Succinate dehydrogenase

Succinate dehydrogenase (SDH) or succinate-coenzyme Q reductase (SQR) or respiratory complex II is an enzyme complex, found in many bacterial cells and in the inner mitochondrial membrane of eukaryotes. It is the only enzyme that participates in both the citric acid cycle and oxidative phosphorylation. Histochemical analysis showing high succinate dehydrogenase in muscle demonstrates high mitochondrial content and high oxidative potential.

In step 6 of the citric acid cycle, SQR catalyzes the oxidation of succinate to fumarate with the reduction of ubiquinone to ubiquinol. This occurs in the inner mitochondrial membrane by coupling the two reactions together.

Mitochondrial and many bacterial SQRs are composed of four structurally different subunits: two hydrophilic and two hydrophobic. The first two subunits, a flavoprotein (SDHA) and an iron-sulfur protein (SDHB), form a hydrophilic head where enzymatic activity of the complex takes place. SDHA contains a covalently attached flavin adenine dinucleotide (FAD) cofactor and the succinate binding site and SDHB contains three iron-sulfur clusters: [2Fe-2S], [4Fe-4S], and [3Fe-4S]. The second two subunits are hydrophobic membrane anchor subunits, SDHC and SDHD. Human mitochondria contain two distinct isoforms of SDHA (Fp subunits type I and type II), these isoforms are also found in Ascaris suum and Caenorhabditis elegans. The subunits form a membrane-bound cytochrome b complex with six transmembrane helices containing one heme b group and a ubiquinone-binding site. Two phospholipid molecules, one cardiolipin and one phosphatidylethanolamine, are also found in the SDHC and SDHD subunits (not shown in the image). They serve to occupy the hydrophobic space below the heme b. These subunits are displayed in the attached image. SDHA is green, SDHB is teal, SDHC is fuchsia, and SDHD is yellow. Around SDHC and SDHD is a phospholipid membrane with the intermembrane space at the top of the image.

Two distinctive ubiquinone binding sites can be recognized on mammalian SDH – matrix-proximal Q_P and matrix-distal Q_D. Ubiquinone binding site Qp, which shows higher affinity to ubiquinone, is located in a gap composed of SDHB, SDHC, and SDHD. Ubiquinone is stabilized by the side chains of His207 of subunit B, Ser27 and Arg31 of subunit C, and Tyr83 of subunit D. The quinone ring is surrounded by Ile28 of subunit C and Pro160 of subunit B. These residues, along with Il209, Trp163, and Trp164 of subunit B, and Ser27 (C atom) of subunit C, form the hydrophobic environment of the quinone-binding pocket Qp. In contrast, ubiquinone binding site Q_D, which lies closer to inter-membrane space, is composed of SDHD only and has lower affinity to ubiquinone.

SDHA provides the binding site for the oxidation of succinate. The side chains Thr254, His354, and Arg399 of subunit A stabilize the molecule while FAD oxidizes and carries the electrons to the first of the iron-sulfur clusters, [2Fe-2S]. This can be seen in image 5.

The succinate-binding site and ubiquinone-binding site are connected by a chain of redox centers including FAD and the iron-sulfur clusters. This chain extends over 40 Å through the enzyme monomer. All edge-to-edge distances between the centers are less than the suggested 14 Å limit for physiological electron transfer. This electron transfer is demonstrated in image 8.

All subunits of human mitochondrial SDH are nuclear encoded. After translation, SDHA subunit is translocated as apoprotein into the mitochondrial matrix. Subsequently, one of the first steps is covalent attachment of the FAD cofactor (covalent flavinylation). This process is enhanced by succinate dehydrogenase assembly factor 2 (SDHAF2; also called SDH5 in yeast and SDHE in bacteria) and by some of the Krebs cycle intermediates. Fumarate most strongly stimulates covalent flavinylation of SDHA. Through studies of the bacterial system, the mechanism of FAD attachment has been shown to involve a quinone:methide intermediate. In mitochondrial, but not bacterial, assembly, SDHA interacts with a second assembly factor called succinate dehydrogenase assembly factor 4 (SDHAF4; called SDH8 in yeast) before it is inserted into the final complex.

Fe-S prosthetic groups of the subunit SDHB are being preformed in the mitochondrial matrix by protein complex ISU. The complex is also thought to be capable of inserting the iron-sulphur clusters in SDHB during its maturation. The studies suggest that Fe-S cluster insertion precedes SDHA-SDHB dimer forming. Such incorporation requires reduction of cysteine residues within active site of SDHB. Both reduced cysteine residues and already incorporated Fe-S clusters are highly susceptible to ROS damage. Two more SDH assembly factors, SDHAF1 (SDH6) and SDHAF3 (SDH7 in yeast), seem to be involved in SDHB maturation in way of protecting the subunit or dimer SDHA-SDHB from Fe-S cluster damage caused by ROS.