title: Sulfite:Cytochrome c oxidoreductase from Thiobacillus novellus. Purification, characterization, and molecular biology of a heterodimeric member of the sulfite oxidase family.
abstract: Direct oxidation of sulfite to sulfate occurs in various photo- and chemotrophic sulfur oxidizing microorganisms as the final step in the oxidation of reduced sulfur compounds and is catalyzed by sulfite:cytochrome c oxidoreductase (EC ). Here we show that the enzyme from Thiobacillus novellus is a periplasmically located alphabeta heterodimer, consisting of a 40.6-kDa subunit containing a molybdenum cofactor and an 8.8-kDa mono-heme cytochrome c(552) subunit (midpoint redox potential, E(m8.0) = +280 mV). The organic component of the molybdenum cofactor was identified as molybdopterin contained in a 1:1 ratio to the Mo content of the enzyme. Electron paramagnetic resonance spectroscopy revealed the presence of a sulfite-inducible Mo(V) signal characteristic of sulfite:acceptor oxidoreductases. However, pH-dependent changes in the electron paramagnetic resonance signal were not detected. Kinetic studies showed that the enzyme exhibits a ping-pong mechanism involving two reactive sites. K(m) values for sulfite and cytochrome c(550) were determined to be 27 and 4 micrometer, respectively; the enzyme was found to be reversibly inhibited by sulfate and various buffer ions. The sorAB genes, which encode the enzyme, appear to form an operon, which is preceded by a putative extracytoplasmic function-type promoter and contains a hairpin loop termination structure downstream of sorB. While SorA exhibits significant similarities to known sequences of eukaryotic and bacterial sulfite:acceptor oxidoreductases, SorB does not appear to be closely related to any known c-type cytochromes.
authors.affiliation: Institut für Mikrobiologie und Biotechnologie, Meckenheimer Allee 168, D-53115 Bonn, Germany.
authors.forename: U
authors.initials: U
authors.lastname: Kappler
authors.forename: B
authors.initials: B
authors.lastname: Bennett
authors.forename: J
authors.initials: J
authors.lastname: Rethmeier
authors.forename: G
authors.initials: G
authors.lastname: Schwarz
authors.forename: R
authors.initials: R
authors.lastname: Deutzmann
authors.forename: A G
authors.initials: AG
authors.lastname: McEwan
authors.forename: C
authors.initials: C
authors.lastname: Dahl
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism; D001483:Base Sequence; D003579:Cytochrome Reductases / Q000737:chemistry / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism; D008969:Molecular Sequence Data; D016415:Sequence Alignment; D013379:Substrate Specificity; D050902:Sulfite Dehydrogenase; D013855:Thiobacillus / Q000201:enzymology*
chemical_list: D001426:Bacterial Proteins; D003579:Cytochrome Reductases; D050902:Sulfite Dehydrogenase