title: The Qrc membrane complex, related to the alternative complex III, is a menaquinone reductase involved in sulfate respiration.
abstract: Biological sulfate reduction is a process with high environmental significance due to its major contribution to the carbon and sulfur cycles in anaerobic environments. However, the respiratory chain of sulfate-reducing bacteria is still poorly understood. Here we describe a new respiratory complex that was isolated as a major protein present in the membranes of Desulfovibrio vulgaris Hildenborough. The complex, which was named Qrc, is the first representative of a new family of redox complexes. It has three subunits related to the complex iron-sulfur molybdoenzyme family and a multiheme cytochrome c and binds six hemes c, one [3Fe-4S](+1/0) cluster, and several interacting [4Fe-4S](2+/1+) clusters but no molybdenum. Qrc is related to the alternative complex III, and we show that it has the reverse catalytic activity, acting as a Type I cytochrome c(3):menaquinone oxidoreductase. The qrc genes are found in the genomes of deltaproteobacterial sulfate reducers, which have periplasmic hydrogenases and formate dehydrogenases that lack a membrane subunit for reduction of the quinone pool. In these organisms, Qrc acts as a menaquinone reductase with electrons from periplasmic hydrogen or formate oxidation. Binding of a menaquinone analogue affects the EPR spectrum of the [3Fe-4S](+1/0) cluster, indicating the presence of a quinone-binding site close to the periplasmic subunits. Qrc is the first respiratory complex from sulfate reducers to have its physiological function clearly elucidated.
authors.affiliation: Instituto de Tecnologia Química e Biológica, Universidade Nova de Lisboa, Oeiras 2780-157, Portugal.
authors.forename: Sofia S
authors.initials: SS
authors.lastname: Venceslau
authors.forename: Rita R
authors.initials: RR
authors.lastname: Lino
authors.forename: Ines A C
authors.initials: IA
authors.lastname: Pereira
mesh_terms: D001665:Binding Sites; D002462:Cell Membrane / Q000201:enzymology*; D003574:Cytochrome c Group / Q000378:metabolism; D016969:Desulfovibrio vulgaris / Q000166:cytology / Q000201:enzymology / Q000254:growth & development; D004579:Electron Transport; D014450:Electron Transport Complex III / Q000378:metabolism*; D005560:Formate Dehydrogenases / Q000378:metabolism; D006859:Hydrogen / Q000378:metabolism; D006864:Hydrogenase / Q000378:metabolism; D019897:Periplasm / Q000201:enzymology; D011808:Quinone Reductases / Q000737:chemistry / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism*; D011809:Quinones / Q000378:metabolism; D017422:Sequence Analysis, DNA; D013431:Sulfates / Q000378:metabolism*
chemical_list: D003574:Cytochrome c Group; D011809:Quinones; D013431:Sulfates; D006859:Hydrogen; C017165:cytochrome c(3); D006864:Hydrogenase; D005560:Formate Dehydrogenases; D011808:Quinone Reductases; D014450:Electron Transport Complex III