Autocatalytic Formation of a Hydroxy Group at Cβ of Trp171 in Lignin Peroxidase

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Abstract
In the high-resolution crystal structures of two lignin peroxidase isozymes from the white rot fungus Phanerochaete chrysosporium a significant electron density at single bond distance from the Cβ of Trp171 was observed and interpreted as a hydroxy group. To further clarify the nature of this feature, we carried out tryptic digestion of the enzyme and isolated the Trp171 containing peptide. Under ambient conditions, this peptide shows an absorbance spectrum typical of tryptophan. At elevated temperature, however, the formation of an unusual absorbance spectrum with λmax = 333 nm can be followed that is identical to that of N-acetyl-α,β-didehydrotryptophanamide, resulting upon water elimination from β-hydroxy tryptophan. The Trp171 containing tryptic peptide isolated from the recombinant and refolded lignin peroxidase produced from Escherichia coli does not contain the characteristic 333 nm absorbance band at any temperature. However, treatment with 3 equiv of H2O2 leads to complete hydroxylation of Trp171. Reducing substrates compete with this process, e.g., in the presence of 0.5 mM veratryl alcohol, about 7 equiv of H2O2 is necessary for complete modification. We conclude that the hydroxylation at the Cβ of Trp171 is an autocatalytic reaction which occurs readily under conditions of natural turnover, e.g., in the ligninolytic cultures of P. chrysosporium, which are known to contain an oxidase-based H2O2-generating system. No dependence on dioxygen was found for this oxidative process. Chemical modification of fungal lignin peroxidase with the tryptophan-specific agent N-bromo succinimide leads to a drastically reduced acitivity with respect to the substrate veratryl alcohol. This suggests that Trp171 is involved in catalysis and that electron transfer from this surface residue to the oxidized heme cofactor is possible under steady-state conditions.