title: Characterization of two ferredoxin-dependent sulfite reductases having different substrate specificity in the red alga Cyanidioschyzon merolae.
abstract: Assimilatory sulfite reductase (SiR) and nitrite reductase (NiR), which are important determinants in biomass productivity, are homologous enzymes that catalyze the reduction of sulfite to sulfide and nitrite to ammonium, respectively. They have a siroheme and a [4Fe-4S] cluster as prosthetic groups in common. The red alga Cyanidioschyzon merolae encodes two SiR-like enzymes, CmSiRA and CmSiRB, which are likely products of recent gene duplication, but no homologues of NiR. The growth in a medium containing nitrate, however, must be supported by a nitrite reducing activity. CmSiRB was not detected in the ammonium medium, but, in the nitrate medium, it was present at a level 1/6 of that of constitutively expressed CmSiRA. Kinetic analysis of the two enzymes showed that CmSiRA has high kcat values with both sulfite and nitrite, but CmSiRB has virtually only the activity of nitrite reduction, although the Km value against nitrite was fairly high in both enzymes. The six amino acid residues that are specific to CmSiRB among various SiR-like enzymes in the active site were mutagenized to mimic partially CmSiRA. Among them, the mutation S217C in CmSiRB partially recovered sulfite reduction activity, suggesting that this residue is a major determinant of substrate specificity.
authors.affiliation: Komaba Organization for Educational Excellence, College of Arts and Sciences, The University of Tokyo, Komaba 3-8-1, Meguro-ku, Tokyo 153-8902, Japan.|Department of Life Sciences, Graduate School of Arts and Sciences, The University of Tokyo, Komaba 3-8-1, Meguro-ku, Tokyo 153-8902, Japan.|JST, CREST, K's Gobancho, 7, Gobancho, Chiyoda-ku, Tokyo 102-0076.
authors.forename: Kohsuke
authors.initials: K
authors.lastname: Sekine
authors.affiliation: Department of Life Sciences, Graduate School of Arts and Sciences, The University of Tokyo, Komaba 3-8-1, Meguro-ku, Tokyo 153-8902, Japan.|JST, CREST, K's Gobancho, 7, Gobancho, Chiyoda-ku, Tokyo 102-0076.
authors.forename: Takashi
authors.initials: T
authors.lastname: Moriyama
authors.affiliation: Institute for Protein Research, Osaka University, Yamadaoka 3-2, Suita-shi, Osaka 565-0871, Japan.
authors.forename: JuYaen
authors.initials: J
authors.lastname: Kim
authors.affiliation: Institute for Protein Research, Osaka University, Yamadaoka 3-2, Suita-shi, Osaka 565-0871, Japan.
authors.forename: Toshiharu
authors.initials: T
authors.lastname: Hase
authors.affiliation: Department of Life Sciences, Graduate School of Arts and Sciences, The University of Tokyo, Komaba 3-8-1, Meguro-ku, Tokyo 153-8902, Japan.|JST, CREST, K's Gobancho, 7, Gobancho, Chiyoda-ku, Tokyo 102-0076.
authors.forename: Naoki
authors.initials: N
authors.lastname: Sato
mesh_terms: D000461:Rhodophyta / Q000201:enzymology*; D013379:Substrate Specificity; D050889:Sulfite Reductase (Ferredoxin) / Q000235:genetics / Q000378:metabolism*; D013447:Sulfites / Q000378:metabolism*
chemical_list: D013447:Sulfites; D050889:Sulfite Reductase (Ferredoxin)
keywords: Cyanidioschyzon merolae; nitrite reductase; red algae; substrate specificity; sulfite reductase