Lignin peroxidase structure and function

Lignin peroxidase (LiP) plays a central role in the biodegradation of the p lant cell wall constituent lignin. LiP is able to oxidize aromatic compound s with redox potentials higher than 1.4 V (NHE) by single electron abstract ion, but the exact redox mechanism is still poorly understood. The finding in our laboratory that the C beta -atom of Trp171 carries a unique modifica tion led us to initiate experiments to investigate the role of this residue . These experiments, employing crystallography, site-directed mutagenesis, protein chemistry, spin-trapping and spectroscopy, yielded the following re sults: (i) Trp171 is stereospecifically hydroxylated at its C beta -atom as the result of an auto-catalytic process, which occurs under turnover condi tions in the presence of hydrogen peroxide. (ii) Evidence for the formation of a Trp171 radical intermediate has been obtained using spin-trapping, in combination with peptide mapping and protein crystallography. (iii) Trp171 is very likely to be involved in electron transfer from natural substrates to the haem cofactor via LRET. (iv) Mutagenetic substitution of Trp171 abo lishes completely the oxidation activity for veratryl alcohol, but not for artificial substrates. (v) Structural changes in response to the mutation a re marginal. Therefore the lack of activity is due to the absence of the re dox active indole side chain.