Succinate:quinone oxidoreductase (SQR) Type A subfamily, Succinate dehydrogenase D (SdhD)-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        80
                 ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 518159844  50 EMAAWLFMRLSGVVLVVLVIGHLLIQLVLDGGvSKIGFAFVAGRWASPFWQVWDLLMLWLAMLHGANGLRTVINDYAERP 129
Cdd:cd03500    1 ESLAWLFQRITGVFLVFLLAGHFWVQHMDNGG-DVIDFAFVANRLASPLWKVWDLLLLVLALLHGGNGLRNILLDYVRRP 79

                         90       100
                 ....*....|....*....|....*..
gi 518159844 130 NTRLWLKGLLYTATVFTILLGTLVIFT 156
Cdd:cd03500   80 RLRRAVKGLLYVAGLLLIVLGTYVIFA 106

Succinate:quinone oxidoreductase (SQR) Type A subfamily, Succinate dehydrogenase D (SdhD)-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        80
                 ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 518159844  50 EMAAWLFMRLSGVVLVVLVIGHLLIQLVLDGGvSKIGFAFVAGRWASPFWQVWDLLMLWLAMLHGANGLRTVINDYAERP 129
Cdd:cd03500    1 ESLAWLFQRITGVFLVFLLAGHFWVQHMDNGG-DVIDFAFVANRLASPLWKVWDLLLLVLALLHGGNGLRNILLDYVRRP 79

                         90       100
                 ....*....|....*....|....*..
gi 518159844 130 NTRLWLKGLLYTATVFTILLGTLVIFT 156
Cdd:cd03500   80 RLRRAVKGLLYVAGLLLIVLGTYVIFA 106

Succinate dehydrogenase/Fumarate reductase transmembrane subunit; This family includes a transmembrane protein from both the Succinate dehydrogenase and Fumarate reductase complexes.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 518159844   35 RKRTKKSPKSTRGNFE--MAAWLFMRLSGVVLVVLVIGHLLIQLVLDGGVSKI-GFAFVAGRWASPFWQVWDLLMLWLAM 111
Cdd:pfam01127   2 RKNRPLSPHLGLYRAHltTWLSILHRITGVALAVLGLIFLLLWLLLLLSLLGPeSYATVVAWLASPVKLILLLLLLLALF 81

                          90       100       110       120
                  ....*....|....*....|....*....|....*....|.
gi 518159844  112 LHGANGLRTVINDYA---ERPNTRLWLKGLLYTATVFTILL 149
Cdd:pfam01127  82 YHAANGIRHLIWDVGfglELKTVRKSGAAVLALSVVLVIVL 122

Succinate:quinone oxidoreductase (SQR) Type A subfamily, Succinate dehydrogenase D (SdhD)-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        80
                 ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 518159844  50 EMAAWLFMRLSGVVLVVLVIGHLLIQLVLDGGvSKIGFAFVAGRWASPFWQVWDLLMLWLAMLHGANGLRTVINDYAERP 129
Cdd:cd03500    1 ESLAWLFQRITGVFLVFLLAGHFWVQHMDNGG-DVIDFAFVANRLASPLWKVWDLLLLVLALLHGGNGLRNILLDYVRRP 79

                         90       100
                 ....*....|....*....|....*..
gi 518159844 130 NTRLWLKGLLYTATVFTILLGTLVIFT 156
Cdd:cd03500   80 RLRRAVKGLLYVAGLLLIVLGTYVIFA 106

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
                 ....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 518159844  86 GFAFVAGRWASPFWQVWDLLMLWLAMLHGANGLRTVINDYAeRPNTRLWLKGLLYTATVFTILLGTLVIF 155
Cdd:cd03493   30 AFAEVVAFLSSPLGKLLYLLLLLALLYHALNGIRHLIWDYG-KGLELKLRKALGYAVLALSVLLTVLLLF 98

Succinate:quinone oxidoreductase (SQR) Type C subfamily, Succinate dehydrogenase D (SdhD) subunit-like; composed of predominantly uncharacterized bacterial proteins with similarity to the E. coli SdhD subunit. One characterized protein is the respiratory Complex II SdhD subunit of the only eukaryotic member, Reclinomonas americana. SQR catalyzes the oxidation of succinate to fumarate coupled to the reduction of quinone to quinol. It 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. E. coli SQR is classified as Type C SQRs because it contains two transmembrane subunits and one heme group. The SdhD and SdhC subunits are membrane anchor subunits containing 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 518159844  86 GFAFVAGRWASPFWQVWDLLMLWLAMLHGANGLRTVINDYAERPNTRLWLKGLLYTATVFTILLGTLVIF 155
Cdd:cd03495   31 SYAEVVAWLAHPFNAILLILTLVSAFYHARLGMQVVIEDYVHSEGLRLALIIAVKLFAIATAAAGIFAIL 100