Dioxygen reduction by bo-type quinol oxidase from Escherichia coli studied by submillisecond-resolved freeze-quench EPR spectroscopy

Characterization of the semiquinone radical stabilized by the cytochrome aa3-600 menaquinol oxidase of Bacillus subtilis

Cytochrome aa(3)-600 is one of the principle respiratory oxidases from Bacillus subtilis and is a member of the heme-copper superfamily of oxygen reductases. This enzyme catalyzes the two-electron oxidation of menaquinol and the four-electron reduction of O(2) to 2H(2)O. Cytochrome aa(3)-600 is of interest because it is a very close homologue of the cytochrome bo(3) ubiquinol oxidase from Escherichia coli, except that it uses menaquinol instead of ubiquinol as a substrate. One question of interest is how the proteins differ in response to the differences in structure and electrochemical properties between ubiquinol and menaquinol. Cytochrome bo(3) has a high affinity binding site for ubiquinol that stabilizes a ubi-semiquinone. This has permitted the use of pulsed EPR techniques to investigate the protein interaction with the ubiquinone. The current work initiates studies to characterize the equivalent site in cytochrome aa(3)-600. Cytochrome aa(3)-600 has been cloned and expressed in a His-tagged form in B. subtilis. After isolation of the enzyme in dodecylmaltoside, it is shown that the pure enzyme contains 1 eq of menaquinone-7 and that the enzyme stabilizes a mena-semiquinone. Pulsed EPR studies have shown that there are both similarities as well as significant differences in the interactions of the mena-semiquinone with cytochrome aa(3)-600 in comparison with the ubi-semiquinone in cytochrome bo(3). Our data indicate weaker hydrogen bonds of the menaquinone in cytochrome aa(3)-600 in comparison with ubiquinone in cytochrome bo(3). In addition, the electronic structure of the semiquinone cyt aa(3)-600 is more shifted toward the anionic form from the neutral state in cyt bo(3).

NO Reduction by Nitric-oxide Reductase from Denitrifying Bacterium Pseudomonas aeruginosa

Nitric-oxide reductase (NOR) of a denitrifying bacterium catalyzes NO reduction to N(2)O at the binuclear catalytic center consisting of high spin heme b(3) and non-heme Fe(B). The structures of the reaction intermediates in the single turnover of the NO reduction by NOR from Pseudomonas aeruginosa were investigated using optical absorption and EPR spectroscopies combined with an originally designed freeze-quench device. In the EPR spectrum of the sample, in which the fully reduced NOR was mixed with an NO solution and quenched at 0.5 ms after the mixing, two characteristic signals for the ferrous Fe(B)-NO and the penta-coordinated ferrous heme b(3)-NO species were observed. The CO inhibition of its formation indicated that two NO molecules were simultaneously distributed into the two irons of the same binuclear center of the enzyme in this state. The time- and temperature-dependent EPR spectral changes indicated that the species that appeared at 0.5 ms is a transient reaction intermediate prior to the N(2)O formation, in good agreement with the so-called "trans" mechanism. It was also found that the final state of the enzyme in the single turnover cycle is the fully oxidized state, in which the mu-oxo-bridged ligand is absent between the two irons of its binuclear center, unlike the resting form of NOR as isolated. On the basis of these present findings, we propose a newly developed mechanism for the NO reduction reaction conducted by NOR.

Role of Tyr-288 at the dioxygen reduction site of cytochrome bo studied by stable isotope labeling and resonance raman spectroscopy

To explore the role of a cross-link between side chains of Tyr-288 and His-284 at the heme-copper binuclear center, we prepared cytochrome bo where d(4)-Tyr, 1-[(13)C]Tyr, or 4-[(13)C]Tyr has been biosynthetically incorporated. Unexpectedly, the d(4)-Tyr-labeled enzyme showed a large decrease in the ubiquinol-1 oxidase and CO binding activities. Optical absorption and resonance Raman spectra identified the defect in the distal side of the heme-copper binuclear center. In the CO-bound d(4)-Tyr-labeled enzyme, a large fraction of the nu((Fe-C)) mode was shifted from the normal 520-cm(-1) band to a broad band centered around 491 cm(-1), as found for the Y288F mutant. Our results suggested that the substitution of ring hydrogens of Tyr-288 with deuteriums slows down the formation of the His-Tyr cross-link essential for dioxygen reduction at the binuclear center.