from an aromatic ring of lignin generates
a structure reacting both as a cation and as a
radical, leading to numerous possible degradation
products. The conversion of LiP into LiP I
requires H 2 O 2 which is generated by extracellular
enzymes such as glucose oxidase or aryl
alcohol oxidase. The latter uses organohalogens
such as 3-chloroanisylalcohol as substrates, and
these may accumulate in the environment
colonized by white-rot fungi (de Jong et al.,
1994b), even though organohalogens have traditionally
been regarded as man-made (anthropogenic)
environmental pollutants.
Manganese peroxidase (MnP) is closely related
to LiP in its protein structure and its catalytic
cycle, except that the co-factor is Mn 2þ instead of
veratryl alcohol. The oxidized Mn 3þ may be
stabilized by organic acids en route to the
lignin substrate, where it catalyses a one-electron
oxidation either directly or via charge transfer
molecules, followed by recycling of Mn 2þ back to
the MnP enzyme. Copper-containing laccases are
a third group of enzymes attacking lignin, and
these are produced by a range of fungi much
wider than that causing white-rot. By reducing
O 2 to H 2 O, laccases are capable of performing a
four-electron oxidation either of a redox carrier
or the final substrate itself. In this way, laccases
may be able to degrade smaller lignin fragments,
although they are probably incapable of a direct
attack on intact lignin, due to steric problems.
The precise role of laccases in lignin degradation