title: Transient-state reduction and steady-state kinetic studies of menaquinol oxidase from Bacillus subtilis, cytochrome aa3-600 nm. Spectroscopic characterization of the steady-state species.
abstract: Cytochrome aa3-600 or menaquinol oxidase, from Bacillus subtilis, is a member of the heme-copper oxidase family. Cytochrome aa3-600 contains cytochrome a, cytochrome a3, and CuB, and each is coordinated via histidine residues to subunit I. Subunit II of cytochrome aa3-600 lacks CuA, which is a common feature of the cytochrome c oxidase family members. Anaerobic reduction of cytochrome aa3-600 by the substrate analogue 2,3-dimethyl-1,4-naphthoquinone (DMN) resolves two distinct kinetic phases by stopped-flow, single-wavelength spectrometry. Global analysis of time-resolved, multiwavelength spectra shows that during these distinct phases cytochromes a and a3 are both reduced. Cyanide binding to cytochrome a3 enhances the fast phase rate, which in the presence of cyanide can be assigned to cytochrome a reduction, whereas cytochrome a3-cyanide reduction is slow. The steady-state activity of cytochrome aa3-600 exhibits saturation kinetics as a function of DMN concentration with a Km of 300 microM and a maximal turnover of 63.5 s(-1). Global kinetic analysis of steady-state spectra reveals a species that is characteristic of a partially reduced oxygen adduct of cytochrome a3-CuB, whereas cytochrome a remains oxidized. Electron paramagnetic resonance (EPR) spectroscopy of the oxidase in the steady state shows the expected signal from ferricytochrome a, and a new EPR signal at g = 2.01. A model of the catalytic cycle for cytochrome aa3-600 proposes initial electron delivery from DMN to cytochrome a, followed by rapid heme to heme electron transfer, and suggests possible origins of the radical signal in the steady-state form of the enzyme.
authors.affiliation: Department of Biochemistry, Queen's University, Kingston, Ontario K7L 3N6, Canada.
authors.forename: N R
authors.initials: NR
authors.lastname: Mattatall
authors.forename: L M
authors.initials: LM
authors.lastname: Cameron
authors.forename: B C
authors.initials: BC
authors.lastname: Hill
mesh_terms: D000693:Anaerobiosis; D001412:Bacillus subtilis / Q000201:enzymology*; D003486:Cyanides / Q000378:metabolism; D004578:Electron Spin Resonance Spectroscopy; D003576:Electron Transport Complex IV / Q000737:chemistry*; D007700:Kinetics; D008024:Ligands; D009285:Naphthoquinones / Q000378:metabolism; D010084:Oxidation-Reduction; D010100:Oxygen / Q000378:metabolism; D011485:Protein Binding; D013053:Spectrophotometry
chemical_list: D003486:Cyanides; D008024:Ligands; D009285:Naphthoquinones; C082578:menaquinol oxidase; D003576:Electron Transport Complex IV; D010100:Oxygen