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It has been shown recently that Trp171 of lignin peroxidase (LiP) is
hydroxylated at the Cbeta position [Blodig, W., Doyle, W. A., Smith, A. T.,
Winterhalter, K., Choinowski, T., and Piontek, K. (1998) Biochemistry 37,
8832-8838]. Comparative experiments, carried out on both wild-type fungal and
recombinant LiP isoenzyme H8 (LiPH8), indicate that the process of hydroxylation
is autocatalytic and that Trp171 may be implicated in catalysis. The role of
this residue has therefore been examined using site-directed mutagenesis to
obtain recombinant enzymes with Trp171 substituted by Phe or Ser (W171F and
W171S LiPH8, respectively). The wild-type recombinant enzyme (LiPH8) was
analyzed in solution using 1H NMR spectroscopy and its integrity confirmed prior
to the kinetic and spectroscopic characterization of LiPH8 mutants. A charge
neutralization mutation in the "classical heme edge" substrate access channel of
LiP, in which Glu146 was substituted by Gly (E146G LiPH8), showed substantial
activity with respect to veratryl alcohol (VA) oxidation and a marked (2.4 pH
units) increase in pKa for the oxidation of a negatively charged difluoroazo
dye. More surprisingly, the Trp171 LiPH8 mutants W171F and W171S LiPH8 were
found to have lost all activity with VA as substrate, and compounds I and II
were unable to react with VA. Both mutants, however, retained substantial
activity with two dye substrates. These data provide the first direct evidence
for the existence of two distinct substrate interaction sites in LiP, a
heme-edge site typical of those encountered in other peroxidases and a second,
novel site centered around Trp171 which is required for the oxidation of VA.
Stopped-flow kinetic studies showed that all the mutants examined reacted
normally with hydrogen peroxide to give a porphyrin cation radical (compound I).
However, the rapid phase of spontaneous compound I reduction (2.3 s-1), typical
of wild-type LiP, was absent in the Trp171 mutants, strongly suggesting that an
electron-transfer pathway must exist within the protein leading from the heme to
a surface site in close proximity to Trp171. The kinetic competence of such a
pathway is dependent on interaction of the enzyme with VA, at or near Trp171.
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