title: Roles of conserved Arg(72) and Tyr(71) in the ascorbate-specific transmembrane electron transfer catalyzed by Zea mays cytochrome b561.
abstract: Cytochromes b561, novel transmembrane electron transport proteins residing in eukaryotic cells, have a number of common features including six transmembrane α-helices and two heme ligation sites. Our recent studies on recombinant Zea mays cytochrome b561 suggested that concerted proton/electron transfer mechanism was functioning in plant cytochromes b561 as well and that conserved Lys(83) on a cytosolic loop had important roles for ascorbate-binding and a succeeding electron transfer. In the present study, we conducted site-directed mutagenesis analyses on conserved Arg(72) and Tyr(71). Removal of a positive charge at Arg(72) did not affect significantly on the final heme reduction level with ascorbate as reductant. However, characteristic pH-dependent initial time-lag upon electron acceptance from ascorbate was completely lost for R72A and R72E mutants. Substitution of Tyr(71) with Ala or Phe affected both on the final heme reduction level and on the pH-dependent initial time-lag, causing acceleration of the electron transfer. These observations were interpreted as existence of specific interactions of Tyr(71) and Arg(72) with ascorbate. However, their mechanistic roles were distinctly different from that of Lys(83), as exemplified by K83A/Y71A double mutant, and might be related for expelling of monodehydroascorbate radical from the substrate-binding site to prevent a back-flow of electrons.
authors.affiliation: Department of Molecular Science and Material Engineering, Graduate School of Science and Technology, Kobe University, 1-1 Rokkodai-cho, Nada-ku, Kobe, Hyogo 657-8501, Japan.
authors.forename: Motiur Md
authors.initials: MM
authors.lastname: Rahman
authors.forename: Nobuyuki
authors.initials: N
authors.lastname: Nakanishi
authors.forename: Yoichi
authors.initials: Y
authors.lastname: Sakamoto
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Hori
authors.forename: Toshiharu
authors.initials: T
authors.lastname: Hase
authors.forename: Sam-Yong
authors.initials: SY
authors.lastname: Park
authors.forename: Motonari
authors.initials: M
authors.lastname: Tsubaki
mesh_terms: D001120:Arginine / Q000737:chemistry*; D001205:Ascorbic Acid / Q000378:metabolism; D001665:Binding Sites; D055162:Biocatalysis; D003573:Cytochrome b Group / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D004579:Electron Transport; D006418:Heme / Q000378:metabolism; D008239:Lysine / Q000737:chemistry; D016297:Mutagenesis, Site-Directed; D014443:Tyrosine / Q000737:chemistry*; D003313:Zea mays
chemical_list: D003573:Cytochrome b Group; C023646:cytochrome b561; D014443:Tyrosine; D006418:Heme; D001120:Arginine; D008239:Lysine; D001205:Ascorbic Acid