The same three putative LRET pathways were identified
in P. eryngii VP, and their eventual operation was
investigated by site-directed mutagenesis by Pérez-
Boada and colleagues (2005). This study definitively
showed that only the pathway starting at the exposed
tryptophan (Trp-164 of isoenzyme VPL) was operative for
oxidation of high-redox-potential aromatic compounds
including veratryl alcohol. The position of this residue at
the vicinity of the haem cofactor is illustrated in Fig. 5,
which also shows a leucine residue involved in the electron
transfer; and the solvent exposed aromatic sidechain
of this tryptophan is shown in Fig. 4 (right). The
catalytic role of a homologous tryptophan has also been
shown in VP from other fungi (Tinoco et al., 2007; Tsukihara
et al., 2008). Formation of the tryptophanyl-free
radical postulated by Du and colleagues (1992) was
confirmed by low-temperature electron paramagnetic
resonance (EPR) of peroxide-activated VP (Pogni et al.,
2006).