Mechanistic studies on bioremediation metalloenzymes

Oakland University, Rochester MI

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Abstract

? DESCRIPTION (provided by applicant): Aromatic compounds are commonly formed during the incomplete burning of fossil fuels, solvents, pesticides and plastics. Their placement within the biosphere either intentionally or accidentally has been a problem for industrialized countries. Bioremediation is the intentional use and manipulation of living organisms to remove environ- mental pollutants. The combination of rapid growth rate, global abundance and high rate of mutations enables bacteria to adapt to utilize pollutants as a carbon source. In aerobic microorganisms, activation of an aromatic substrate is usually effected by hydroxylation of the ring and subsequent dearomatization. Ring-fission dioxygenases that catalyze these reactions contain Fe3+ ions (intradiol dioxygenases) or Fe2+ ions (extradiol cleaving enzymes). A third Class of dioxygenases has been recently identified. These enzymes belong to the cupin super- family, which is characterized by a six-stranded ?-barrel fold and conserved amino acid motifs providing 3His or 2- or 3His-1Glu ligand environments to metal ions. The enzymes gentisate 1,2-dioxygenase (GDO) and salicylate 1,2-dioxygenase (SDO) belong to this new class and contain a 3-His metal binding site. Some mechanistic work has been reported for GDO, however, essentially no mechanistic work for SDO has been published. This provides an opportunity to investigate the mechanism for SDO using model studies since it is unlikely to be the same as GDO due to structural differences in the substrates. Additionally, a scarcely characterize bioremediation enzyme (2,4'-dihydroxyacetophenone dioxygenase, DAD) capable of oxidizing 2,4'-dihydroxyacetophenone (DHAP) has been identified. This cupin enzyme contains a 3-His-1-Glu active site. Synthetic model studies can assist in elucidating the mechanism for this new enzyme as well. In this proposal we aim to synthesize iron compounds using organic ligands of varying properties. We will synthesize three nitrogen and three nitrogen-one-carboxylate ligands. These ligands will be reacted with iron(II) and iron(III) to generate model systems with tunable coordination geometry and electronic properties. The complexes will be systematically studied to generate correlations be- tween the model complex structure and spectroscopic and physical properties. Compounds which are deemed good structural and spectroscopic models for selected bioremediation enzymes will be studied for biomimetic activity. Next, we will probe the reactivity of model complexes towards aromatic ring-containing compounds. Assays will be performed to test the complexes for both stoichiometric and catalytic dioxygenase activity. If intermediates are observed at low temperature, we will attempt to isolate and characterize these species. Such information will provide mechanistic insights relevant to bioremediation enzymes thus providing a better understanding of how these new classes of enzymes performs their function.