Page 29 - Cascade_Biocatalysis_Integrating_Stereoselective_and_Environmentally_Friendly_Reactions

1.3 The Ligninolytic Enzymatic Consortium 5

cation may act as a redox mediator [20]. Manganese peroxidases (MnPs) generate
3+
Mn , which upon chelation with organic acids (e.g., oxalate synthesized by fungi)
attacks phenolic lignin structures; in addition, MnP can also oxidize nonphenolic
compounds via lipid peroxidation [21]. Versatile peroxidases (VPs) combine the
catalytic activities of LiP, MnP, and generic peroxidases to oxidize phenolic and
nonphenolic lignin units [22]. Some fungal oxidases produce the H O necessary
2
2
for the activity of peroxidases. Among them, aryl-alcohol oxidase (AAO) transforms
benzyl alcohols to the corresponding aldehydes; glyoxal oxidase (GLX) oxidizes
3+
glyoxal producing oxalate, which in turn chelates Mn ; and then methanol oxidase
(MOX) converts methanol into formaldehyde; all the above oxidations are coupled
with O reduction of H O . Other enzymes such as cellobiose dehydrogenase
2
2
2
(CDH) have been indirectly implicated in lignin degradation. This is because of
CDH ability to reduce both ferric iron and O -generating hydroxyl radicals via
2
Fenton reaction. These radicals are strong oxidizers that act as redox mediators
playing a fundamental role during the initial stages of lignin polymer decay, when
the small pore size of the plant cell wall prevents the access of fungal enzymes [23].
The same is true for laccases, whose substrate spectrum can be broadened in the
presence of natural mediators to act on nonphenolic parts of lignin [24].
High-redoxpotentiallaccases and peroxidases/peroxygenases are of great biotech-
nological interest [25, 26]. With minimal requirements and high redox potentials
(up to +790 mV for laccases and over +1000 mV for peroxidases), these enzymes
can oxidize a wide range of substrates, finding potential applications in a variety of
areas, which are as follows:

←−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
Figure 1.2 General view of the plant cell wall AAD and AAO (13, 14). Methanol resulting
and the action of the ligninolytic enzymatic from demethoxylation of aromatic radicals (6)
consortium. The lignin polymer is oxidized is oxidized by MOX to produce formaldehyde
by white-rot fungi laccases and peroxidases, and H O (15). Fungi also synthesize glyoxal,
2
2
producing nonphenolic aromatic radicals (1) which is oxidized by GLX to produce H O 2
2
and phenoxy radicals (2). Nonphenolic aro- and oxalate (16), which in turn chelate Mn 3+
matic radicals can suffer nonenzymatic mod- ions produced by MnP (17). The Mn 3+ chelated
ifications such as aromatic ring cleavage (3), with organic acids acts as a diffusible oxidant
ether breakdown (4), C –C cleavage (5), and for the oxidation of phenolic compounds (2).
β
α
demethoxylation (6). The phenoxy radicals (2) The reduction of ferric ions present in wood
can repolymerize on the lignin polymer (7) or is mediated by the superoxide radical (18)
be reduced to phenolic compounds by AAO and they are re-oxidized by the Fenton reac-
(8) (concomitantly with aryl alcohol oxidation). tion (19) to produce hydroxyl radicals, which
These phenolic compounds can be re-oxidized are very strong oxidizers that can attack the
by fungal enzymes (9). In addition, phenoxy lignin polymer (20). AAO, aryl-alcohol oxidase;
radicals can undergo C –C cleavage to pro- AAD, aryl-alcohol dehydrogenase; GLX, glyoxal
β
α
duce p-quinones (10). Quinones promote the oxidase; LiP, lignin peroxidase; MnP, man-
production of superoxide radicals via redox ganese peroxidase; MOX, methanol oxidase;
cycling reactions involving QR, laccases, and QR, quinone reductase; VP, versatile peroxi-
peroxidases (11, 12). The aromatic aldehydes dase. (Figure adapted from [18, 19].) (Source:
released from C –C cleavage, or synthesized Bidlack, J.M. et al. 1992 [18], Fig. 1, p. 1.
α
β
by fungi, are involved in the production of H O Reproduced with permission of the Oklahoma
2 2
via another redox cycling reaction involving Academy of Science.)