As noted above, laccases were the first ligninolytic
enzymes to be investigated, and had been known in plants for
many years. Nevertheless, the first molecular structure of a
complete fungal laccase was published only in 2002. That
year, the crystal structures of the laccases from the basidiomycete
T. versicolor, and the ascomycete Melanocarpus
albomyces were reported(Fig. 5C). The first structure
of a bacterial laccase was published one year later.
A previously reported laccase structure corresponded to an
inactive form due to the loss of a copper ion during deglycosylation
to obtain suitable crystals for X-ray diffraction.
The active site of laccases includes four copper ions. Type-I
copper (right sphere in Fig. 5C) acts as electron acceptor
from substituted phenols or amines (the typical laccase substrates);
and type-II copper, which transfers the electrons to
the final acceptor, dioxygen, which is reduced to water. The
two type-III coppers act as intermediates in the electron
transfer pathway that also includes one cysteine and two histidine
protein residues. The molecular environment of laccase
type-I copper seems to regulate the redox potential of
the enzyme. The fact that laccase can use atmospheric
oxygen as the final electron acceptor represents a considerable
advantage for industrial and environmental applications
compared with peroxidases, which require a continuous supply
of H2O2. Taking into account that the advantage of peroxidases
is their higher redox potential, engineering the active
site of laccases to obtain high redox potential variants would
be of considerable biotechnological interest.