Dioxygen activation and substrate hydroxylation by the hydroxylase component of toluene/O-xylene monooxygenase from pseudomonas sporium OX1

Abstract

Non-heme carboxylate-bridged diiron centers in the hydroxylase components of the bacterial multicomponent monooxygenases activate dioxygen at structurally homologous active sites. Catalysis requires the management of four substrates: electrons, protons, dioxygen, and hydrocarbons. Protein component complexes control the delivery of these substrates to the diiron center in the hydroxylase ensuring selective hydrocarbon oxidation. A detailed mechanistic understanding of structural and chemical consequences of such interactions is a significant challenge. This thesis begins with an overview of our current understanding of these processes. The discussion is primarily on the methane monooxygenase systems (MMO) because these have been the most extensively studied BMMs to date. Recent results for the toluene/o-xylene monooxygenase (ToMO) and phenol hydroxylase systems from Pseudomonas sporium OX1 are also briefly summarized, the former being the research focus of this dissertation. Restricting access to the diiron center in ToMOH and other non-heme carboxylate-bridged diiron proteins was proposed to facilitate observation of oxygenated intermediates. To examine this hypothesis, dioxygen activation in ToMOH mutants that were predicted to occlude this channel was investigated by rapid-freeze quench (RFQ) EPR, Mossbauer, and ENDOR spectroscopy and stoppedflow optical spectroscopy. For the I100W mutant, a transient species is observed with an absorption maximum at 500 nm. EPR and Mossbauer spectra of RFQ samples identified this species as a diiron(III,IV) cluster spin-coupled to a neutral W radical. ENDOR spectra of this intermediate confirmed the protonation state and type of the amino acid radical and also identified a labile terminal water or hydroxide on the diiron center.

(cont.) Decay of this intermediate results in hydroxylation of the W radical. A diamagnetic precursor to the mixed-valent diiron(III,IV) center was also observed at an earlier time-point, with Mossbauer parameters typical of high-spin FeIII. We have tentatively assigned this antiferromagnetically-coupled diiron(III) intermediate as a peroxo-bridged cluster. A similar diiron(III) species is observed in RFQ Mossbauer samples from the reaction of reduced wild type hydroxylase with dioxygen. Substrate accelerates the decay rate of this species, providing evidence for the diiron(III) transient as the active oxidant. Under steady state conditions, hydrogen peroxide was generated in the absence of substrate. The oxidized hydroxylase also decomposed hydrogen peroxide to liberate dioxygen if no reducing equivalents were present. This catalase activity suggests that dioxygen activation could be reversible. The linear free energy relationship determined from steady state hydroxylation of para substituted phenols has a negative slope. A value of ? < 0 is indicative of electrophilic attack on the aromatic substrate by the oxidizing diiron(III) intermediate. The results from these steady state and pre-steady experiments provide compelling evidence that the diiron(III) transient is the active oxidant in ToMO and is a peroxodiiron(III) transient, despite differences between the optical and Mossbauer spectroscopic parameters and those of other peroxodiiron(III) centers. Enzymatic oxidation of the radical clock substrate probe, norcarane, by ToMO gives rise to both desaturation and hydroxylation products, norcarenes and norcaranols respectively.

(cont.) Norcarenes are better substrates for this enzyme system than norcarane, producing additional oxidation products. In all, more than twenty oxidation products were characterized in these reaction mixtures, half of which arose from norcarene oxidation. Accounting for these secondary oxidation products, we determined that no substrate radical intermediates with a significant lifetime (t < 25 ps) are formed during catalysis.