Oxidation and nitration of mononitrophenols by a DyP-type peroxidase

Substantial conversion of nitrophenols, typical high-redox potential phenolic substrates, by heme peroxidases has only been reported for lignin peroxidase (LiP) so far. But also a dye-decolorizing peroxidase of Auricularia auricula-judae (AauDyP) was found to be capable of acting on (i) ortho-nitrophenol (oNP), (ii) meta-nitrophenol (mNP) and (iii) para-nitrophenol (pNP). The pH dependency for pNP oxidation showed an optimum at pH 4.5, which is typical for phenol conversion by DyPs and other heme peroxidases. In the case of oNP and pNP conversion, dinitrophenols (2,4-DNP and 2,6-DNP) were identified as products and for pNP additionally p-benzoquinone. Moreover, indications were found for the formation of random polymerization products originating from initially formed phenoxy radical intermediates. Nitration was examined using 15N-labeled pNP and Na14NO2 as an additional source of nitro-groups. Products were identified by HPLC-MS, and mass-to-charge ratios were evaluated to clarify the origin of nitro-groups. The additional nitrogen in DNPs formed during enzymatic conversion was found to originate both from 15N-pNP and 14NO2Na. Based on these results, a hypothetical reaction scheme and a catalytically responsible confine of the enzyme's active site are postulated.