Abstract
Yeast NADPH-sulfite reductase (hydrogen suifide:NADP + oxidoreductase, EC 1.8.1.2) is a complex hemoflavoprotein. The Stokes radius was determined to be 80 Å by gel filtration and the molecular weight was estimated to be 604000. The minimal molecular weight calculated from flavin and heme content was 306000, indicating that this enzyme contains two FAD, two FMN and two hemes per molecule. The enzyme consists of two types of subunit, α and β, having molecular weights of 116000 and 167000, respectively. The subunit structure is suggested to be α 2 β 2. The secondary structure, the amino acid composition and the isoelectric point were also investigated. The K m values for sulfite and NADPH were 17 μM and 10 μM, respectively. Sulfite and NADPH affected the reaction velocity to give parallel Lineweaver-Burk plots, indicating the involvement of the ‘ping-pong’ mechanism in the overall reaction. NADP + inhibited the reaction competitively with NADPH and noncompetitively with sulfite. Inhibition by sulfide was partially noncompetitive with both substrates but very weak. These results are discussed and compared with sulfite reductase from Escherichia coli.
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Schedule a callMore From: Biochimica et Biophysica Acta (BBA) - Protein Structure and Molecular Enzymology
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