title: Is a Q-cycle-like mechanism operative in dihaemic succinate:quinone and quinol:fumarate oxidoreductases?
abstract: Succinate:quinone (SQR) and quinol:fumarate oxidoreductases (QFR) are members of the same enzyme family. These are membrane bound enzymes anchored to the membrane by one or two subunits that may contain two, one or no haems. For the dihaemic enzymes the electron pathway from the flavin at the catalytic centre to the quinones remains to be established. Taking into account that the two haems are located on opposite sites of the membrane, and the possible presence of two quinone binding sites, also located on opposite sides of the membrane, we re-hypothesise the presence of a Q-cycle type mechanism in these enzymes. Such a mechanism can explain an active functional role for two haems and two quinone binding sites, allowing SQR to conserve energy. With this testable hypothesis we intend to challenge the discussion and drive further experimentation to unravel the functional mechanism of SQRs and QFRs.
authors.affiliation: Instituto de Tecnologia Química e Biológica, Universidade Nova de Lisboa, Rua da Quinta Grande 6, 2780-156, Oeiras, Portugal. mpereira@itqb.unl.pt
authors.forename: Manuela M
authors.initials: MM
authors.lastname: Pereira
authors.forename: Miguel
authors.initials: M
authors.lastname: Teixeira
mesh_terms: D000595:Amino Acid Sequence; D016227:Benzoquinones / Q000378:metabolism; D001665:Binding Sites; D042963:Electron Transport Complex II; D006418:Heme / Q000032:analysis*; D017136:Ion Transport; D008956:Models, Chemical; D008969:Molecular Sequence Data; D009097:Multienzyme Complexes / Q000737:chemistry / Q000378:metabolism*; D010088:Oxidoreductases / Q000737:chemistry* / Q000378:metabolism*; D010507:Periodicity; D011522:Protons; D016415:Sequence Alignment; D013385:Succinate Dehydrogenase / Q000737:chemistry / Q000378:metabolism*
chemical_list: D016227:Benzoquinones; D009097:Multienzyme Complexes; D011522:Protons; C004532:quinone; D006418:Heme; D010088:Oxidoreductases; D042963:Electron Transport Complex II; C424907:quinol fumarate reductase; D013385:Succinate Dehydrogenase