The Effect of Manganese on the Oxidation of Chemicals by Lignin Peroxidase

It has recently been discovered that lignin peroxidase isozyme H2 (LiPH2) has the ability to oxidize Mn²⁺ (Khindaria et al., 1995). Furthermore, at pH 4.5, the physiological pH of Phanerochaete chrysosporium, LiPH2 oxidizes Mn²⁺ at a much faster rate (25 times) than veratryl alcohol (VA). The ability of Mn²⁺ to act as a redox mediator for indirect oxidations catalyzed by LiPH2 was therefore investigated. In the presence of physiologically relevant levels of oxalate and Mn²⁺, the rate of LiPH2- catalyzed oxidation of all substrates studied was dramatically increased. Up to 10-fold stimulations were observed compared to the rates of oxidation of substrate in either the presence or absence of VA. We propose that the stimulation is due to the ability of LiPH2 to oxidize Mn²⁺, producing the strong oxidant Mn³⁺, at a high rate. The rates of oxidation of the substrates showed a hyperbolic dependence on Mn²⁺ in the presence of oxalate, a chelator which was required for maximal activity. The oxalate dependence of the oxidation rates correlated well with the concentration of the 1:1 complex of Mn²⁺-oxalate. The relative concentrations of the substrates and H₂O₂ and the rate constants for their reactions with Mn3+ determined which chemical was oxidized by the enzymatically produced Mn³⁺. The importance of the ability of Mn²⁺-oxalate to stimulate the oxidation of chemicals by LiPH2 is discussed.