Spectroscopic and computational studies of the intradiol and extradiol dioxygenases: Understanding oxygen activation by ferrous and ferric non-heme iron active sites.

摘要

Mononuclear non-heme iron enzymes catalyze a wide range of biological reactions involving O2. A sub-class of these enzymes is the catechol dioxygenases. They are mostly found in soil bacteria but related enzymes are also found in humans. They catalyze the ring cleavage of catecholic substrates by inserting both atoms of O2 into the aromatic ring. Based on the position of ring cleavage, the catechol dioxygenases are further divided into intradiol and extradiol dioxygenases. These enzymes have different residues around the iron center and differ in metal oxidation states and reaction mechanisms. Intradiol dioxygenases employ a Fe3+ center to activate the substrate for direct attack by O2 and insert O2 between the vicinal hydroxyl groups to yield muconic acids derivatives. Alternatively, extradiol dioxygenases use a Fe2+ center to activate O 2 and incorporate both atoms of O2 adjacent to the vicinal diols to yield muconic semialdehyde products. The significance of these enzymes lies in their role in bioremediation, and mutations in human enzymes are associated with genetic diseases.; Oxygen intermediates are often too labile to be trapped for spectroscopic studies; therefore we applied a combination of experiments and theoretical calculations on a series of anaerobic species to evaluate the similarities and differences in these enzymes. To understand how the Fe3+ site activates the substrate in intradiol dioxygenases, we characterized the geometric and electronic structures of the anaerobic enzyme-substrate complex by using variable-temperature variable-field magnetic circular dichroism spectroscopy and density functional theory (DFT) calculations. We further developed a theoretical model to explain how triplet O2 interacts with singlet catechol in this formally spin-forbidden reaction. To understand how the Fe 2+ site activates O2 in extradiol dioxygenases, we used NO as an O2 analogue to study potential FeO2 intermediates in the protein-catalyzed reactions. We developed an experimentally calibrated DFT protocol for {lcub}FeNO{rcub}7 complexes using a well-characterized model system. By applying this DFT methodology on the enzyme-nitrosyl complexes and complementing the results by spectroscopic data, we have laid the foundation necessary for the investigation of the O2 reaction coordinate of extradiol dioxygenases and identification of factors governing the specificity of ring cleavage in the catechol dioxygenases.