Nitroalkane oxidase (NAO) catalyzes the oxidation of neutral nitroalkanes to aldehydes and ketones with oxygen consumption and the production of hydrogen peroxide and nitrite. The enzyme is a flavoprotein from the fungus Fusarium oxysporum. The active site base, Asp402, abstracts one proton from the substrate to give a carbanion which then attacks the flavin adenine dinucleotide (FAD). The three dimensional crystal structure of NAO shows that Arg409 is 3.6 Å from Asp402. When Arg409 is mutated to Lys, the rate constant for proton abstraction decreases 100-fold. The three-dimensional structure of NAO also reveals the existence of a tunnel which extends from the protein exterior and terminates at the FAD N5 atom and the residues Asp402 and Phe401. We mutated amino acids in the tunnel into tryptophan, phenylalanine and leucine. The L99W, S276W and S276A enzymes showed the biggest decreases in both kcat and kcat/Km; these amino acids are closest to the FAD molecule and the active site. Mutation of amino acids farther away from the active site showed very small changes in the kinetic parameters. Ser276 is hydrogen bonded to Asp402 in the wild-type enzyme. When this amino acid is mutated to alanine or tryptophan, k3, the rate constant for proton abstraction, decreases around 35 fold. Asp402, Arg409 and Ser276 constitute a catalytic triad in the active site of nitroalkane oxidase, and both Arg409 and Ser276 are important for positioning Asp402 and catalysis.

Paderborn, Universiẗat, Diss., 2003.

京都大学 / 新制・課程博士 / 博士(工学) / 甲第23910号 / 工博第4997号 / 新制||工||1780(附属図書館) / 京都大学大学院工学研究科材料化学専攻 / (主査)教授 中尾 佳亮, 教授 松原 誠二郎, 教授 杉野目 道紀 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

The central topic of this dissertation is to seek the answer to the question: Is the single transition state model appropriate for (1) the proton transfer reactions of nitroalkanes and (2) the aromatic nucleophilic reactions of trinitroarenes? If not, what are the real mechanisms? This objective has been accomplished by careful kinetic and mechanistic studies which take advantage of modern digital acquisition of absorbance - time data, combined with extensive new data analysis of results from pseudo-first-order kinetic measurements. Several new analysis procedures for pseudo-first-order kinetics that are capable of distinguishing between single-step and multi-step mechanisms have been introduced and intensively confirmed during the application in the kinetic study of the target reactions. The procedures include (1) half-life dependence of apparent rate constant, (2) sequential linear pseudo-first-order correlation, (3) approximate instantaneous rate constant analysis, and (4) time-dependent apparent kinetic isotope effects. Various conventional and nonconventional pseudo-first-order kinetic analyses of the reactions of nitroalkanes in aqueous solutions revealed that the reactions are complex and involve kinetically significant intermediates. The spectral evidence for the formation of reactive intermediates was obtained by carrying out the kinetic experiments at the isosbestic points where changes in reactant and product absorbance cancel. The apparent deuterium kinetic isotope effects studies indicated that the deuterium kinetic isotope effects are not associated with the formation of the intermediates. The observations of anomalous Brønsted exponents previously found for this reaction series could be rationalized well with the complex mechanisms proposed in this work, which indicate that the nitroalkane anomaly does not exist, but arises from an interpretation based upon the incorrect assumption that the reactions follow a simple one-step mechanism. Simulations revealed that the generally accepted competitive mechanism was not appropriate to describe the Meisenheimer complex formation during the reaction of 2,4,6-trinitroanisole with methoxide ion in methanol. This conclusion is supported by the conventional pseudo-first-order kinetic analysis. A reversible consecutive mechanism that accounts for the kinetic behavior has been proposed, which involves an intermediate dianion complex that is formed reversibly from the initial 1,3-σ-complex, and then eliminates methoxide ion to form the 1,1-σ-complex product.