Mechanisms by which soluble (i.e., dissolved and colloidal) humic acids (HA) and fulvic acids (FA) alter the transformation kinetics of aromatic substrates by oxidoreductive enzymes such as lignin peroxidase and laccase are not well characterized nor have been quantitatively compared. Hypothesized mechanisms are that soluble HA and FA (1) deactivate the enzyme; (2) act as a competitive substrate; or (3) sequester the aromatic substrate. In this study the transformation of pentachlorophenol (PCP) by purified laccase isolated from Trametes versicolor at 28°C and pH 5.0, in combination with Aldrich HA, Leonardite HA, Suwannee River FA, or Waskish peat FA, were used as a model system to quantitatively test and compare these hypotheses. The four HA and FA did not deactivate laccase after adding catechol as the enzyme's substrate. Slower PCP transformation rates by laccase in the presence of HA and FA were found with higher inhibitor binding constants of HA and FA (Ki, ranging from 3.63±0.78 to 6.47±1.07 μg mL−1) and sorption coefficients of HA and FA to PCP (Kdom, log Kdom ranging from 4.09 to 4.81). Comparison of experimental PCP transformation rates in the presence of soluble HA and FA with predicted transformation rates of dissolved PCP (i.e., PCP not associated with soluble HA and FA) indicated that soluble HA and FA modified PCP transformation by lacasse through sequestration and by acting as a competitive substrate.
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