Conversion of Lignin Peroxidase Compound III to Active Enzyme by Cation Radicals

It has been previously reported that the catalytically inactive compound III form of lignin peroxidase is formed during the oxidation of certain chemicals such as phenols (P. J. Harvey and J. M. Palmer, 1990, J. Biotechnol. 13, 169-179). Here we provide evidence that the cation radicals of methoxybenzenes such as 1,2,4,5-tetramethoxybenzene (TMB) and veratryl alcohol promote the oxidative conversion of compound III back to ferric enzyme. Two kinetic phases were observed during the oxidation of TMB by lignin peroxidase. In the first phase the formation of TMB cation radical and compound III were observed simultaneously. The second phase involved a rapid disappearance of compound III and the TMB cation radical. Ferric enzyme appeared concomitantly with the disappearance of compound III. The TMB cation radical, generated electrochemically, was able to convert compound III to ferric enzyme. Comparative studies using veratryl alcohol were performed and supported the idea that the cation radical of these chemicals are capable of reactivating compound III. The significance of these reactions with respect to lignin peroxidase catalysis are discussed.