Lignin peroxidase oxidation of veratryl alcohol: effects of the mutants H82A, Q222A, W171A, and F267L

The site-directed mutations H82A and Q222A (residues near the heme access channel), and W171A and F267L (residues near the surface of the protein) were introduced into the gene encoding lignin peroxidase (LiP) isozyme H8 from Phanerochaete chrysosporium. The variant enzymes were produced by homologous expression in P. chrysosporium, purified to homogeneity, and characterized by kinetic and spectroscopic methods. The molecular masses, the pIs, and the UV-vis absorption spectra of the ferric and oxidized states of these LiP variant enzymes were similar to those of wild-type LiP (wtLiP), suggesting the overall protein and heme environments were not significantly affected by these mutations. The steady-state and transient-state parameters for the oxidation of veratryl alcohol (VA) by the H82A and Q222A variants were very similar to those of wtLiP, demonstrating that these residues are not involved in VA oxidation and that the heme access channel is an unlikely site for VA oxidation. In contrast, the W171A variant was unable to oxidize VA, confirming the apparent essentiality of Trp171 in VA oxidation by LiP. The kinetic rates of spontaneous LiP compound I reduction in the absence of VA were similar for W171A and wild-type LiP, suggesting that there may not be a radical formed on the Trp171 residue of LiP in the absence of VA. For the F267L variant, both the K(m app) value in the steady state and the apparent dissociation constant (K(D)) for compound II reduction were greater than those for wtLiP. These results indicate that the site including W171 and F267, rather than the heme access channel, is the site of VA binding and oxidation in LiP. Whereas Trp171 appears to be essential for VA oxidation, it apparently is not independently responsible for the spontaneous decomposition of oxidized intermediates. The nearby Phe267 apparently is also involved in VA binding.