Three mammalian cytochromes b561 are ascorbate-dependent ferrireductases

摘要: Cytochromes b(561) are a family of transmembrane proteins found in most eukaryotic cells. Three evolutionarily closely related mammalian cytochromes (chromaffin granule cytochrome b, duodenal and lysosomal b) were expressed Saccharomyces cerevisiaeDeltafre1Deltafre2 mutant, which lacks almost all its plasma membrane ferrireductase activity, to study their ability reduce ferric iron (Fe(3+)). The expression each these was able rescue the growth defect Deltafre1Deltafre2 mutant cells iron-deficient conditions, suggesting involvement metabolism. Plasma activities measured using intact yeast Each showed significant FeCN Fe(3+)-EDTA reductase that dependent on presence intracellular ascorbate. Site-directed mutagenesis b conducted identify amino acids indispensable for activity. Among more than 20 conserved or partially investigated, mutations four His residues (H47, H83, H117 H156), one Tyr (Y66) Arg (R67) completely abrogated whereas (R149), Phe (F44), Ser (S115), Trp (W119), Glu (E196), Gln (Q131) affected activity some degree. These may affect heme coordination, ascorbate binding, and/or substrate binding. Possible roles discussed. This demonstrates ascorbate-dependent members proteins.