title: Reactivity, secondary structure, and molecular topology of the Escherichia coli sulfite reductase flavodoxin-like domain.
abstract: The flavodoxin-like domain, missing in the three-dimensional structure of the monomeric, simplified model of the Escherichia coli sulfite reductase flavoprotein component (SiR-FP), has now been expressed independently. This 168 amino acid protein was named SiR-FP18 with respect to its native molecular weight and represents the FMN-binding domain of SiR-FP. This simplified biological object has kept the main characteristics of its counterpart in the native protein. It could incorporate FMN exclusively and stabilize a neutral air-stable semiquinone radical. Both the radical and the fully reduced forms of SiR-FP18 were able to transfer their electrons to DCPIP or cytochrome c quantitatively. SiR-FP18 was able to form a highly stable complex with SiR-HP, the hemoprotein component of the sulfite reductase containing an iron-sulfur cluster coupled to a siroheme. In agreement with the postulated catalytic cycle of SiR-FP, only the fully reduced form of SiR-FP18 could transfer one electron to SiR-HP, the transferred electron being localized exclusively on the heme. As isolated SiR-FP18 has kept the main characteristics of the FMN-binding domain of the native protein, a structural analysis by NMR was performed in order to complete the partial structure obtained previously. Structural modeling was performed using sequence homologues, cytochrome P450 reductase (CPR; 29% identity) and bacterial cytochrome P450 (P450-BM3; 26% identity), as conformational templates. These sequences were anchored using common secondary structural elements identified from heteronuclear NMR data measured on the protein backbone. The resulting structural model was validated, and subsequently refined using residual (C(alpha)-C', N-H(N), and C'-H(N)) dipolar couplings measured in an anisotropic medium. The overall fold of SiR-FP18 is very similar to that of bacterial flavodoxins and of the flavodoxin-like domain in CPR or P450-BM3.
authors.affiliation: Laboratoire de Chimie et Biochimie des Centres Rédox Biologiques, CEA-Grenoble, DBMS/CB, UMR 5047 CNRS-CEA-UJF, 17, Avenue des Martyrs, 38054 Grenoble Cedex 9, France.
authors.forename: Ludovic
authors.initials: L
authors.lastname: Champier
authors.forename: Nathalie
authors.initials: N
authors.lastname: Sibille
authors.forename: Beate
authors.initials: B
authors.lastname: Bersch
authors.forename: Bernhard
authors.initials: B
authors.lastname: Brutscher
authors.forename: Martin
authors.initials: M
authors.lastname: Blackledge
authors.forename: Jacques
authors.initials: J
authors.lastname: Covès
mesh_terms: D000595:Amino Acid Sequence; D001483:Base Sequence; D002846:Chromatography, Affinity; D017931:DNA Primers; D004578:Electron Spin Resonance Spectroscopy; D004926:Escherichia coli / Q000201:enzymology*; D005418:Flavodoxin / Q000378:metabolism*; D006860:Hydrogen Bonding; D008969:Molecular Sequence Data; D050862:Oxidoreductases Acting on Sulfur Group Donors / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism*; D017510:Protein Folding; D017433:Protein Structure, Secondary; D011993:Recombinant Fusion Proteins / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism
chemical_list: D017931:DNA Primers; D005418:Flavodoxin; D011993:Recombinant Fusion Proteins; D050862:Oxidoreductases Acting on Sulfur Group Donors