Kinetic analysis of the noncompetitive inhibition of the lignin-peroxidase-catalyzed reaction by oxalic acid

Oxalic acid was found to inhibit noncompetitively the Cα-Cβ bond cleavage of veratrylglycerol catalyzed by a lignin peroxidase (LiP) isozyme of the white-rot fungus P. chrysosporium. With greater amounts of oxalic acid in the LiP system, the substrate was not converted to veratraldehyde but was almost all recovered. Oxalic acid was shown to be decomposed to CO 2 during the enzymatic reaction. The results clearly indicate that oxalic acid reduced the cation radical intermediate formed in the reaction back to the substrate to block the production of veratraldehyde. A novel equation has been derived to explain the mechanism for this unique non-competitive inhibition that is different from the classical noncompetitive one. The inhibition constant K i obtained here, which is different from the classical inhibition constant K i , is defined as the ratio of the rate constant (k p ) for product formation to the rate constant (k i ) for the reduction of the cation radical to the substrate.