succinate dehydrogenase, cytochrome b556 subunit; In E. coli and many other bacteria, two small, hydrophobic, mutually homologous subunits of succinate dehydrogenase, a TCA cycle enzyme, are SdhC and SdhD. This family is the SdhC, the cytochrome b subunit, called b556 in bacteria and b560 in mitochondria. SdhD (see TIGR02968) is called the hydrophobic membrane anchor subunit, although both SdhC and SdhD participate in anchoring the complex. In some bacteria, this cytochrome b subunit is replaced my a member of the cytochrome b558 family (see TIGR02046). [Energy metabolism, TCA cycle]

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1141145752   3 KQRPVNLDLTTISMPATAKASIFHRVTGVALFFALTFVIWAWSESLSSAESFEFVKGLFNGFIAKFIAWGTVSVLGYHLI 82
Cdd:TIGR02970   1 KQRPLSPHLQIYRFPITAILSILHRITGVLLFFGLPFLLWWLSLSLSSPESFATVHALLSSPLGKLILWGLLWALLYHLL 80

                          90       100       110       120
                  ....*....|....*....|....*....|....*....|
gi 1141145752  83 GGIRHMIMDMGHWEELESGNLSAKVSMALGVVFAVLAGVW 122
Cdd:TIGR02970  81 AGIRHLLWDLGYGLELKSARISAWVVLVLSLVLTILAGIW 120

Succinate:quinone oxidoreductase (SQR) Type C subfamily, Succinate dehydrogenase C (SdhC) subunit; composed of bacterial SdhC and eukaryotic large cytochrome b binding (CybL) proteins. SQR catalyzes the oxidation of succinate to fumarate coupled to the reduction of quinone to quinol. Members of this family reduce high potential quinones such as ubiquinone. SQR is also called succinate dehydrogenase or Complex II, and is part of the citric acid cycle and the aerobic respiratory chain. SQR is composed of a flavoprotein catalytic subunit, an iron-sulfur protein and one or two hydrophobic transmembrane subunits. Proteins in this subfamily are classified as Type C SQRs because they contain two transmembrane subunits and one heme group. The heme and quinone binding sites reside in the transmembrane subunits. The SdhC or CybL protein is one of the two transmembrane subunits of bacterial and eukaryotic SQRs. The two-electron oxidation of succinate in the flavoprotein active site is coupled to the two-electron reduction of quinone in the membrane anchor subunits via electron transport through FAD and three iron-sulfur centers. The reversible reduction of quinone is an essential feature of respiration, allowing transfer of electrons between respiratory complexes.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1141145752   5 RPVNLDLTTISMPATAKASIFHRVTGVALFFALTFVIWAWSESLSSAESFEFVKGLFNGFIAKFIAWGTVSVLGYHLIGG 84
Cdd:cd03499     1 RPLSPHLTIYRPPLTAILSILHRITGVALFLGLPLLLWWLLASLSSPESFESVSALLGSWLGKLVLFGLTWALFYHLLNG 80

                          90       100       110
                  ....*....|....*....|....*....|....*..
gi 1141145752  85 IRHMIMDMGHWEELESGNLSAKVSMALGVVFAVLAGV 121
Cdd:cd03499    81 IRHLIWDLGKGLELKTVYKSGYAVLVLSVVLTVLLGI 117

succinate dehydrogenase, cytochrome b556 subunit; In E. coli and many other bacteria, two small, hydrophobic, mutually homologous subunits of succinate dehydrogenase, a TCA cycle enzyme, are SdhC and SdhD. This family is the SdhC, the cytochrome b subunit, called b556 in bacteria and b560 in mitochondria. SdhD (see TIGR02968) is called the hydrophobic membrane anchor subunit, although both SdhC and SdhD participate in anchoring the complex. In some bacteria, this cytochrome b subunit is replaced my a member of the cytochrome b558 family (see TIGR02046). [Energy metabolism, TCA cycle]

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1141145752   3 KQRPVNLDLTTISMPATAKASIFHRVTGVALFFALTFVIWAWSESLSSAESFEFVKGLFNGFIAKFIAWGTVSVLGYHLI 82
Cdd:TIGR02970   1 KQRPLSPHLQIYRFPITAILSILHRITGVLLFFGLPFLLWWLSLSLSSPESFATVHALLSSPLGKLILWGLLWALLYHLL 80

                          90       100       110       120
                  ....*....|....*....|....*....|....*....|
gi 1141145752  83 GGIRHMIMDMGHWEELESGNLSAKVSMALGVVFAVLAGVW 122
Cdd:TIGR02970  81 AGIRHLLWDLGYGLELKSARISAWVVLVLSLVLTILAGIW 120

Succinate:quinone oxidoreductase (SQR) Type C subfamily, Succinate dehydrogenase C (SdhC) subunit; composed of bacterial SdhC and eukaryotic large cytochrome b binding (CybL) proteins. SQR catalyzes the oxidation of succinate to fumarate coupled to the reduction of quinone to quinol. Members of this family reduce high potential quinones such as ubiquinone. SQR is also called succinate dehydrogenase or Complex II, and is part of the citric acid cycle and the aerobic respiratory chain. SQR is composed of a flavoprotein catalytic subunit, an iron-sulfur protein and one or two hydrophobic transmembrane subunits. Proteins in this subfamily are classified as Type C SQRs because they contain two transmembrane subunits and one heme group. The heme and quinone binding sites reside in the transmembrane subunits. The SdhC or CybL protein is one of the two transmembrane subunits of bacterial and eukaryotic SQRs. The two-electron oxidation of succinate in the flavoprotein active site is coupled to the two-electron reduction of quinone in the membrane anchor subunits via electron transport through FAD and three iron-sulfur centers. The reversible reduction of quinone is an essential feature of respiration, allowing transfer of electrons between respiratory complexes.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1141145752   5 RPVNLDLTTISMPATAKASIFHRVTGVALFFALTFVIWAWSESLSSAESFEFVKGLFNGFIAKFIAWGTVSVLGYHLIGG 84
Cdd:cd03499     1 RPLSPHLTIYRPPLTAILSILHRITGVALFLGLPLLLWWLLASLSSPESFESVSALLGSWLGKLVLFGLTWALFYHLLNG 80

                          90       100       110
                  ....*....|....*....|....*....|....*..
gi 1141145752  85 IRHMIMDMGHWEELESGNLSAKVSMALGVVFAVLAGV 121
Cdd:cd03499    81 IRHLIWDLGKGLELKTVYKSGYAVLVLSVVLTVLLGI 117

Succinate:quinone oxidoreductase (SQR) and Quinol:fumarate reductase (QFR) family, transmembrane subunits; SQR catalyzes the oxidation of succinate to fumarate coupled to the reduction of quinone to quinol, while QFR catalyzes the reverse reaction. SQR, also called succinate dehydrogenase or Complex II, is part of the citric acid cycle and the aerobic respiratory chain, while QFR is involved in anaerobic respiration with fumarate as the terminal electron acceptor. SQRs may reduce either high or low potential quinones while QFRs oxidize only low potential quinols. SQR and QFR share a common subunit arrangement, composed of a flavoprotein catalytic subunit, an iron-sulfur protein and one or two hydrophobic transmembrane subunits. The structural arrangement allows efficient electron transfer between the catalytic subunit, through iron-sulfur centers, and the transmembrane subunit(s) containing the electron donor/acceptor (quinol or quinone). The reversible reduction of quinone is an essential feature of respiration, allowing the transfer of electrons between respiratory complexes. SQRs and QFRs can be classified into five types (A-E) according to the number of their hydrophobic subunits and heme groups. This classification is consistent with the characteristics and phylogeny of the catalytic and iron-sulfur subunits. Type E proteins, e.g. non-classical archael SQRs, contain atypical transmembrane subunits and are not included in this hierarchy. The heme and quinone binding sites reside in the transmembrane subunits. Although succinate oxidation and fumarate reduction are carried out by separate enzymes in most organisms, some bifunctional enzymes that exhibit both SQR and QFR activities exist.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1141145752  24 IFHRVTGVALFFALTFVIWAWSESLSSAESFEFVKGLFNGFIAKFIAWGTVSVLGYHLIGGIRHMIMDMGHWEELESGNL 103
Cdd:cd03493     1 ILHRITGVALLLFLPLHLLGLLALLGGPYAFAEVVAFLSSPLGKLLYLLLLLALLYHALNGIRHLIWDYGKGLELKLRKA 80

                          90
                  ....*....|....*...
gi 1141145752 104 SAKVSMALGVVFAVLAGV 121
Cdd:cd03493    81 LGYAVLALSVLLTVLLLF 98

Succinate:quinone oxidoreductase (SQR) Type A subfamily, Succinate dehydrogenase C (SdhC)-like subunit; SQR catalyzes the oxidation of succinate to fumarate coupled to the reduction of quinone to quinol. Members of this subfamily reduce low potential quinones such as menaquinone and thermoplasmaquinone. SQR is also called succinate dehydrogenase or Complex II, and is part of the citric acid cycle and the aerobic respiratory chain. SQR is composed of a flavoprotein catalytic subunit, an iron-sulfur protein and one or two hydrophobic transmembrane subunits. Members of this subfamily are similar to the Thermoplasma acidophilum SQR and are classified as Type A because they contain two transmembrane subunits as well as two heme groups. Although there are no structures available for this subfamily, the presence of two hemes has been proven spectroscopically for T. acidophilum. The two membrane anchor subunits are similar to the SdhD and SdhC subunits of bacterial SQRs, which contain heme and quinone binding sites. The two-electron oxidation of succinate in the flavoprotein active site is coupled to the two-electron reduction of quinone in the membrane anchor subunits via electron transport through FAD and three iron-sulfur centers. The reversible reduction of quinone is an essential feature of respiration, allowing transfer of electrons between respiratory complexes.

                          10        20        30        40        50        60        70
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....
gi 1141145752  22 ASIFHRVTGVALFFALTFVIWAWSESLSSAESFEFVKGLFNGFIAKFIAWGTVSVLGYHLIGGIRHMIMDMGHW 95
Cdd:cd03501     3 AWVLHRITGVVILFYLFLHVLDLSSLRRGPETYNAVIATYKSPIFKLGEFGLVAAVVFHALNGIRLILVDFGSG 76